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Lattice dynamics of ZnAl2O4 and ZnGa2O4 under high pressure
Abstract
In this work we present a first-principles density functional study of the vibrational properties of ZnAl2O4 and ZnGa2O4 as function of hydrostatic pressure. Based on our previous structural characterization of these two compounds under pressure, herewith, we report the pressure dependence on both systems of the vibrational modes for the cubic spinel structure, for the CaFe2O4-type structure (Pnma) in ZnAl2O4 and for marokite (Pbcm) ZnGa2O4. Additionally we report a second order phase transition in ZnGa2O4 from the marokite towards the CaTi2O4-type structure (Cmcm), for which we also calculate the pressure dependence of the vibrational modes at the Γ point. Our calculations are complemented with Raman scattering measurements up to 12 GPa that show a good overall agreement between our calculated and measured mode frequencies.
... In the last few decades, research has been focused on understanding the theoretical and experimental insight about the lattice dynamics of the ZnGa 2 O 4 spinel [14,15]. Lopez et al. have studied the phase transitions in ZnAl 2 O 4 and ZnGa 2 O 4 by first-principle studies and observed a second-order phase transition in ZnGa 2 O 4 spinel at a pressure of 40 GPa [15]. ...
... In the last few decades, research has been focused on understanding the theoretical and experimental insight about the lattice dynamics of the ZnGa 2 O 4 spinel [14,15]. Lopez et al. have studied the phase transitions in ZnAl 2 O 4 and ZnGa 2 O 4 by first-principle studies and observed a second-order phase transition in ZnGa 2 O 4 spinel at a pressure of 40 GPa [15]. In another work, Liang et al. investigated structural and elastic properties of ZnX 2 O 4 (X = Al, Cr, Ga) spinels up to high pressure of 50 GPa. ...
... The first-order Raman mode However, there is no contribution in the Raman spectra from the motion of the Ga 3+ located on octahedral sites [26]. The peak at 714 cm − 1 arises from the A 1g mode, while the T 2g modes are responsible for the Raman peaks at 466 cm − 1 and 611 cm − 1 , respectively [15]. The peak at 419 cm − 1 correspond to A 2u Raman mode of the ZnGa 2 O 4 spinel [35]. ...
The role of sintering on the microstructure and mechanical properties of ZnGa2O4 pellets has been systematically investigated in this work. The hardness (H) and elastic modulus (E) of the sintered pellets are obtained via quasi-static and dynamic nanoindentation. The force versus displacement curves revealed the elastic-plastic behaviour of the ZnGa2O4 ceramics. The H and E values of the sintered pellets vary within 5.29–3.94 GPa and 149.25–111.04 GPa, respectively. A decreasing trend is observed in both H and E of the sintered pellets with increased sintering duration, which could be ascribed to the grain size of the sintered products. In addition, the viscoelastic properties of the sintered ZnGa2O4 ceramics were examined via dynamic nanoindentation. The results from the dynamic nanoindentation testing reveal that ZnGa2O4 ceramics show a reverse indentation size effect (RISE). Further, a high storage modulus of 121 GPa is obtained with a corresponding low damping factor. Thus, the above investigations not only provide a new insight into the microstructural and mechanical properties of ZnGa2O4 spinel but also open avenues for its wider commercial applications.
... Since ZGGO is isostructural with ZGO, as seen in the XRD results, similar vibrational modes are expected for both compounds. Consequently, the signature of the vibrational modes was based on those of the ZGO [30]. Considering the reduced cell, ZGO has two formula units, corresponding to 14 atoms. ...
... Therefore, a total of 42 vibrational modes are generated. The total number of vibrational modes at the first Brillouin zone centre described by the irreducible representation of O h point group is [30,31]: ...
... The remaining T 1g mode is an acoustic phonon and all the other ones are silent modes, thus not contributing to the Raman nor the IR spectra. Therefore, at → k = → 0 , five Raman active modes are expected: A 1g , E g , and 3T 2g , with E and T modes doubly and triply degenerated, respectively [30,31]. In spinel oxides, the highest frequency vibrational mode, assigned as A 1g , is characteristic of stretching vibrations of O atoms inside the octahedral unit GaO 6 [30,32]. ...
Zn1+xGa2-2xGexO4 (ZGGO:Cr)-persistent phosphor, with a molar fraction, x, of x = 0.1, doped with a 0.5% molar of chromium, was synthesised via solid-state reaction at 1350 °C for 36 h. X-ray diffraction measurements and Raman spectroscopy evidence a single crystalline phase corresponding to the cubic spinel structure. Room temperature (RT) photoluminescence (PL) and afterglow decay profiles were investigated using above and below bandgap excitation. In both cases, persistent PL was observed for almost 8 h, mainly originating from a Cr3+ defect, the so-called N2 optical centre. RT PL excitation and diffuse reflectance allow identification of the best pathways of Cr3+ red/NIR emission, as well as estimation of the ZGGO bandgap energy at 4.82 eV. An in-depth investigation of the observed luminescence at 15 K and temperature‑dependent PL under site‑selective excitation reveals the spectral complexity of the presence of several optically active Cr3+ centres in the ZGGO host that emit in almost the same spectral region. Furthermore, the temperature dependence of the R‑lines’ intensity indicates the existence of thermal populating processes between the different optical centres. Such observations well account for a wide distribution of defect trap levels available for carrier capture/release, as measured by the persistent luminescence decay, from which the carriers are released preferentially to the N2 Cr3+-related optical centre.
... (b) Schematic view of the orthorhombic hexagonal phase (CaMn2O4-like) of ZnGa2O4. The atom color palettes for Zn, Ga, and O are yellow, blue, and red, respectively [46] (with copyright permission from John Wiley and Sons, 2020). (c) The structure of CaTi2O4 showing TiO6 octahedra in blue and calcium ions as green spheres [47] (with copyright permission from Elsevier, 2020). ...
... (b) Schematic view of the orthorhombic hexagonal phase (CaMn 2 O 4 -like) of ZnGa 2 O 4 . The atom color palettes for Zn, Ga, and O are yellow, blue, and red, respectively [46] (with copyright permission from John Wiley and Sons, 2020). (c) The structure of CaTi 2 O 4 showing TiO 6 octahedra in blue and calcium ions as green spheres [47] (with copyright permission from Elsevier, 2020). ...
... High-pressure XRD studies reveal that ZnGa 2 O 4 undergoes two transitions. The first transition occurs in ZnGa 2 O 4 from the cubic spinel (Fd3m) toward the orthorhombic CaMn 2 O 4 -type structure (Pbcm) under hydrostatic pressure of 33.4 GPa, which was studied by Moreno et al. [46] where they concluded that phonon frequencies from the orthorhombic CaMn 2 O 4 -type structure (Pbcm) phase of ZnGa 2 O 4 have a noteworthy change at 42.5 GPa. CaMn 2 O 4 (Pbcm) and CaTi 2 O 4 -type (Cmcm) structures exist simultaneously at this pressure, but the CaTi 2 O 4 -type (Cmcm) structure is more stable than the CaMn 2 O 4 (Pbcm) one. ...
Spinel ZnGa2O4 has received significant attention from researchers due to its wide bandgap and high chemical and thermal stability; hence, paving the way for it to have potential in various applications. This review focuses on its physical, optical, mechanical and electrical properties, contributing to the better understanding of this material. The recent trends for growth techniques and processing in the research and development of ZnGa2O4 from bulk crystal growth to thin films are discussed in detail for device performance. This material has excellent properties and is investigated widely in deep-ultraviolet photodetectors, gas sensors and phosphors. In this article, effects of substrate temperature, annealing temperature, oxygen partial pressure and zinc/gallium ratio are discussed for device processing and fabrication. In addition, research progress and future outlooks are also identified.
... According to the XRD data, Fig. 3(a), gahnite crystals appear during the heat-treatment at 750 • С. It is known that five active modes A 1 g + E g + 3 F 2 g should be observed in the Raman spectra of normal spinels [67][68][69][70][71]. Their wavenumbers are in the following sequence, F 2 g (1) < E g < F 2 g (2) < F 2 g (3) < A 1 g . ...
... [68]. According to Refs [69,70], in the Raman spectrum of gahnite, the low-frequency F 2 g (1) mode (196 cm -1 ) is assigned to a complete translation of the ZnO 4 within the spinel structure, while the phonon modes with higher frequencies are due to O and Al, with a major contribution of O than Al; the frequencies over 729 cm -1 are predominantly due to O motions. The disordering can cause a breakdown in the Raman selection rules; the inversion degrees cause the appearance of an additional peak at ~700 -730 cm -1 attributed to A 1 g character and assigned to the Al-O stretching vibration of AlO 4 groups created by redistribution of some aluminum ions from octahedral to tetrahedral sites [71]. ...
In order to develop glass-ceramics containing ions of variable valence in lower oxidation states, it is important to know how changing the redox conditions of glass melting affects structure and properties of glass-ceramics. The zinc aluminosilicate glass nucleated by TiO2 was melted with and without addition of As2O3 and heat-treated from 720° to 1350°C to obtain gahnite-based glass-ceramics. DSC, XRD analysis, Raman spectroscopy and TEM studies revealed that variation of glass melting redox conditions affects kinetics of liquid phase separation and rutile crystallization, composition and structure of gahnite and rutile, crystallization of the residual glass, and does not affect kinetics of gahnite crystallization, gahnite and rutile fractions and structure of glass-ceramics. In glass-ceramics prepared from glasses melted without As2O3, absorption in the visible and near-IR spectral ranges is due to octahedrally coordinated Ti3+ ions in gahnite nanocrystals. The study is important for development of rare-earth-free phosphors.
... [23][24][25][26][27] Preudhomme and Tarte 28 provided an early IR study of vibrational modes in II-III spinels. L opez et al. 27 and L opez-Moreno et al. 29 reported Raman-active modes, IR-active TO modes, and silent modes from density functional theory (DFT) calculations. L opez et al. 27 and Van Gorkom et al. 24 reported experimental Raman scattering results without identifying all Raman-active modes. ...
... Note the diversity of the participating constituents and the complexity of their respective displacement pattern for all modes, where a simple subdivision into vibrations within or among sub-blocks within the spinel structure cannot be upheld. Our results for the IR-active TO mode frequencies as well as for the Raman-active and silent modes are consistent with the DFT-calculated phonon modes frequencies reported by L opez et al. 27 and L opez-Moreno et al. 29 (Table I). Calculated LO modes have not been reported yet, and neither were the intensities of the IR-active modes or activities of the Raman-active modes. ...
