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We have investigated the local structure around the Mn atom on highly
diluted manganites with formal Mn3+ and Mn3.5+
ions by x-ray absorption fluorescence spectra. Both x-ray absorption
near-edge structure and extended x-ray absorption fine structure were
recorded for LaMn1-xGaxO3 ( x=0.8
and 0.9) and
La0.9Ca0.1Mn0.2Sc0.8O3
compounds at room tempe...
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Context 1
... XANES spectra of LaMn 1−x Ga x O 3 x = 0.6, 0.8, and 0.9 and La 0.9 Ca 0.1 Mn 0.2 Sc 0.8 O 3 samples are shown in Fig. 1. The spectra present a main resonance at the edge and a prepeak structure around 15 eV below the edge. All the Ga-substituted samples show the same absorption edge posi- tion E 0 . On the other hand the absorption threshold for the Sc substituted sample shifts to higher energies by about 2 eV. This shift agrees with the expected formal ...
Context 2
... XANES spectra show a prepeak structure inset Fig. 1 whose intensity decrease with the Ga content being nearly constant for the highest dilutions. This prepeak structure in transition-metal oxides is ascribed to 1s → 3d transitions ei- ther quadrupole allowed usually very weak or dipole for- bidden. The latter becomes allowed due to the mixing of the Mn 3d states via O 2p ones. The ...
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... On the other hand, the substituted Ga 3+ ions in the LaMnO3 perovskite crystal lattice behave as acceptor dopants replacing the Mn 3+ ions in the octahedral B-site [22], which leads to the compensation of the charge imbalance following their oxidation. As a result, gallium doping slightly reduces the octahedral distortion and diminishes the Jahn-Teller effect by enhancing electronic correlations [23,24], resulting in a larger increase in the electrical conductivity [25] of the LMO system doped with Ga ions, as can be observed in Figure 5. The yttrium ions replace the lanthanum ions in the tetrahedral A-site of the LMO perovskite crystal lattice [26], and as a result, the yttrium substitution leads to a lowering of the mean ionic size at the La-site, which leads to distortion of the MnO6 octahedra. ...
... On the other hand, the substituted Ga 3+ ions in the LaMnO 3 perovskite crystal lattice behave as acceptor dopants replacing the Mn 3+ ions in the octahedral B-site [22], which leads to the compensation of the charge imbalance following their oxidation. As a result, gallium doping slightly reduces the octahedral distortion and diminishes the Jahn-Teller effect by enhancing electronic correlations [23,24], resulting in a larger increase in the electrical conductivity [25] of the LMO system doped with Ga ions, as can be observed in Figure 5. The yttrium ions replace the lanthanum ions in the tetrahedral A-site of the LMO perovskite crystal lattice [26], and as a result, the yttrium substitution leads to a lowering of the mean ionic size at the La-site, which leads to distortion of the MnO 6 octahedra. ...
The synthesis, thermal behavior and electrical properties of a series of undoped and 1% Ga- or Y-doped lanthanum manganite compounds, obtained via the sol–gel technique, are reported. Scanning electron microscopy (SEM/EDX), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) analyses were performed on all synthesized samples. The XRD results confirmed a good crystallinity for all studied samples, and a change in the crystal structure of Ga- or Y-doped lanthanum manganite (Pm-3m space group) was observed compared to the pristine sample (R-3c space group). Thermal analysis highlighted a different behavior of the doped samples compared to the undoped sample, observed by the different mass losses in the analyzed temperature range. For these materials, it is shown for the first time that the static electrical conductivity, σDC, of Ga- or Y-doped LaMnO3 compounds increases compared to the σDC of the pristine sample, and the thermal activation energy of the process of electrical conduction, EA,cond, increases linearly with the temperature for all three studied samples.
... This explanation contrasts with the experimental determination of the local geometrical structure of the Mn atom by x-ray absorption spectroscopy (XAS) for LaMn 1-x Ga x O 3 . It has been shown that the tetragonal distortion of MnO 6 octahedron continuously decreases with the gallium content in such a way that for x > 0.5 MnO 6 distortion disappears and becomes regular, simultaneously to a long-range FM ground state [26,27]. Dynamical tetragonal distortions have been discarded as well, considering the shortest time of the x-ray absorption process [28]. ...
... Similar structural parameters were obtained within the experimental error for the two sets of fits. Fits for selected compounds are shown in Fig. 6 and Table II summarizes the relevant structural parameters for [29], whereas in the LaMn 1-x Ga x O 3 , this MnO 6 distortion continuously diminishes and disappears for x > 0.5 [26,27]. ...
... As we showed in Refs. [26,27], the orthorhombic distortion in LaMnO 3 decreases continuously after Ga substitution and it disappears for x > 0.5, despite that the formal valence of the Mn atom is 3+. Besides, for x < 0.5, the local Jahn-Teller distortion remains ordered as in LaMnO 3 . ...
