No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Ab initio study of the high-pressure phases and dynamical properties of ZnAl2O4 and ZnGa2O4
Abstract
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
... 8.05 [27] 0.27 [27] 183 [27 ] 8.02 [31] 1.90 [31] 8.09 [28] 0.26 [31] 206.91 [30] 1.93 [31] 8.02 [31] 219.65 [31] 8.34 [10] 0.26 [10] 233 [10] 1.95 [10] 2.00 [10] 8.29 [13] 0.26 [13] 218.93 [13] 1.95 [13] 1.99 [13] 8.33 [32] 0.26 [32] 1.97 [32] 1.99 [32] calc. ...
... 8.05 [27] 0.27 [27] 183 [27 ] 8.02 [31] 1.90 [31] 8.09 [28] 0.26 [31] 206.91 [30] 1.93 [31] 8.02 [31] 219.65 [31] 8.34 [10] 0.26 [10] 233 [10] 1.95 [10] 2.00 [10] 8.29 [13] 0.26 [13] 218.93 [13] 1.95 [13] 1.99 [13] 8.33 [32] 0.26 [32] 1.97 [32] 1.99 [32] calc. ...
... 8.05 [27] 0.27 [27] 183 [27 ] 8.02 [31] 1.90 [31] 8.09 [28] 0.26 [31] 206.91 [30] 1.93 [31] 8.02 [31] 219.65 [31] 8.34 [10] 0.26 [10] 233 [10] 1.95 [10] 2.00 [10] 8.29 [13] 0.26 [13] 218.93 [13] 1.95 [13] 1.99 [13] 8.33 [32] 0.26 [32] 1.97 [32] 1.99 [32] calc. ...
This paper performs first-principles calculations to study the structural, mechanical and electronic properties of the spinels ZnAl2O4, ZnGa2O4 and ZnCr2O4, using density functional theory with the plane-wave pseudopotential method. Our calculations are in good agreement with previous theoretical calculations and the available experimental data. The studies in this paper focus on the evolution of the mechanical properties of ZnAl2O4, ZnGa2O4 and ZnCr2O4 under hydrostatic pressure. The results show that the cubic phases of ZnAl2O4, ZnGa2O4 and ZnCr2O4 become unstable at about 50 GPa, 40 GPa and 25 GPa, respectively. From analysis of the band structure of the three compounds at equilibrium volume, it obtains a direct band gap of 4.35 eV for ZnAl2O4 and 0.89 eV for ZnCr2O4, while ZnGa2O4 has an indirect band gap of 2.73 eV.
... AB2O4 compounds are investigated widely for the aforementioned purpose, because they have structural dependence under pressure. This material can be found in many geological settings of the earth's crust and mantle, in lunar rocks and meteorites [49]. The zinc atoms are located at the Wyckoff positions, 8a (1/8, 1/8, 1/8) tetrahedral sites, while the gallium atoms take position at the 16d (1/2, 1/2, 1/2) octahedral sites and the oxygen atoms are at 32e (u, u, u), where u is approximately 1/4 [45]. ...
... AB 2 O 4 compounds are investigated widely for the aforementioned purpose, because they have structural dependence under pressure. This material can be found in many geological settings of the earth's crust and mantle, in lunar rocks and meteorites [49]. The zinc atoms are located at the Wyckoff positions, 8a (1/8, 1/8, 1/8) tetrahedral sites, while the gallium atoms take position at the 16d (1/2, 1/2, 1/2) octahedral sites and the oxygen atoms are at 32e (u, u, u), where u is approximately 1/4 [45]. ...
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.
... High pressure X-ray diffraction studies of these two compounds have shown that while ZnAl 2 O 4 does not undergo any phase transition till 43 GPa [4], ZnGa 2 O 4 undergoes two phase transitions towards the tetragonal spinel (I4 1 /amd) and marokite (Pbcm) structures around 34 and 55 GPa, respectively [5]. Recently, S. López et al. have performed first principles calculations to study the stability of the spinel structures of ZnAl 2 O 4 and ZnGa 2 O 4 under high pressure and phonon frequencies in the Γ point at zero pressure [6,7]. It has been predicted that ZnAl 2 O 4 should undergo a pressure-induced a) b) c) d) phase transition from the cubic spinel towards the orthorhombic CaFe 2 O 4 -type (Pnma) structure around 38.5 GPa while ZnGa 2 O 4 should undergo a pressure-induced phase transition from the cubic spinel towards the orthorhombic CaMn 2 O 4 -type (marokite) structure around 33.4 GPa. ...