Infrared-active lattice mode properties of melt-grown high-quality single bulk crystals of ZnGa2O4 are investigated by combined spectroscopic ellipsometry and density functional theory computation analysis. The normal spinel structure crystals are measured by spectroscopic ellipsometry at room temperature in the range of 100 cm–1–1200 cm–1. The complex-valued dielectric function is determined from a wavenumber-by-wavenumber approach, which is then analyzed by the four-parameter semi-quantum model dielectric function approach augmented by impurity mode contributions. We determine four infrared-active transverse and longitudinal optical mode pairs, five localized impurity mode pairs, and the high frequency dielectric constant. All four infrared-active transverse and longitudinal optical mode pairs are in excellent agreement with results from our density functional theory computations. With the Lyddane–Sachs–Teller relationship, we determine the static dielectric constant, which agrees well with electrical capacitance measurements performed on similarly grown samples. We also provide calculated parameters for all Raman-active and for all silent modes and, thereby, provide a complete set of all symmetry predicted Brillouin zone center modes.
... In particular, wave velocities or compressional velocities along a axis is almost 30 % larger than that along b axis. Olivine silicates also show anisotropies on the elastic constants being: C 11 > C 33 > C 22 [12,13]. ...
... It is found that temperature and pressure have greater effect on large Mg-O bonds than the short Mg-O bonds [16]. In addition to Olivine-type compounds, Spinel-type aluminates have been broadly studied under high-pressure conditions [22,23]. The same can be stated for Sinhalite [24]. ...
The present work reports the complete study of structural, vibrational, mechanical, and electronic properties of BeAl2O4 (known as Chrysoberyl) using first-principles computing methods. The calculated ground-state properties agree quite well with previous experiments. The computed phonon dispersion curves do not show imaginary frequencies confirming the dynamical stability. In addition, the calculated elastic constants also ensure the mechanical stability through fulfillment of mechanical stability criteria. Apart from that, the theoretically determined phonon frequencies agree quite well with previous Raman and infrared experiments at ambient conditions. Various thermodynamic properties are also being calculated as a function of temperature. The thermal expansion computed within the quasi-harmonic approximation is found to be positive in agreement with the experiments. From the electronic structure we can see that Chrysoberyl possess a direct band gap of 8.3 eV with the oxygen-2p states dominating close to fermi level in the valence band. The influence of pressure on different properties is discussed. The highlight of the present work is the presence of optical isotropy in Chrysoberyl although the crystal structure is highly anisotropic.
... Wang et al. 2002b), ZnFe 2 O 4 (0.52–1.88, Wang et al. 2003 ), ZnAl 2 O 4 (0.9–1.5, Moreno et al. 2011 ), ZnGa 2 O 4 (1.0–1.8, Moreno et al. 2011), MgCr 2 O 4 (1.27–2.03, ...
... Wang et al. 2003 ), ZnAl 2 O 4 (0.9–1.5, Moreno et al. 2011 ), ZnGa 2 O 4 (1.0–1.8, Moreno et al. 2011), MgCr 2 O 4 (1.27–2.03, Yong et al. 2012), CoCo 2 O 4 (0.57– 1.32, Bai et al. 2012). ...
The vibrational properties of synthetic galaxite, MnAl2O4, were investigated at various pressures (0–29.7 GPa) and temperatures (80–973 K), respectively. The Raman frequencies of all observed bands for galaxite continuously increase with increasing pressure and decrease with increasing temperature, respectively. The quantitative analysis shows that the lowest frequency T2g mode has the smallest isothermal mode Grüneisen parameter. Combined with previous studies, the thermal Grüneisen parameter of galaxite is determined to be 1.23(5). The quantitative analysis of temperature dependences of Raman bands yields that the lowest frequency T2g mode has the largest isobaric mode Grüneisen parameter. The intrinsic anharmonic mode parameters are also calculated and nonzero, indicating an existence of intrinsic anharmonicity for MnAl2O4 spinel.
... The first-order Raman active modes in relation with spinel ZnGa 2 O 4 structure were observed at 712 cm À1 (A 1g ), 609 cm À1 (T 2g ), and 468 cm À1 (3T 2g ) which corresponds to Zn 21 ions in the tetrahedral sites and independent from the motions of Ga 31 ions in the octahedral sites. 45,46 On the other hand, the low frequency modes at around 131 cm À1 (T 2u ), 167 cm À1 (T 1u : infrared active), and 183 cm À1 (T 2g : Raman active) were Zn ions related modes as mentioned in the study of Lopez-Moreno et al. 45 Furthermore, as illustrated in Fig. 3, doping Co atoms raises the infrared active mode at around 540 cm À1 (4T 1u ); a finding reported in a previous study as well. 46,47 Increasing the Co amount in the lattice disorders the structure. ...
... The first-order Raman active modes in relation with spinel ZnGa 2 O 4 structure were observed at 712 cm À1 (A 1g ), 609 cm À1 (T 2g ), and 468 cm À1 (3T 2g ) which corresponds to Zn 21 ions in the tetrahedral sites and independent from the motions of Ga 31 ions in the octahedral sites. 45,46 On the other hand, the low frequency modes at around 131 cm À1 (T 2u ), 167 cm À1 (T 1u : infrared active), and 183 cm À1 (T 2g : Raman active) were Zn ions related modes as mentioned in the study of Lopez-Moreno et al. 45 Furthermore, as illustrated in Fig. 3, doping Co atoms raises the infrared active mode at around 540 cm À1 (4T 1u ); a finding reported in a previous study as well. 46,47 Increasing the Co amount in the lattice disorders the structure. ...
... Te samples all Wavenumber (cm -1 ) 600 700 800 900 1000 S15 S14 S13 S12 S11 S10 S09 S08 S07 S06 S05 S04 S03 S02 S01 Figure 3: Infrared spectra of gahnite. [15], so the above characteristic absorption peaks are caused by the Al-O bending vibration of the (AlO 6 ) octahedron, which belongs to the characteristic Raman spectral peaks of gahnite. Te Raman spectral peak data of gahnite in this work were compared with those of ZnAl 2 O 4 crystals (theory), natural blue spinel (Fe, Zn chromogenic), natural blue spinel (Co chromogenic), natural pink spinel (Cr chromogenic), and synthetic spinel (see Table 4). ...
Gem-quality blue octahedral crystalline gahnite was produced in Nigeria. This paper investigated gemological and spectroscopic characteristics by basic gemological experiments, electron probes, infrared reflectance spectroscopy, laser Raman spectroscopy, photoluminescence spectroscopy, and ultraviolet-visible spectroscopy. The results show that the refractive index (RI) of Nigerian gahnite is 1.792∼1.794, and the specific gravity is 4.45∼4.66, with no fluorescence. The main chemical composition is ZnAl2O4, accounting for 93.57%, and the rest is mainly FeAl2O4, which also contains Na, Mg, Co, Mn, Cr, Cu, Si, K, and Ca elements. The infrared spectra showed midinfrared absorption bands near 510 cm−1, 559 cm−1, and 664 cm−1 in the fingerprint region, corresponding to the Zn-O stretching vibration, bending vibration, and Al-O bending vibration, respectively. The Raman spectra showed three of the five Raman active modes of the spinel group, with characteristic Raman absorption peaks located at 418 cm−1, 508 cm−1, and 660 cm−1, corresponding to Eg, T2g(2), and T2g(3) modes, respectively, and the comparison revealed a higher degree of Zn and Al ordering in this paper for gahnite. The photoluminescence spectra show the common Cr3+-activated fluorescence splitting peaks of natural spinel, of which the 686 nm (R-line) fluorescence peak is obvious and sharp. The UV-vis absorption spectra located at 444 nm and 489 nm are the most obvious, which are caused by the d-d electron leap of TFe2+ (5E ⟶ 5T2), and the blue-gray tones of the samples are mainly caused by the spin-forbidden electronic transitions in TFe2+ and MFe2+ ↔ MFe3+; the weak absorption peak at 609 nm was determined to be associated with Co2+ by derivative spectra.
... Similar behavior have been observed before. [31,32] The Raman modes observed at~338 cm À 1 and at~496 cm À 1 could be due to the second order vibration mode of ZnÀ O bond (A 1 ) and stretching vibration of the ZnÀ O bond respectively. [33] The Raman bands corresponding to the corner shared NbO 6 octahedra are observed at~820 cm À 1 (υ 1 (A 1g ) strong), 612 cm À 1 (υ 2 (E g ) very weak), and a shoulder at 375 cm À 1 (υ 5 (T 2g )). ...
The non ‐ centrosymmetric tetragonal inverse spinel structure of LiZnNbO4 has been explored with a view to prepare new colored compounds. The substitution of Co²⁺, Ni²⁺, Fe²⁺, Mn²⁺, and Cu²⁺ ions were attempted in the place of Zn²⁺ ions and Sb⁵⁺ ions in place of Nb⁵⁺ ions. The studies indicated that 0.75 Zn²⁺ ions in LiZnNbO4 can be replaced by Co²⁺ ions and 0.5 Zn²⁺ ions in LiZnNb0.5Sb0.5O4 compound. The substitution of Co²⁺ ions gives rise to different shades of blue color in Li(Zn1‐xCox)NbO4 compounds and from ink blue to blue‐green color in Li(Zn1‐xCox)(Nb0.5Sb0.5)O4 compounds. The different colors observed in the present study were explained by the traditional allowed d‐d transitions as well as the metal‐to‐metal charge transfer (MMCT) transitions involving Nb⁵⁺ (4d⁰) ions and partially filled 3d electrons. The SHG studies indicate that the prepared compounds are SHG active. All the compounds exhibit reasonable dielectric behavior with low loss. The XPS studies confirm the oxidation states of the different substituted ions. Raman studies indicate variations in the bands due to the substitutions in the parent LiZnNbO4 phase. Magnetic studies on the Co²⁺ ions substituted compounds suggest antiferromagnetic behavior.
... The previous report showed that the phonon modes with low frequencies are owing to motions of Zn ions and the high-frequency modes are due to motions of O and Al ions [38,39]. In this material, the high-frequency mode at 662 cm −1 is caused by the motion of O atoms in the AlO 6 octahedral [40]. ...
Most promising oxide thermoelectric materials such as perovskites, layered oxide materials, Al-doped ZnO, etc., have been reported. In the present work, Zn1-xCuxAl2O4 (0.0≤ x ≤0.1) samples were synthesized by a simple hydrothermal method. The structural, optical, morphological, and TE properties of Zn1-xCuxAl2O4 (0.0≤ x ≤0.1) have been investigated. XRD analysis reveals that ZnAl2O4 has a single-phase cubic structure and Cu is completely dissolved in the ZnAl2O4 lattice. Thermal analysis shows that ZnAl2O4 has high thermal stability up to 1000 °C. From the UV-Vis DRS analysis, the energy band gap of ZnAl2O4 decreased from 3.30 eV to 2.82 eV with increasing the content of Cu. Carrier concentration and mobility of the samples were measured by the Hall effect. The values of a carrier concentration of undoped ZnAl2O4 and Zn0.9Cu0.1Al2O4 are obtained to be 3.836×1013 cm-3 and 3.3×1016 cm-3 at 313 K and 9.6×1013 cm-3 for pure and 5.5×1016 cm-3 for Zn0.9Cu0.1Al2O4 at 673 K. TE properties of the synthesized samples have been analyzed as a function of temperature. With the optimum values of Seebeck coefficient and electrical conductivity, Zn0.9Cu0.1Al2O4 shows the highest power factor of 0.50 μW/mK2 while the pure ZnAl2O4 shows a maximum power factor of 0.19 μW/mK2 at 673 K. The Zn0.9Cu0.1Al2O4 exhibits a relatively high zT of 2.4×10-4 at 673 K, while pure ZnAl2O4 has a zT value of 0.4×10-4 at 673 K. The obtained values reveal the improvement of TE properties by increasing the Cu content in the sample.
... For ZnAl 2 O 4 , the group theory predicts four infrared active modes around 222, 496, 548, and 666 cm −1 . 41 For aluminate spinel, the octahedral groups AlO 6 generate stretching, bending, and asymmetrical bending modes in the region 220−700 cm −1 . The tetrahedral coordination AlO 4 is expected to produce vibrational frequencies in the 700−850 and 250−320 cm −1 ranges. ...
Deep-UV (180-280 nm) phosphors have attracted tremendous interest in tri-band-based white light-emitting diode (LED) technology, bio- and photochemistry, as well as various medical fields. However, the application of many UV-emitting materials has been hindered due to their poor thermal or chemical stability, complex synthesis, and environmental harmfulness. A particular concern is posed by the utilization of rare earths affected by rising price and depletion of natural resources. As a consequence, the development of phosphors without rare-earth elements represents an important challenge. In this work, as a potential UV-C phosphor, undoped ZnAl2O4 fibers have been synthesized by a cost-efficient wet chemical route. The rare-earth-free ZnAl2O4 nanofibers exhibit a strong UV emission with two bands peaking at 5.4 eV (230 nm) and 4.75 eV (261 nm). The emission intensity can be controlled by tuning the Zn/Al ratio. A structure-property relationship has been thoroughly studied to understand the origin of the UV emission. For this reason, ZnAl2O4 nanofibers have been analyzed by X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and Raman spectroscopy techniques showing that a normal spinel structure of the synthesized material is preserved within a wide range of Zn/Al ratios. The experimental evidence of a strong and narrow band at 7.04 eV in the excitation spectrum of the 5.4 eV emission suggests its excitonic nature. Moreover, the 4.75 eV emission is shown to be related to excitons perturbed by lattice defects, presumably oxygen or cation vacancies. These findings shed light on the design of UV-C emission devices for sterilization based on a rare-earth-free phosphor, providing a feasible alternative to the conventional phosphors doped with rare-earth elements.
... A set of phonon modes (peaking at 738.2, 771.2, and 794.3 cm −1 ) assigned to A 1g Raman-active modes were observed, which indicates that the materials have a lot of ZnO 4 and GeO 4 groups with symmetric stretching vibrations. 53,54 In a pure rhombohedral phase, the first-order Raman modes are related to the tetrahedral sites, indicating the formation of ZnO 4 and GeO 4 tetrahedrons in 900°C annealed ZGO:Mn,Li,K crystals prepared by hydrothermal method. 53 Compared with the samples before annealing, the phonon vibration energy of the samples after annealing shows a slight decrease ( Figure S6). Figure 4a shows the PL excitation (PLE) and PL spectra of these two series of as-synthesized and annealed ZGO:Mn,Li and ZGO:Mn,Li,K phosphors. ...
Fluorescence latent fingerprinting (LFP) imaging is
a convenient and frequently used in situ detection technique. In
recent years, various types of phosphors have been widely used for
LFP imaging. However, traditional phosphor powders are suffering
from a formidable challenge to design effective fluorescent LFP
powders with low background interference and high detection
sensitivity. In this work, we have synthesized Zn2GeO4:Mn,Li,K
(ZGO:Mn,Li,K) nanophosphors with the high persistent luminescence
(PersL) brightness and long duration using hydrothermal
method by optimizing synthesis conditions. The ZGO:Mn,Li,K
phosphors, with a perceptible green PersL, have been successfully
used for developing LFPs on non-fluorescent substrate, fluorescent
substrate, and PersL substrate without special equipment. Besides
the level 1 and level 2 details, the other details of LFPs even exceeding level 3 details including the ridge width and boundary
streamline of LFPs and the sweat pore feature are distinguishable. These findings show that the ZGO:Mn,Li,K phosphors, with
multimode luminescence and high detection sensitivity, will promise a more sophisticated luminescent label for LFP detection on
various substrates.
... For ZnGa 2 O 4 current studies are limited to room temperature optical measurements and density functional theory (DFT) calculations. [32][33][34][35][36][37] Recently, Hilfiker et al. used a combined density functional theory calculation and spectroscopic ellipsometry analysis approach to determine the optical properties for room temperature ZnGa 2 O 4 . Spectroscopic ellipsometry analysis relies on a line shape analysis of the dielectric function, where multiple line shape methods may be applied to interpret the electronic transitions. ...
We report the elevated temperature (22 °C [Formula: see text] T [Formula: see text] 600 °C) dielectric function properties of melt grown single crystal ZnGa 2 O 4 using a spectroscopic ellipsometry approach. A temperature dependent Cauchy dispersion analysis was applied across the transparent spectrum to determine the high-frequency index of refraction yielding a temperature dependent slope of 3.885(2) × 10 ⁻⁵ K ⁻¹ . A model dielectric function critical point analysis was applied to examine the dielectric function and critical point transitions for each temperature. The lowest energy M 0 -type critical point associated with the direct bandgap transition in ZnGa 2 O 4 is shown to red-shift linearly as the temperature is increased with a subsequent slope of −0.72(4) meV K ⁻¹ . Furthermore, increasing the temperature results in a reduction of the excitonic amplitude and increase in the exciton broadening akin to exciton evaporation and lifetime shortening. This matches current theoretical understanding of excitonic behavior and critically provides justification for an anharmonic broadened Lorentz oscillator to be applied for model analysis of excitonic contributions.
... Toutefois, certaines études mettent en évidence la variation de structure de certaines phases spinelles à température ambiante, lorsqu'ils sont soumis à une pression élevée. Ce phénomène a été étudié sur les spinelles directes ZnGa2O4 et ZnAl2O4 [204], ainsi que, plus récemment, sur le spinelle inverse Mg2TiO4 [205]. ...
Depuis quelques années, les céramiques transparentes sont largement étudiées pour leurs propriétés optiques et mécaniques et leurs applications dans les domaines de la photonique (laser de puissance, …) ou militaire (radôme, fenêtre, …). Par rapport aux monocristaux dont elles reproduisent les caractéristiques techniques, leur synthèse est facilitée, plus rapide et moins onéreuse. Aujourd’hui, elles sont le plus souvent synthétisées par des méthodes de frittage sous charge (Spark Plasma Sintering, Hot Isostatic Pressing ou Hot Pressing). Il s’agit par exemple d’Al2O3, de YAG ou encore de MgAl2O4. Ce dernier cristallise sous forme de spinelle cubique et sa composition permet notamment l’obtention d’une fenêtre de transmission large (de 0,25 à 9 μm). D’autres espèces de même structure cristalline, telles que ZnAl2O4, sont également étudiées dans la littérature. Au cours de cette thèse, nous nous sommes concentrés sur la synthèse de nouvelles céramiques transparentes ZnGa2O4 par Spark Plasma Sintering. Plusieurs voies de synthèse ont été étudiés, notamment en modifiant la poudre initialement introduite dans la matrice graphite. Le frittage « pseudo-réactif » d’un mélange ZnO-Ga2O3, la densification d’une phase mère ZnGa2O4 ainsi que la consolidation de cristaux nanométriques ZnGa2O4 obtenus par lixiviation d’une vitrocéramique ont permis d’obtenir ces céramiques transparentes par SPS. La faisabilité de ces différentes synthèses a donc été montrée. Des céramiques Cr3+:ZnGa2O4 et Ni2+:ZnGa2O4 ont également pu être obtenues par frittage « pseudo-réactif » et leurs propriétés optiques ont été étudiées.
... Infrared and Raman active phonon mode properties have been explored both computationally and experimentally. [10][11][12][13][14][15] Stokey et al. reported the first complete set of infrared and Raman active mode parameters for ZGO using a combined spectroscopic ellipsometry and density functional theory (DFT) computational analysis. 16 Electronic band structure properties, band-to-band transitions, and crystal structure 17 have been computationally studied previously (see, e.g., Zeraga et al. 18 and Galazka et al. 4 ). ...
We determine the dielectric function of the emerging ultrawide bandgap semiconductor ZnGa2O4 from the near-infrared (0.75 eV) into the vacuum ultraviolet (8.5 eV) spectral regions using spectroscopic ellipsometry on high quality single crystal substrates. We perform density functional theory calculations and discuss the band structure and the Brillouin zone Γ-point band-to-band transition energies, their transition matrix elements, and effective band mass parameters. We find an isotropic effective mass parameter (0.24 me) at the bottom of the Γ-point conduction band, which equals the lowest valence band effective mass parameter at the top of the highly anisotropic and degenerate valence band (0.24 me). Our calculated band structure indicates the spinel ZnGa2O4 is indirect, with the lowest direct transition at the Γ-point. We analyze the measured dielectric function using critical-point line shape functions for a three-dimensional, M0-type van Hove singularity, and we determine the direct bandgap with an energy of 5.27(3) eV. In our model, we also consider contributions from Wannier–Mott type excitons with an effective Rydberg energy of 14.8 meV. We determine the near-infrared index of refraction from extrapolation (1.91) in very good agreement with results from recent infrared ellipsometry measurements ( ε ∞ = 1.94) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)].
... Unlike CM and CF structures, most CT phases have not been discovered at room temperature but instead under higher temperatures [17][18][19] . In addition to the orthorhombic structures, a tetragonal structure (space group I4 1 /amd) was reported for post-spinel phases at high pressure conditions as an intermediate phase [20][21][22][23][24][25] . The post-spinel structures attract considerable attention because they are isostructural with ringwoodite, which is the most abundant phase, comprising approximately 50-60% by volume, in the Earth's transition zone (400-600 km in depth) 26 . ...