... According to our previous calculations [6,7], a phase transition in ZnAl 2 O 4 from the cubic spinel towards the CaFe 2 O 4 -type structure (Pnma) should occur at 38.5 GPa. Similarly, a phase transition in ZnGa 2 O 4 from the cubic spinel towards the CaMn 2 O 4 -type structure (Pbcm) should occur at 33.4 GPa. ...
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]. ...
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.
... The replacement of Co atoms in the lattice was also analyzed using Raman spectra, shown in Fig. 3. The spinel structure related phonon modes in AB 2 O 4 structures with space group Fd3m were classified as follows [42][43][44] : ...
... The replacement of Co atoms in the lattice was also analyzed using Raman spectra, shown in Fig. 3. The spinel structure related phonon modes in AB 2 O 4 structures with space group Fd3m were classified as follows [42][43][44] : ...
The present study describes magnetic interactions in (Zn1–x
Co
x
)Ga2O4 (x = 0.05, 0.10, and 0.20) particles dependant on Co atoms in both tetrahedral and octahedral sites. The effects of substituted Co atoms to magnetic character are analyzed using Curie–Weiss law. The ferromagnetic character is found dominant in (Zn1–x
Co
x
)Ga2O4 semiconductors for x values lower than 0.10; in addition, a specific hysteresis with 139 ± 50 Oe coercivity is observed for 5% Co-doped ZnGa2O4. The high Co amount in tetrahedral site increased the number of antiferromagnetic couplings and the hysteresis at 300 K disappeared for (Zn0.80Co0.20)Ga2O4 particles. Furthermore, the Co+3 ions in the octahedral site decreased µeff values, per Co amounts, in the range of 4.89 ± 0.01 µB/Co to 4.44 ± 0.02 µB/Co, because of enhancing paramagnetic behaviors.
The structural, electronic, and vibrational properties of two leading representatives of the Zn‐based spinel oxides class, normal ZnX2O4 (X = Al, Ga, In) and inverse Zn2MO4 (M = Si, Ge, Sn) crystals, were investigated. In particular, density functional theory (DFT) was combined with different exchange‐correlation functionals: B3LYP, HSE06, PBE0, and PBESol. Our calculations showed good agreement with the available experimental data, showing a mean percentage error close to 3% for structural parameters. For the electronic structure, the obtained HSE06 band‐gap values overcome previous theoretical results, exhibiting a mean percentage error smaller than 10.0%. In particular, the vibrational properties identify the significant differences between normal and inverse spinel configurations, offering compelling evidence of a structure‐property relationship for the investigated materials. Therefore, the combined results confirm that the range‐separated HSE06 hybrid functional performs the best in spinel oxides. Despite some points that cannot be directly compared to experimental results, we expect that future experimental work can confirm our predictions, thus opening a new avenue for understanding the structural, electronic, and vibrational properties in spinel oxides. The predictive power of the density functional theory (DFT) method is dependent on the relative precision of the exchange‐correlation functional. In this study, a performance test was carried out in order to analyze the description of structural, electronic, and vibrational properties of normal and inverted spinel. Theoretical results indicate that HSE06 performs the best in reproducing experimental findings for such a class of compounds, being the most suitable choice for the in silico prediction of spinel oxide properties.