... Unlike CM and CF structures, most CT phases have not been discovered at room temperature but instead under higher temperatures [17][18][19] . In addition to the orthorhombic structures, a tetragonal structure (space group I4 1 /amd) was reported for post-spinel phases at high pressure conditions as an intermediate phase [20][21][22][23][24][25] . The post-spinel structures attract considerable attention because they are isostructural with ringwoodite, which is the most abundant phase, comprising approximately 50-60% by volume, in the Earth's transition zone (400-600 km in depth) 26 . ...
Synthetic Mg2TiO4 qandilite was investigated to 50 and 40.4 GPa at room temperature using Raman spectroscopy and X-ray diffraction, respectively. The Raman measurements showed that cubic Mg2TiO4 spinel transforms to a high pressure tetragonal (I41/amd, No.141) phase at 14.7 GPa. Owing to sluggish kinetics at room temperature, the spinel phase coexists with the tetragonal phase between 14.7 and 24.3 GPa. In the X-ray diffraction experiment, transformation of the cubic Mg2TiO4 to the tetragonal structure was complete by 29.2 GPa, ~5 GPa higher than the transition pressure obtained by Raman measurements, owing to slow kinetics. The obtained isothermal bulk modulus of Mg2TiO4 spinel is KT0 = 148(3) GPa when KT0’ = 6.6, or KT0 = 166(1) GPa when KT0’ is fixed at 4. The isothermal bulk modulus of the high-pressure tetragonal phase is calculated to be 209(2) GPa and V0 = 270(2) Å3 when KT0’ is fixed at 4, and the volume reduction on change from cubic to tetragonal phase is about 9%. The calculated thermal Grüneisen parameters (γth) of cubic and tetragonal Mg2TiO4 phases are 1.01 and 0.63. Based on the radii ratio of spinel cations, a simple model is proposed to predict post-spinel structures.
... The spectra are weakly dependent on the starting composition. The two most intense modes are due to the motion of Al and O atoms [13]. ...
... The spectra are weakly dependent on the starting composition. The two most intense modes are due to the motion of Al and O atoms [57]. ...
Transparent cobalt-doped zinc aluminate spinel (gahnite), Co²⁺:ZnAl2O4, ceramics were fabricated by hot pressing of (Zn,Co)Al2O4/ZnF2 nanopowders at 1520 °C for 4 h. A novel approach was suggested for the preparation of (Zn,Co)Al2O4 precursor: the (Zn,Co)Al2O4 powders were synthesized by mixing an alcohol solution of aluminium isopropoxide with a joint aqueous solution of zinc formate (the source of zinc), cobalt nitrate and zinc fluoride (the sintering additive) followed by drying and calcination in air. The study of the thermal behavior of precursors revealed an optimum calcination temperature (700 °C) preventing the loss of the sintering additive (ZnF2). The effect of the ZnF2 content (3–10 wt%) on the microstructure, the cobalt doping concentration, absorption and luminescence of ceramics was systematically studied. The ceramics exhibit a close-packed microstructure with a mean grain size of 50–70 μm and high in-line transmission (about 84% at ∼2 μm). The actual concentration of Co²⁺ ions in tetrahedral (Td) sites of gahnite is lower than the Co²⁺ doping level and it decreases with the ZnF2 content. We propose a possible mechanism of this variation including both the losses of Co²⁺ via evaporation of the intermediate CoF2 phase and a partial location of Co²⁺ ions in octahedral (Oh) sites of gahnite due to its partly inverse spinel structure. The developed ceramics are promising for saturable absorbers of erbium lasers.
... Studies on the stability of the spinel structured materials under high pressure, viz., pressure dependent phase transition in ZnAl 2 O 4 and ZnGa 2 O 4 have been performed in several works [1,2]. Room-temperature angledispersive x-ray diffraction measurements on spinel ZnGa 2 O 4 upto 56 GPa shows two structural phase transition; one, at 31.2 GPa where ZnGa 2 O 4 undergoes a transition from the cubic spinel structure to a tetragonal spinel structure and the other at 55 GPa where transition to the orthorhombic marokite structure (CaMn 2 O 4 -type) takes place. ...
This work is pertaining to the synthesis of fine magnesium aluminate spinel (MgAl 2 O 4 ) powders of varied trivalent:bivalent cation ratio along line of homogeneity of the solid solution (MgO. xAl 2 O 3 , x = 1, 1.25, 1.50, 1.75, and 2) via gel combustion method. Magnesium- and aluminum- nitrate were used as the oxidants in combustion reaction fuelled by urea in combination with stoichiometric formaldehyde solution acting as reductant. Synthesized powders were characterized in terms of microscopic analysis and optical absorbance measurements. The cation ratio, through a change in gel structure influences the nature of crystallization of the product, while on the other hand does not affect grain shapes and sizes. Distinct enhancement in both absorption intensity and the corresponding estimated energy band gap has been observed against increasing excess than stiochiometric alumina concentrations. Evaluated optical band gaps were widened in proportion to the Al: Mg ratio which may be attributed to Burstein-Moss effect in consequence of substitutional insertion of introduced Al ³⁺ ions in spinel lattices.
... Hence, Raman spectra of spinel only shows five modes (underlined and bold notations) whose wavenumbers are in order of F 2g (1) < E g < F 2g (2) < F 2g (3) < A 1g ). [32][33][34][35][36][37] However, the presence of vacancies and structural disordering can break down the Raman selection rule and trigger new phonon modes that are not predicted by the group theory. Therefore, the degree and variation in (dis)orderness can be detected via (dis)appearance and variation in Raman peaks. ...
Spinels (AB2O4) form a niche class of ceramics, which is rich in structural (dis)ordering due to the inherent mixing between the constituent tetrahedral and octahedral sites. The cations (A and B) can form antisite defects under the influence of external parameters like pressure, temperature and nuclear irradiation. The current study reports the formation and evolution of disorder-order structural transition in hydrothermally prepared zinc aluminate spinel ZnAl2O4. The effect of final calcination temperature (300–900 °C for 9 h) on the degree of cation ordering has been investigated with powder X-ray diffraction, Raman and ²⁷Al solid-state NMR spectroscopy. Rietveld refinement revealed a gradual disorder to order structural transition accompanied by lower inversion parameter (is) and smaller lattice parameter (a) with higher calcination temperature. It was further affirmed by Raman analysis and solid-state NMR spectroscopy probing the ZnO4 and AlO4 tetrahedra in spinel. Independent of the degree of cation ordering, nanometric particle size with high surface area was observed in ZnAl2O4 spinel.
... In our measurements there were not any additional bands in Raman spectra, indicating the presence of impurities. In particular, there were no expected bands coming from Al 2 ZnO 4 spinels [22]. This means that the Al doping leads to appearance of oxygen vacancies, and consequently, to disturbances of the Zn-O stoichiometry. ...
In this paper the preparation process and optical characterization of pure and Al3+ doped zinc oxide (Al:ZnO) coatings will be presented. ZnO based materials have been studied extensively due to their potential applications in optoelectronic devices as conductive gas sensors, transparent conductive, electrodes, solar cell windows, varistors, UVfilters or photovoltaic cells. It is II–VI semiconductor with wide-band gap of 3.37 eV and large exciton binding energy of 60meV. It is possible to improve the conductivity of ZnO coating by intentionally doping ZnO with aluminium ions during preparation process. Such transparent and conducting thin films, known as AZO (Aluminium Zinc Oxide) films, are very good candidate for application as transparent conducting materials in many optoelectronic devices. The well-known sol-gel method is used for preparation of solution, coated on glass substrates by dip coating process. Prepared samples were investigated by Raman and UV-VIS spectroscopy. Transmittance as well as specular and diffuse reflectance spectroscopy methods were used for studies of optical parameters. We found that Al admixture influences on optical bandgap of ZnO. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
... The postspinel orthorhombic CaTi 2 O 4 structure shows 18-24 Raman active modes [46], while none were identified during the metallization process at HP. Further investigations were performed by in situ optical microscopic imaging the sample during compression inside the DAC, as shown in Fig. 7. Figures 7(a)-7(d) correspond to the regions of OA, AB, BC, and CD in Fig. 6, respectively. ...
The effect of pressure on the electronic properties and crystal structure in a mixed spinel ferrite Zn0.2Mg0.8Fe2O4 was studied for the first time up to 48 GPa at room temperature using x-ray diffraction, Raman spectroscopy, and electrical transport measurements. The sample was cubic (spinel-type Fd3¯m) at ambient pressure and underwent a pressure-induced structural transition to an orthorhombic phase (CaTi2O4−typeBbmm) at 21 GPa. This structural transformation corresponded to a first-order phase transition that involved 7.5% molar volume shrinkage. The onset of the Mott insulator-metal transition (IMT) around 20 GPa was due to a spin crossover mechanism that led to the Fe3+ magnetic moment collapse. All the Raman modes disappeared at high pressures, which supported metallization. Analysis of structural and electrical transport measurements showed a simultaneous volume collapse and sharp IMT within a narrow pressure range. The orthorhombic high-pressure phase was found to have a higher conductivity than the cubic phase. The pressure dependence of the conductivity supported the metallic behavior of the high-pressure phase.
... Interesting studies of the microscopic crystal field effects on the Cr 3 þ energy levels in MgAl 2 O 4 can be found in Ref. [38]. Experimental studies of high-pressure effects on the lattice vibrations of ZnAl 2 O 4 and ZnGa 2 O 4 were described in Ref. [39]. The superposition model of crystal field was used to calculate the Cr 3 þ energy levels in several spinels in Ref. [40]. ...
Details of preparation, spectroscopic and structural studies along with crystal field calculations for two Cr 3 þ doped spinels MgAl 2 O 4 and ZnAl 2 O 4 are given in the present paper. Both compounds show efficient red emission at about 685 nm, which is due to the 2 E g-4 A 2g spin-forbidden transition of Cr 3 þ ions located at the sites with D 3d local symmetry. Analysis of structure of the CrO 6 clusters was performed; comparison of the crystal field effects in both compounds revealed that the low-symmetry splitting of the orbital triplet states is more pronounced in ZnAl 2 O 4. Both compounds show potential for applications as red-emitting phosphors.
... Interesting studies of the microscopic crystal field effects on the Cr 3 þ energy levels in MgAl 2 O 4 can be found in Ref. [38]. Experimental studies of high-pressure effects on the lattice vibrations of ZnAl 2 O 4 and ZnGa 2 O 4 were described in Ref. [39]. The superposition model of crystal field was used to calculate the Cr 3 þ energy levels in several spinels in Ref. [40]. ...
... Similar to the Raman-active modes of CaFe 2 O 4 -type phases (Kojitani et al. 2003(Kojitani et al. , 2013Kolev et al. 2003;López-Moreno et al. 2011), the orthorhombic β-CaCr 2 O 4 is associated with the following Raman-active modes: ...
In situ high-pressure synchrotron X-ray diffraction and Raman spectroscopic studies of orthorhombic CaFe2O4-type β-CaCr2O4 chromite were carried out up to 16.2 and 32.0 GPa at room temperature using multi-anvil apparatus and diamond anvil cell, respectively. No phase transition was observed in this study. Fitting a third-order Birch–Murnaghan equation of state to the P–V data yields a zero-pressure volume of V
0 = 286.8(1) Å3, an isothermal bulk modulus of K
0 = 183(5) GPa and the first pressure derivative of isothermal bulk modulus K
0′ = 4.1(8). Analyses of axial compressibilities show anisotropic elasticity for β-CaCr2O4 since the a-axis is more compressible than the b- and c-axis. Based on the obtained and previous results, the compressibility of several CaFe2O4-type phases was compared. The high-pressure Raman spectra of β-CaCr2O4 were analyzed to determine the pressure dependences and mode Grüneisen parameters of Raman-active bands. The thermal Grüneisen parameter of β-CaCr2O4 is determined to be 0.93(2), which is smaller than those of CaFe2O4-type CaAl2O4 and MgAl2O4.
Zn and Al-incorporated ZnGa2O4 films (ZGO:Zn and ZGO:Al) were deposited on sapphire substrates using radio-frequency magnetron sputtering in this work. The effects of Zn and Al incorporation on the microstructural and optoelectronic properties of ZGO films were systematically investigated and compared with each other. This investigation revealed that incorporation of Al enhanced the characteristics of the ZGO film, whereas incorporation of Zn affected the characteristics of the ZGO film. The annealed ZGO:Al film revealed high crystallinity, average transmittance of more than 82%, and wide-bandgap of 5.18 eV. First principles calculations were performed to investigate the defects associated with the ZGO:Al structure. The Al incorporated ZGO PD exhibited an optimum metal-semiconductor-metal photodetector achievement with a photo/dark current ratio of ∼10⁴ order and responsivity of 3.01 A/W (at 3 V and 220 nm), demonstrating the responsivity of ZGO PD (0.25 A/W) can be increased up to 12 times by incorporating Al in ZGO films. This enhancement in the characteristics of the ZGO:Al film can be ascribed to the high crystalline nature and the formation of significant amount of oxygen vacancies in this film. These results demonstrate the potential of Al incorporated ZGO films in wide-bandgap devices and deep-ultraviolet applications.
A complex ZnO/ZnAl2O4 heterostructures thin films on glass and Si(111) substrates have been successively obtained by a soft ultrasonic spray pyrolysis (USP) method deposition using the Zn/Al molar ratios concentrations of 0.07/0.13 and 0.1/0.1, respectively. According to (XRD) an ordered zinc oxide (ZnO) and zinc aluminate (ZnAl2O4) structures deposited onto glass from the air annealing at 500 °C during 2 hours was observed and confirmed by the (EDX), (FTIR) and Raman spectroscopy techniques. The estimated crystallites size and stress values of ZnO and ZnAl2O4 in the ZnO/ZnAl2O4/glass film were 19 nm/0.469 GPa and 11 nm/-0.292 GPa, respectively. The lower Zn/Al molar ratio around 0.035/0.06 produced only ZnO as a single phase, suggesting the Al insufficient quantity. The Si(100) substrate with 0.07 Zn molarity conducted to the Zn2SiO4/ZnO/ZnAl2O4 composite. The Raman integrated intensity bands of ZnO and ZnAl2O4 increases with increasing Zn to Al molar ratio (0.1/0.1 comparatively to 0.07/0.13). The ZnO&ZnAl2O4 crystallinity enhances as Zn molarity increases. The ZnO films in the composites grow with (002) texture. The TC(hkl) value indicated that ZnAl2O4 in the ZnO/ZnAl2O4/glass layer is polycrystalline preferentially oriented along the (311) plane. Spinel ZnAl2O4 oxide onto Si(111) substrate grown according to the (220) orientation. Crystallites are larger in ZnO/ZnAl2O4/Si than in ZnO/ZnAl2O4/glass. The ZnO/ZnAl2O4 film onto glass substrate is transparent in the visible and near infrared regions and sensitive to UV absorption, as characterized by UV-Vis spectroscopy. The ZnO and ZnAl2O4 Eg values in the ZnO/ZnAl2O4/glass composite were 3.25 and 3.88 eV, respectively.
We report magnetic properties in the random spinel magnet CoGa2O4. Rietveld analysis of the x-ray diffraction profile for CoGa2O4 reveals that the Co and Ga ions are distributed randomly in the tetrahedral A sites and octahedral B sites in the cubic spinel structure. CoGa2O4 exhibits a spin-glass transition at TSG=8.2K that is confirmed by measurements of the dc and ac susceptibilities and thermoremanent magnetization (TRM) that develops below TSG. From the frequency dependence of the freezing temperature Tf for CoGa2O4, it is indicated that the relaxation time τ(T) follows a Vogel-Fulcher law τ=τ0exp[−Ea/kB(T−T0)]. An analysis of specific heat suggested that a doublet ground state of the octahedrally coordinated Co2+ was stabilized by spin-orbit and crystal field couplings. The relaxation rate of TRM is considerably enhanced at TSG and decays rapidly above and below TSG. The time course of TRM is reproduced by nonexponential relaxation forms, such as a stretched exponential (Kohlrausch) as well as Ogielski and Weron relaxation forms. This behavior is displayed universally in glass systems, and the characteristic parameters associated with these functions were reasonable.
Development of new near infrared luminescent (NIR) emitters improves our understanding of their fundamental structure - property relationships. The ability to use eficcient energy transfer to convert ultraviolet or visible light photons to enhance the NIR emission has attracted a great deal of attention in down-conversion applications. Taking advantage of sol-gel impregnation process and growth of materials along a support or template, core-shell structured nanofibers of ZnAl2O4 – based ceramic doped with Cerium and Neodymium were synthesized with the help of an elaborated facile and cost-efficient strategy. . The color-tunable emissions make this material a suitable host for a wide range of applications, e.g, bio-imaging, security markers, imaging devices, optical coatings, and solar cells. This research correlates the defects and the remarkable optical properties of the developed structures. Specified conditions of sol-gel processing combined with the incorporation of various rare-earths concentrations provide possibility to tune the ratio between Ce3+ and Ce4+ in the nanofibers with an average diameter of 50 nm and, therefore, their functional response. It is important to clarify the role of trivalent and tetravalent Cerium cations on the modulation of NIR emission to establish the luminescence mechanism. The NIR emitter luminescent compound ZnAl2O4 : Nd, Ce, which adopts a spinel-type structure, is studied by X-ray absorption near-edge. For the first time, this study reveals the energy transfer from Ce3+ to Nd3+ and the enhacement of the NIR emission due to the presence of Ce4+ in ZnAl2O4 : Nd, Ce spinel compound. Cytotoxicity analyses suggest the viability of the synthesized nanofibers, which opens new avenues in bio-imaging applicattions.
Infrared-active lattice mode properties of melt-grown high-quality single bulk crystals of ZnGa2O4 are investigated by combined spectroscopic ellipsometry and density functional theory computation analysis. The normal spinel structure crystals are measured by spectroscopic ellipsometry at room temperature in the range of 100 cm–1–1200 cm–1. The complex-valued dielectric function is determined from a wavenumber-by-wavenumber approach, which is then analyzed by the four-parameter semi-quantum model dielectric function approach augmented by impurity mode contributions. We determine four infrared-active transverse and longitudinal optical mode pairs, five localized impurity mode pairs, and the high frequency dielectric constant. All four infrared-active transverse and longitudinal optical mode pairs are in excellent agreement with results from our density functional theory computations. With the Lyddane–Sachs–Teller relationship, we determine the static dielectric constant, which agrees well with electrical capacitance measurements performed on similarly grown samples. We also provide calculated parameters for all Raman-active and for all silent modes and, thereby, provide a complete set of all symmetry predicted Brillouin zone center modes.
Structural, elastic, electronic and vibrational properties of XAl2O4 (X = Ca, Sr and Cd) compounds with orthorhombic structure are studied by first principles method within generalized gradient approximation. The calculated negative formation enthalpy for each compounds indicates the thermodynamical stability of the studied phase. Band structure calculations reveal that CaAl2O4, SrAl2O4 and CdAl2O4 compounds have a direct band gap of 4.86, 4.54 and 2.46 eV, respectively. Besides, from the analysis of the band gap values, one can notice that the replacement of Ca atoms by Sr and Cd atoms in these compounds reduces the band gap energy values. It is also observed that the CaAl2O4, SrAl2O4 and CdAl2O4 compounds are less compressible along b-axis and their compressibility decreases in the sequence SrAl2O4 > CdAl2O4 > CaAl2O4. Similarly, it is also noticeable that the CaAl2O4 compound have more resisting power against the monoclinic shear distortion along {100} plane and along {110} direction compared to CdAl2O4 and SrAl2O4 compounds. Moreover, Cauchy pressures confirm that the CaAl2O4 and SrAl2O4 compounds are ductile while the CdAl2O4 compound is brittle in nature. This fit very well with the forecast from B/G relation. The calculated elastic constants and the phonon dispersion relations of the studied compounds show that these compounds are both mechanically and dynamically stable. Moreover the temperature dependence of the specific heat and entropy have been discussed in detail and calculated Debye temperature is in good agreement with the related study in literature.
In the present study, the pulsed laser ablation in water technique was used to produce chromium-doped ZnGa 2 O 4 nanostructures from a sintered pellet target. The materials obtained mainly comprised the cubic spinel crystalline structure of ZnGa 2 O 4 and the monoclinic β-Ga 2 O 3 as a minority phase (3%). Intra-ionic Cr ³⁺ red luminescence from the ions in the spinel structure was the dominant recombination process in the bulk target and nanostructures. Overlapping of the Cr-related emitting centers was identified and the samples exhibited persistent red luminescence, which is of interest for bioimaging. Moreover, the temperature-dependent luminescence demonstrated the versatility of the nanostructures obtained, which can be further explored as temperature sensors based on the low energy shift of the zero phonon line peak positions as the temperature increased.