This chapter is the review of the computational methods applied to the transition metal oxides most abundant in heterogeneous catalysis and is focused on the influence of the environment on the transition metal cation properties. The shortcomings of the most commonly used DFT level of theory are discussed, and its extensions towards more realistic environment are presented. The modern reactive force-field methods are also mentioned. The embedding schemes most commonly found in the quantum-chemical or classical description of the heterogeneous processes are discussed. The errors stemming from the non-completeness of the basis function, i.e. the basis set superposition error, found in the calculations with atomic basis, and the Pulay stress, occurring in the planewave calculations, together with remedies, are briefly described. It is shown that in all discussed systems, i.e. \( {\mathrm {CeO}}_{2}\), \({\mathrm {TiO}}_{2}\), \({\mathrm {ZrO}}_{2}\), zeolites, d-electron metal spinels, and \({\mathrm {V}}_{2}\mathrm{O}_{5}\), the appropriately applied Hubbard DFT GGA+U methods are successful for the compromise between computational cost and resultant accuracy. The much more time-consuming hybrid functionals give slightly more accurate results and, moreover, are more universal in the sense that they do not need calibration against experiment contrary to DFT+U where the Hubbard correction needs to be carefully selected for modelling particular properties.
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.
First-principles calculations are performed to investigate the elastic, phonon and thermodynamic properties of ZnAl2O4 and ZnAl2S4 structures. The equations of state are fitted by a four-parameter Birch-Murnaghan equation upon the first-principles energy vs. volume data. Three independent elastic constants c11, c12 and c44 at zero pressure are determined by strain energy vs. strain relationships, and the two structures are both mechanically stable. Elastic properties including bulk moduli, shear moduli, Young's moduli, Poisson's ratios and anisotropy values for both phases are estimated by Voigt-Reuss-Hill averaging scheme. The mechanical properties such as ductility and brittleness are further analyzed. The density functional perturbation theory is utilized to calculate the phonon properties and both phases are found to be dynamically stable. The phonon and Debye models are used to predict the thermodynamic properties such as the Gibbs free energies, entropies and specific heats at constant pressure for ZnAl2O4 and ZnAl2S4 compounds.
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.
The free energy and other thermodynamic properties of hexagonal-close-packed iron are calculated by direct ab initio methods over a wide range of pressures and temperatures relevant to the Earth’s core. The ab initio calculations are based on density-functional theory in the generalized-gradient approximation, and are performed using the projector augmented wave approach. Thermal excitation of electrons is fully included. The Helmholtz free energy consists of three parts, associated with the rigid perfect lattice, harmonic lattice vibrations, and anharmonic contributions, and the technical problems of calculating these parts to high precision are investigated. The harmonic part is obtained by computing the phonon frequencies over the entire Brillouin zone, and by summation of the free-energy contributions associated with the phonon modes. The anharmonic part is computed by the technique of thermodynamic integration using carefully designed reference systems. Detailed results are presented for the pressure, specific heat, bulk modulus, expansion coefficient and Grüneisen parameter, and comparisons are made with values obtained from diamond-anvil-cell and shock experiments.
Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA VA and IIB VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.
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.
We report the partial phonon densities of states (DOS) of iron sulfide, a possible component of the rocky planet's core, measured by the 57Fe nuclear resonant inelastic x-ray scattering and calculate the total phonon DOS under pressure. From the phonon DOS, we drive thermodynamic parameters. A comparison of the observed and estimated compressibilities makes it clear that there is a large pure electronic contribution in the observed compressibility in the metallic state. Our results present the observation of thermodynamic parameters of iron sulfide with the low-spin state of an Fe2+ ion at the high density, which is similar to the condition of the Martian core.
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.
We have used density functional theory to investigate the stability of MgAl2O4 polymorphs under pressure. Our results can reasonably explain the transition sequence of MgAl2O4 polymorphs observed in previous experiments. The spinel phase (stable at ambient conditions) dissociates into periclase and
corundum at 14GPa. With increasing pressure, a phase change from the two oxides to a calcium-ferrite phase occurs, and finally
transforms to a calcium-titanate phase at 68GPa. The calcium-titanate phase is stable up to at least 150GPa, and we did
not observe a stability field for a hexagonal phase or periclase+Rh2O3(II)-type Al2O3. The bulk moduli of the phases calculated in this study are in good agreement with those measured in high-pressure experiments.
Our results differ from those of a previous study using similar methods. We attribute this inconsistency to an incomplete
optimization of a cell shape and ionic positions at high pressures in the previous calculations.