LiZn1-xMxNbO4 (M = Co, Ni) (x = 0–0.06) systems were fabricated by a facile solid-state reaction method. Structure and property relationships of spinel structured LiZn1-xMxNbO4 were investigated systematically. Appropriate amount of Co²⁺ and Ni²⁺ greatly improved the dielectric loss of LiZnNbO4 ceramics. While, the dielectric loss deteriorated seriously when the doping content exceeded x = 0.02. The origin of dielectric loss in LiZn1-xMxNbO4 ceramics was investigated systematically. Moreover, the theoretical dielectric constant and linear expansion coefficient were calculated on the bases of the crystallographic parameters from XRD refinement. The temperature coefficient of resonant frequency calculated by the P-V theory agreed well with the test values. Due to the small doping content, the change in chemical bonds was negligible. Density became the major factor determining the variation of dielectric constant in LiZnNbO4 ceramics. At last, excellent microwave dielectric properties were obtained: Ts = 1010 °C, εr = 15.25, Qf = 107,000 GHz, τf = −63.3 ppm/°C for LiZn0.98Co0.02NbO4 and Ts = 995 °C, εr = 14.85, Qf = 104,000 GHz, τf = −61.7 ppm/°C for LiZn0.98Ni0.02NbO4.
A swarm of pegmatitic bodies occurs in the central portion of the Mineiro Belt at the meridional border of the São Francisco craton (Brazil), forming the São Jõao del Rei Pegmatitic Province. These bodies are genetically related to one common source, the granitic magmatism precursor to the Rita'polis granitoid, whose age of 2121 6 7 Ma marks the last pulse of paleoproterozoic plutonism in the Mineiro Belt. A selected group of gahnite crystals from several pegmatite bodies hosted in the Brumado metadiorite, Rita'polis metagranitoid, and Rio das Mortes amphibolites and phyllites has been analyzed by optical stereomicroscopy, Raman spectroscopy, SEM/EDS, and EMPA/WDS. The gahnites have compositions defined by the ranges Ghn65.52-85.38Spl0-2.16Hc13.71-31.92Jac0-0.92Mfr0-1.22Glx0-1.11(in mol.%), plot almost exclusively on the gahnite-hercynite axis, and fall into the granitic pegmatite field in the gahnite-spinel-hercynite ternary diagram. The composition of gahnite varies depending on the country rock the pegmatite cuts, and the interaction between the pegmatite and host rocks is more evident in grains found in pegmatite bodies associated with amphibolites due to higher Fe and Mg in these rocks. These compositions also indicate a higher degree of fractionation of the pegmatites hosted by the Brumado metadiorite in comparison to those hosted by the Rio das Mortes metavolcano-sedimentary sequence (amphibolites and phyllites) and the Rita'polis metagranitoid, reflecting different crustal levels of emplacement of these pegmatitic bodies.
Zn–Al2O3 supports were prepared by impregnating commercial γ-Al2O3 powders with different amounts of Zn(NO3)2, followed by calcination in air at 500 or 800 °C. Rh/Zn–Al2O3 catalysts were then prepared by impregnating Zn–Al2O3 supports with Rh(NO3)3 followed by calcination in air at 500 °C. The catalysts and/or supports were characterized by ICP-OES, XRD, N2 adsorption, Raman spectroscopy, TEM-EDX, XPS, CO2-TPD, H2-TPR, and O2-TPD, and the catalytic performance of supported Rh catalysts in N2O decomposition was tested. It is concluded that the support can be described as ZnO/Al2O3 (ZnO supported on Al2O3) when calcining Zn(NO3)2/Al2O3 at 500 °C, whereas ZnAl2O4 spinel forms on the Al2O3 surface at 800 °C. Rh/Zn–Al2O3 catalysts are much more active than Rh/Al2O3 and Rh/ZnO. The best catalyst (Rh/Zn–Al2O3-800 with 1 wt% Rh and 1 wt% Zn) has the smallest Rh2O3 particle size and can desorb O2 at lower temperature than other catalysts. Both factors may be important for achieving high activity in N2O decomposition.
The local structural changes within the normal spinel structure of a ZnAl2O4 host caused by the titanium incorporation as studied by the Raman spectroscopy and computationally using the density functional theory have been analyzed. The lattice dynamics calculation of the ZnAl2O4 phonons were performed within the density functional theory for the 3D periodic crystal structures. Band structure calculations by the grid-based PAW method predicted a direct band gap in the pure ZnAl2O4 sample to be 4.732 eV, while doping with Ti⁴⁺ produced a density of states in the middle of the gap. The lattice dynamics calculations using Gaussian-type wavefunctions as basis set and a local density approximation gave a good agreement between observed and predicted Raman bands. Doping with Ti⁴⁺ causes the infrared active T1u phonons as well as the inactive phonons (T1g, A2u, Eu and T2u symmetries) to appear in the Raman spectra and breaking of the symmetry selection rules.
The performance of pure and Pd-doped aluminate spinel catalysts (i.e., MAl2O4 and MAl1.95Pd0.05O4, where M = Cu, Co, Zn) for the selective catalytic reduction of NO by H2 (H2-SCR) were investigated in this paper. The catalytic performance over MAl2O4 is poor but can be improved significantly by incorporating Pd into the lattice, resulting in NO conversion over Co-AlPd, Zn-AlPd and Cu-AlPd catalysts of approximately 95%, 90.5% and 84%, respectively, in the presence of 2% O2 at a low temperature range of 100-350?C. Both the pure and Pd-doped aluminate spinel catalysts showed the same sequences with respect to activity: Co-Al > Zn-Al > Cu-Al and Co-AlPd > Zn-AlPd > Cu-AlPd, indicating that the selection of divalent metal M is of essential importance in designing spinel catalysts and in modifying their SCR performance. Co-AlPd catalyst showed stable activity in the presence of 3% and 5% H2O at 250?C, whereas the SCR reaction was promoted and a slight positive influence of the NO conversion was observed with a further increase of the H2O concentration to 5%. The presence of 100 ppm SO2 in the feed resulted in almost a 23% NO reduction at 250?C for the Co-AlPd catalyst, whereas only a 1.2% decrease of NO conversion was observed with further increase in the SO2 concentration to 150 ppm, and the NO conversion recovered rapidly to approximately 82% after removing the SO2 from the feed stream.
Advances in the accuracy and efficiency of ab initio calculations have allowed detailed studies of properties of material under pressure. In this chapter we present an overview of the theoretical work done from first principles on the structural, elastic, electronic, and vibrational properties of spinels under pressure as well as pressure-induced post-spinel phases. Theoretical results are compared with experimental data when available. As there is a lack in the study of vibrational properties of thiospinels under pressure, we also report an analysis of the vibration modes for the cubic spinel CdIn2S4.
The electronic structure, density of state, population analysis and optical properties of the ZnAl2O4 have been calculated using the first-principle density function theory method. The results indicate that ZnAl2O4 is a direct semiconductor material with a band gap of 3.91 eV. The valence band is composed of the density of states of O2p, Zn3d electrons and the conduction band is composed of Al3s, 3p electrons. ZnAl2O4 is a compound which have both ion bond and covalent bond. Moreover, dielectric functions, refractive index and extinction coefficient of ZnAl2O4 were analyzed in terms of calculated band structure and density of state. The static dielectric constant ε1(0) and the reflectivity n0 are 3.35 and 1.83, respectively.
Sm3+-doped ZnAl2O4 phosphor was synthesized by citrate sol-gel method and characterized using X-ray diffraction and scanning electron microscopy to identify the crystalline phase and determine the particle size. Photoluminescence (PL) studies on the sample showed emission peaks at 563, 601, 646 and 707 nm with λ
ex = 230 nm corresponding to the 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2 and 4G5/2 → 6H11/2 transitions, respectively, due to Sm3+ ions. PL lifetime decay studies confirmed that Sm3+ ions partly entered into the lattice by replacing Al3+ ions and remaining located at the surface of ZnAl2O4 host matrix. Thermally stimulated luminescence (TSL) studies of γ-irradiated Sm3+-doped ZnAl2O4 sample showed two glow peaks at 440 and 495 K, the former being most intense than the latter. The trap parameters were determined using different heating rate methods. Spectral characteristics of the TSL glow showed emission around 565, 599 and 641 nm, indicating the role of Sm3+ ion as the luminescent centre. A probable mechanism for the prominent TSL glow peak, observed at 440 K, was proposed. CIE chromaticity coordinates for the system was evaluated, which suggested that Sm3+-doped ZnAl2O4 could be employed as a potential orange red-emitting phosphor.
In this chapter, we present an overview of the effects of pressure on the crystalline structure and physical properties of oxygen-based spinels and other related oxides. Recent X-ray diffraction and Raman spectroscopy studies are summarized. A brief description of pressure-driven transitions and post-spinel structures is also provided. We also compare the response to high-pressure of several spinel oxides. We conclude with an examination of elastic and magnetic properties.
The phosphors ZnGa2O4 were synthesized via high temperature solid-state reaction. The crystal structure, photoluminescence, persistent luminescence, and photocatalytic properties of ZnGa2O4 were studied in detail. The x-ray diffraction patterns showed that some remaining phases of ZnO and β-Ga2O3 appeared with the excess amount of ZnO and Ga2O3, respectively. The results of the Raman spectra indicated that the first order Raman active modes of ZnGa2O4 were attributed to O2− ions and Zn2+ ions in tetrahedral sites. The phosphors exhibited a broad-band emission around 430 nm, which could be ascribed to the Ga-O transition of regular octahedral sites in the spinel lattice of ZnGa2O4. It also exhibited the emission peak around 430 nm shift to longer wavelength with the amount of the excess ZnO. The persistent luminescence of ZnGa2O4 could be observed for 10 min by naked eyes at room temperature under 254 nm ultraviolet (UV) excitation. In addition, photocatalytic activity test showed that ZnGa2O4 exhibited excellent photocatalytic activity for the degradation of Rhodamine B by the UV irradiation. It was indicated that the traps played an important role in trapping the electrons or holes to decrease the combination of the holes or electrons producing by the irradiation.
Room-temperature angle-dispersive x-ray diffraction measurements on spinel ZnGaO up to 56 GPa show evidence of two structural phase transformations. At 31.2 GPa, ZnGaO undergoes a transition from the cubic spinel structure to a tetragonal spinel structure similar to that of ZnMnO. At 55 GPa, a second transition to the orthorhombic marokite structure (CaMnO-type) takes place. The equation of state of cubic spinel ZnGaO is determined: V = 580.1(9) ³, B = 233(8) GPa, B' = 8.3(4), and B'' = -0.1145 GPa¹ (implied value); showing that ZnGaO is one of the less compressible spinels studied to date. For the tetragonal structure an equation of state is also determined: V = 287.8(9) ³, B = 257(11) GPa, B' = 7.5(6), and B'' = -0.0764 GPa¹ (implied value). The reported structural sequence coincides with that found in NiMnO and MgMnO.
Single-crystal Raman and infrared reflectivity data including high pressure results to over 200 kbar on a natural, probably fully ordered MgAl2O4 spinel reveal that many of the reported frequencies from spectra of synthetic spinels are affected by disorder at the cation sites. The spectra are interpreted in terms of factor group analysis and show that the high energy modes are due to the octahedral internal modes, in contrast to the behavior of silicate spinels, but in agreement with previous data based on isotopic and chemical cation substitutions and with new Raman data on gahnite (∼ ZnAl2O4) and new IR reflectivity data on both gahnite and hercynite (∼Fe0.58Mg0.42Al2O4). Therefore, aluminate spinels are inappropriate as elastic or thermodynamic analogs for silicate spinels.
Fluorescence sideband spectra yield complementary information on the vibrational modes and provide valuable information on the acoustic modes at high pressure. The transverse acoustic modes are nearly pressure independent, which is similar to the behavior of the shear modes previously measured by ultrasonic techniques. The pressure derivative of all acoustic modes become negative above 110 kbar, indicating a lattice instability, in agreement with previous predictions. This lattice instability lies at approximately the same pressure as the disproportionation of spinel to MgO and Al2O3 reported in high temperature, high pressure work.
In this work we present a first-principles density functional study of the electronic, vibrational, and structural properties of ZnGa2O4 and ZnAl2O4 spinel structures. Here we focus our study in the evolution of the structural properties under hydrostatic pressure. Our results show that ZnGa2O4 under pressure has a first-order phase transition to the marokite (CaMn2O4) structure, which is in good agreement with recent angle-dispersive x-ray diffraction experiments. We also report a similar study for the ZnAl2O4 spinel; we found that this compound under pressure has a first-order phase transition to the orthorhombic CaFe2O4-type structure. Our results in both compounds support, under nonhydrostatic condition, the possibility of a second-order phase transition from the cubic spinel to the tetragonal spinel as reported experimentally in ZnGa2O4.
The phonon spectrum of ZnAl2O4 spinel was investigated jointly by inelastic neutron-scattering and first-principles calculations. The results permit an assessment of important mechanical and thermodynamical properties such as the bulk modulus, elastic constants, lattice specific heat, vibration energy, and Debye temperature. The observed generalized phonon density of states shows a gapless spectrum extending to a cutoff energy of ∼840 cm-1. The theoretical results reproduce all of the features of the phonon density of states. The calculated Raman-and infrared-active phonon frequencies agree well with the data in the literature. A comparison of the lattice dynamics of ZnAl2O4 and MgAl2O4 spinels was carried out using a simple rigid-ion model, which shows that the major difference in the phonon frequencies of the two materials can be accounted for by the mass effects between the Zn and Mg ions.
We have measured the specific heat of zinc-blende ZnS for several isotopic compositions and over a broad temperature range (3–1100 K). We have compared these results with calculations based on ab initio electronic band structures, performed using both local-density approximation and generalized gradient approximation exchange-correlation functionals. We have compared the lattice dynamics obtained in this manner with experimental data and have calculated the one-phonon and two-phonon densities of states. We have also calculated mode-Grüneisen parameters at a number of high-symmetry points of the Brillouin zone. The electronic part of our calculations has been used to investigate the effect of the 3d core electrons of zinc on the spin-orbit splitting of the top valence bands. The effect of these core electrons on the band structure of the rocksalt modification of ZnS is also discussed.
The structural behavior of synthetic gahnite (ZnAl2O4) has been investigated by X-ray powder diffraction at high pressure (0–43 GPa) and room temperature, on the ID9 beamline
at ESRF. The equation of state of gahnite has been derived using the models of Birch–Murnaghan, Vinet and Poirier–Tarantola,
and the results have been mutually compared (the elastic bulk modulus and its derivatives versus P determined by the third-order Birch–Murnaghan equation of state are K
0=201.7(±0.9) GPa, K
′
0=7.62(±0.09) and K
″
0=−0.1022 GPa−1 (implied value). The compressibilities of the tetrahedral and octahedral bond lengths [0.00188(8) and 0.00142(5) GPa−1 at P=0, respectively], and the␣polyhedral volume compressibilities of the four-␣and␣sixfold coordination sites [0.0057(2) and
0.0041(2) GPa−1 at P=0, respectively] are discussed.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
The exact density functional for the ground-state energy is strictly self-interaction-free (i.e., orbitals demonstrably do not self-interact), but many approximations to it, including the local-spin-density (LSD) approximation for exchange and correlation, are not. We present two related methods for the self-interaction correction (SIC) of any density functional for the energy; correction of the self-consistent one-electron potenial follows naturally from the variational principle. Both methods are sanctioned by the Hohenberg-Kohn theorem. Although the first method introduces an orbital-dependent single-particle potential, the second involves a local potential as in the Kohn-Sham scheme. We apply the first method to LSD and show that it properly conserves the number content of the exchange-correlation hole, while substantially improving the description of its shape. We apply this method to a number of physical problems, where the uncorrected LSD approach produces systematic errors. We find systematic improvements, qualitative as well as quantitative, from this simple correction. Benefits of SIC in atomic calculations include (i) improved values for the total energy and for the separate exchange and correlation pieces of it, (ii) accurate binding energies of negative ions, which are wrongly unstable in LSD, (iii) more accurate electron densities, (iv) orbital eigenvalues that closely approximate physical removal energies, including relaxation, and (v) correct longrange behavior of the potential and density. It appears that SIC can also remedy the LSD underestimate of the band gaps in insulators (as shown by numerical calculations for the rare-gas solids and CuCl), and the LSD overestimate of the cohesive energies of transition metals. The LSD spin splitting in atomic Ni and $s$-${}d$ interconfigurational energies of transition elements are almost unchanged by SIC. We also discuss the admissibility of fractional occupation numbers, and present a parametrization of the electron-gas correlation energy at any density, based on the recent results of Ceperley and Alder.
The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blöchl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
An in-situ Raman Spectroscopic study was conducted to explore the pressure-induced phase transformation of CaMn2O4 to pressures of 73.7GPa. Group theory yields 24 Raman active modes for CaMn2O4, of which 20 are observed at ambient conditions. With the slight compression below 5GPa, the pressure-induced contraction compensates the structural distortion induced by a Jahn–Teller (JT) effect, resulting in the occurrence of the zero pressure shifts of the JT-related Raman modes. Upon elevation of pressure to nearby 35GPa, these Raman modes start to display a significant variation in pressure shift, implying the appearance of a pressure-induced phase transformation. Group factor analyses on all possible structure polymorphs indicate that the high-pressure phase is preferentially assigned to an orthorhombic structure, having the CaTi2O4 structure. The cooperative JT distortion is continuously reduced in the CaMn2O4 polymorph up to 35GPa. Beyond 35GPa, it is found that the JT effect was completely suppressed by pressure in the newly formed high-pressure phase. Upon release of pressure, this high-pressure phase transforms to the original CaMn2O4 phase, and continuously remains stable to ambient conditions.
An in-situ Raman spectroscopic study was conducted to explore the pressure induced phase transformation of spinel-type ferrite ZnFe2O4. Results indicate that ferrite ZnFe2O4 initially transforms to an orthorhombic structure phase (CaFe2O4-polymorph) at a pressure of 24.6 GPa. Such a phase transformation is complete at 34.2 GPa, and continuously remains stable to the peak pressure of 61.9 GPa. The coexistence of the two phases over a wide range of pressure implies a sluggish mechanism upon the spinel-to-orthorhombic phase transition. Upon release of pressure, the high pressure ZnFe2O4 polymorph is quenchable at ambient conditions.
A study of the infrared and Raman spectra of single crystals and powders of ZnGa2O4 yields the following k = 0 phonon frequencies: 175 cm−1 (T1u), 328 cm−1 (T1u), 420 cm−1 (T1u), 570 cm−1 (T1u), 467 cm−1 (T2g), 611 cm−1 (T2g), 638 cm −1 (Eg) and 714 cm−1 (A1g).
The results are compared with the frequencies of the vibronic sidebands of the Cr3+R line emission as observed in ZnGa2O4: Cr3+. It is found that the strogest vibronics of the R lines are due to coupling of the Cr3+ electrons with the T1u modes of the ZnGa2O4 lattice, especially with the two higher frequency T1u modes.
It has been observed that the fcc spinel NiMn2O4 transforms to a tetragonal structure at about 12 GPa. The tetragonal phase does not revert to the cubic phase upon decompression and its unit-cell constants at ambient pressure are a0=8.65(8) and c0=7.88(15)Å (distorted fcc). Within the experimental uncertainty, there is no volume change at the transition. The c/a ratio of the tetragonal spinel is almost independent of pressure and equal to 0.91. The phase transition is attributed to the Jahn-Teller-type distortion and the ionic configuration can be assumed as (Mn3+)tetr[Ni2+Mn3+]oct. The bulk modulus of the cubic phase is 206(4) GPa.
While the first comparison of shallow bound exciton photoluminescence between natural Si and highly enriched 28Si dramatically demonstrated the importance of inhomogeneous isotope broadening, the transitions in 28Si were in fact too narrow to be resolved with the then available instrumental resolution of 0.014 cm−1. We report results for the boron bound exciton transition in highly enriched 28Si using a novel apparatus for photoluminescence excitation spectroscopy based on a tuneable single-frequency laser source with sub-MHz resolution. Twenty well-resolved doublets, exhibiting a 10B–11B isotope splitting, are observed in the new spectra for 28Si with isotopic enrichment >99.99%. Linewidths as narrow as 0.0012 cm−1 (150 neV) full width at half maximum are observed for the most highly enriched sample.
X-ray photoelectron spectroscopy, magnetic measurements, and a single-crystal x-ray structure determination at normal pressure have shown that Jahn-Teller active manganese ions in ZnMn2O4 are present in one valence state (III) on the octahedral sites of the spinel structure. The high-pressure behavior of ZnMn2O4 was investigated up to 52 GPa using the energy-dispersive x-ray diffraction technique and synchrotron radiation. The structural first-order phase transition from the body-centered to primitive-tetragonal cell takes place at Pc=23GPa. The high-pressure phase is metastable down to normal pressure. The c/a ratio reduces from 1.62 to 1.10 above Pc and remains nearly pressure independent in the high-pressure phase. The transition is attributed to the changes in electron configuration of the Mn3+ ions. According to the crystal field theory, the eg electron of octahedrally coordinated Mn3+ is either in the dz2 orbital or in the dx2-y2. In the first configuration the MnO6 octahedron will be elongated and this is the case at normal pressure, while the second configuration gives the flattened octahedron. In the high-pressure phase some proportion of the eg electrons of the Mn3+ ions is moved to the dx2-y2 level, which is revealed as an abrupt fall of observed magnitude of the distortion of the bulk crystal above Pc.
An improved calibration curve of the pressure shift of the ruby R1 emission line was obtained under quasi-hydrostatic conditions in the diamond-window, high-pressure cell to 800 kbar. Argon was the pressure-transmitting medium. Metallic copper, as a standard, was studied in situ by X ray diffraction. The reference pressure was determined by calibration against known equations of state of the copper sample and by previously obtained data on silver.
An in situ Raman spectroscopic study was conducted to investigate the pressure-induced phase transformation in the synthetic ZnCr2O4 spinel up to pressures of 70 GPa at room temperature. Results indicate that ZnCr2O4 spinel starts to transform to the CaFe2O4 (or CaTi2O4) structure at 17.5 GPa, and such a phase transformation is complete at 35 GPa. The coexistence of two phases over a wide range of pressure implies a sluggish mechanism upon phase transformation. No experimental evidence was observed to support the theoretical simulation with the dissociation of ZnCr2O4 to ZnO and Cr2O3 at 34 GPa. Moreover, enhancement of the intensity of the Raman peak at 642 cm(-1) at either elevated pressures or temperatures is most likely caused by an enhanced order-disorder effect. Upon release of pressure, the recovered phase may exhibit an inverse spinel structure, which differs from the initial normal spinel structure. (C) 2002 Elsevier Science (USA).
In this paper, we report ab initio calculations of the vibrational and structural properties of ZnAl2O4 and ZnGa2O4 spinel structures. The calculated vibrational modes at zero pressure, at the Γ point and the complete phonon spectrum of both compounds are presented. Also, we report our findings for the high-pressure structure
Eu3+-doped zinc aluminate (ZnAl2O4) nanorods with a spinel structure were successfully synthesized via an annealing transformation of layered precursors obtained by a homogeneous coprecipitation method combined with surfactant assembly. These spinel nanorods, which consist of much finer nanofibres together with large quantities of irregular mesopores and which possess a large surface area of 93.2 m2 g−1 and a relatively narrow pore size distribution in the range of 6–20 nm, are an ideal optical host for Eu3+ luminescent centres. In this nanostructure, rather disordered surroundings induce the typical electric-dipole emission of Eu3+ to predominate and broaden.
An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
Spinel-type ZnGa2O4 and ZnAl2O4 powders doped with Eu3+ and Tb3+ ions, including dual doping, have been prepared by solid phase reactions and by deposition from chemical solutions. XRD analysis demonstrates the high quality of spinel grains constituting the powder. The luminescence shows that the rare earth ions are incorporated in the defect regions at the grain boundaries. The emission spectra of the samples with europium are characterized by an intense emission in red region due to the 5D0 → 7F1,2 transitions of Eu3+ ions, whereas in the case of terbium the highest intensity corresponds to the green emission due to the 5D4 → 7F5 transitions of Tb3+ ions. The possibility to change the color of the emission from green to red by the variation of Tb and Eu doping concentration is demonstrated. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
The optical spectra of zinc aluminate (ZnAl2O4), zinc gallate (ZnGa2O4), and zinc aluminogallate (ZnAlGaO4) spinel powders were studied at wavelengths in the range of 250-900 nm using reflectance spectroscopy. The ZnAl2O4 and ZnGa2O4 powders were synthesized by using conventional ceramic processing techniques and had systematic variations in the molar ratio of ZnO to M2O3 (M = Al or Ga). The cubic spinel crystal structure of each composition was confirmed via powder X-ray diffractometry. The ZnAl2O4 powders showed optical properties in the ultraviolet wavelength region and had combined characteristics that were similar to that of ZnO (wurtzite structure) and Al2O3 (corundum structure), which result from the similar local environments of the zinc and aluminum cations within the cubic spinel crystal structure. A mechanically induced optical absorption (optomechanical effect) in the ultraviolet wavelength region was also observed in ZnAl2O4. The ZnGa2O4 powder followed a similar behavior, with the exception that the optomechanical effect did not occur in the gallate. The ZnAlGaO4 showed optical spectra that were intermediate to that of the endpoint compositions.
High-quality bulk ZnGa2O4 has been synthesized from equimolar mixtures of ZnO and Ga2O3 by the conventional solid-state method. For the first time, the sample has been characterized in detail to confirm the formation of pure single phase of spinel ZnGa2O4. The formation of ZnGa2O4 has been confirmed by sintering the mixtures of ZnO and Ga2O3 at different temperatures, ranging from 900–1200 C. It is observed that the single phase of ZnGa2O4 has been formed at and above 1000 C sintering temperature for 24 h. The crystallinity and phase formation of this single phase has been confirmed by X-ray diffraction. X-ray photoelectron spectroscopic studies have been carried out for bulk ZnGa2O4 sintered at 1000 C for 24 h which showed 14% Zn, 28% Ga and 58% O, indicating stoichiometric ZnGa2O4. A new parameter, the energetic separation between the Zn 2p3/2 and Ga 2p3/2 peaks, has been used as a sensitive tool to distinguish between a complete formation of ZnGa2O4 compound and a mixture of ZnO and Ga2O3 powders. Surface morphology studies by scanning electron microscopy reveal that the formation of ZnGa2O4 takes place in mosaic rod-like structure. The purity of the compound has also been checked by the energy dispersive X-ray method, indicating the absence of foreign ions and the ratio of zinc to gallium has been calculated and found to be 1 : 2, indicating stoichiometric ZnGa2O4.
In this paper, a study was made on the electroconductive property of the compound ZnGa2O4, a double oxide with a spinel configuration. Polycrystalline specimen of ZnGa2O4 were calcined, sintered and exposed to H2 annealing. A sample of ITO was also readied in a similar manner for the purpose of analogous testing. Using a Simadzu UV-365 double-beam spectrophotometer, the absorption end of ZnGa2O4 was determined at about 250 nm with shorter wavelength than ITO and ZnO. In addition, the conventional four-point probe method was used to quantify the dc electrical conductivity of the specimen at 77 to 295 K. The H2-annealed ZnGa2O4 displayed a weak semiconductor behavior with a conductivity of 3 × 101 s cm-1 at 295 K. Based on the results, ZnGa2O4 spinel is a potential UV-transparent electronic conductor for photoelectronic equipment.
The effects of uniaxial stress on the pressure-induced α→ω transition in pure titanium (Ti) are investigated by means of angle dispersive X-ray diffraction in a diamond-anvil cell. Experiments under four different pressure environments reveal that: (1) the onset of the transition depends on the pressure medium used, going from 4.9 GPa (no pressure medium) to 10.5 GPa (argon pressure medium); (2) the α and ω phases coexist over a rather large pressure range, which depends on the pressure medium employed; (3) the hysteresis and quenchability of the ω phase is affected by differences in the sample pressure environment; and (4) a short-term laser heating of Ti lowers the α→ω transition pressure. Possible transition mechanisms are discussed in the light of the present results, which clearly demonstrate the influence of uniaxial stress in the α→ω transition.
We present a detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set. We will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temperature density-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N-atoms(2) scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge density including a new special 'preconditioning' optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. We have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio molecular-dynamics package), The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
We present ab initio quantum-mechanical molecular-dynamics calculations based on the calculation of the electronic ground state and of the Hellmann-Feynman forces in the local-density approximation at each molecular-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using subspace alignment. This approach avoids the instabilities inherent in quantum-mechanical molecular-dynamics calculations for metals based on the use of a fictitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows us to perform simulations over several picoseconds.
We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal–amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nosé dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.
- A Yang
A. Yang, Phys. Rev. B 80(19), 195203 (2009).
- E Rusu
E. Rusu et al., Phys. Status Solidi C 6, 1199 (2009).
- D Errandonea
- Y Meng
- M Somayazulu
- D Hüsermann
D. Errandonea, Y. Meng, M. Somayazulu, and D. Hüsermann, Physica B 355, 116 (2005).
- A Phani
A. Phani et al., J. Mater. Sci. 33, 3969 (1998).
- D Errandonea
D. Errandonea et al., Phys. Rev. B 79(2), 024103 (2009).
- T Omata
- N Ueda
- K Ueda
- H Kawazoe
T. Omata, N. Ueda, K. Ueda, and H. Kawazoe, Appl. Phys. Lett. 64, 1077 (1994).
- Z Wang
- D Schiferl
- Y Zhao
- H C O'neill
Z. Wang, D. Schiferl, Y. Zhao, and H. C. O'Neill, J. Phys. Chem. Solids 64, 2517 (2003).
- G V Gorkom
- J Haanstra
- H Boom
G. V. Gorkom, J. Haanstra, and H. Boom, J. Raman Spectrosc. 1, 513 (1973).
has been supported by CONACyT México under the project J-59853 acknowledge the support of the CONACyT México by the postdoctoral fellowship
- Proalmex-Daad-Conacyt Spain
- Cgstic S L Cinvestav
A.H.R. has been supported by CONACyT México under the project J-59853-F, PROALMEX-DAAD-CONACYT,
Spain and CGSTIC – Cinvestav. S. L.-M. acknowledge the support of the CONACyT México by the postdoctoral fellowship. F.J.M., D.E., P. R.-H. and A. M. acknowledge the financial support of the MICINN of Spain under grants Nos.
- P Blochl
P. Blochl, Phys. Rev. B 50, 17953 (1994).
- S López
S. López et al., Phys. Rev. B 79, 214103 (2009).
- B Cheng
- S Qu
- H Zhou
- Z Wang
B. Cheng, S. Qu, H. Zhou, and Z. Wang, Nanotech. 17, 2982 (2006).
- M Cardona
M. Cardona et al., Phys. Rev. B 81(7), 075207 (2010).
- S Sampath
- J Cordaro
S. Sampath and J. Cordaro, J. Am. Ceram. Soc. 81, 649 (1998).
- H Monkhorst
- J Pack
H. Monkhorst and J. Pack, Phys. Rev. B 13, 5188 (1976).
- S Åsbrink
S.Åsbrink et al., Phys. Rev. B 60(18), 12651-12656 (1999).
- G Kresse
- J Hafner
G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993).
- G Kresse
- J Hafner
G. Kresse and J. Hafner, Phys. Rev. B 49, 14251 (1994).
- G Kresse
- J Furthmüller
G. Kresse and J. Furthmüller,Comput. Math. Sci. 6, 15 (1996).