We have investigated the role of Mn3+ Jahn-Teller distortion in driving ferromagnetism in the LaMn1-xScxO3 series. The replacement of Mn by Sc in LaMnO3 decreases the orthorhombic distortion of the Pbnm cell, but the unit cell remains distorted even in the LaScO3 sample. The analysis of the x-ray diffraction patterns indicates a continuous evolution from the typical Jahn-Teller distorted octahedron in LaMnO3 into a nearly regular one in LaScO3. Surprisingly, x-ray absorption spectroscopy measurements at the Mn and Sc K edges reveal the stability of both Jahn-Teller distorted MnO6 octahedron and nearly regular ScO6 octahedron along the whole Sc-substituted series. Moreover, the structure is described as a random distribution of Jahn-Teller distorted MnO6 octahedra spatially ordered as in LaMnO3 and nearly regular ScO6 octahedra. This result contrasts with the pseudocubic phase and the appearance of regular MnO6 octahedra in LaMn1-xGaxO3 for x > 0.5. Thus the occurrence of Jahn-Teller distortion strongly depends on the distorted orthorhombic crystallographic structure of the matrix in which the Mn3+ atom is allocated. Besides, a ferromagnetic ground state is observed for x > 0.5 in both series independently of the presence (or not) of Jahn-Teller distortions around Mn3+, which discards either the spin flipping or the vibronic superexchange models proposed for the ferromagnetism in LaMn1-xBxO3 (B = Sc or Ga).
... The Mn-doped LaGaO 3 shows a very high value of resistivity that is comparable to that of good dielectric systems [31]. Moreover, even though there exists significant literature on Mn-doped LaGaO 3 [28][29][30][31][32][33], to the best of our knowledge this material has not been studied for MD coupling. Keeping this in view, here we investigate and report the effect of Mn doping at the Ga site on the MD properties of LaGaO 3 . ...
The polycrystalline sample of LaGa1-xMnxO3 (0<x<0.2) has been prepared by
solid state reaction route. The phase purity of these samples has been
confirmed by powder x-ray diffraction experiments carried out on BL-12 at
Indus-2 synchrotron radiation source. The room temperature
magneto-capacitance/magneto-dielectric (MC/MD) study on these samples has been
carried out in the frequency range of 20 Hz to 5 MHz. The MD effect about 4% at
f = 100 kHz and H = 0.4 Tesla has been observed at room temperature. With
increasing applied magnetic field the value of dielectric constant
({\epsilon}r) increases significantly throughout the frequency range i.e. up to
5 MHZ. The change in {\epsilon}r with magnetic field is significant (~1.8%)
even at higher frequencies such as at 1 MHz. The observed results are analyzed
in the framework of Maxwell-Wagner behavior and the intrinsic
magneto-dielectric effect; i.e. by systematically analyzing magneto-capacitance
(MC) and dielectric loss as a function of frequency. Our results and analysis
suggests that at lower frequencies i.e. up to ~30 kHz the MD effect is
dominated by Maxwell-Wagner effects where as at higher frequencies the observed
MD coupling might be the intrinsic property of these samples and appears to be
dominated by magnetic field induced dipolar relaxation. Our results of
intrinsic and significant MD coupling at room temperature, illustrate a new
path for the next generation MD materials for device applications.
... In both series the main structural change produced by Ga dilution is to progressively eliminate the long-range ordered transition metal in the LaFeO 3 and LaMnO 3 lattices giving way to isolated Fe 3+ and Mn 3+ ions, respectively. The FeO 6 octahedra remain regular in the whole range of composition, while the tetragonal distortion of the MnO 6 octahedra in LaMnO 3 decreases continuously as Ga replaces Mn, becoming almost symmetric octahedra for x 0.6 [17,18]. ...
... The Ga atoms occupy the Fe sites and the Fe(Ga)-O distances indicate the maintenance of regular oxygen octahedra around Fe(Ga) for the whole range 0 x 0.9. Powder LaMn 1−x Ga x O 3 (x = 0, 0.1, 0.2, 0.3, 0.5, 2/3, 0.8, 0.9) samples were also synthesized by means of a conventional ceramic procedure and preparation details as well as information on how the structure is affected by Ga dilution can be found in [17,18]. ...
... However, since gallium dilution in LaMnO 3 induces structural changes in the lattice, the latter study can only be used as a theoretical characterization of the spectral features but not as a simulation of the behaviour observed in the experimental XANES spectra of LaMn 1−x Ga x O 3 with Ga substitution. The replacement of Mn by Ga induces a decrease of the orthorhombic distortion of the cell in LaMnO 3 , the tetragonal distorted Mn octahedra being practically regular for gallium concentration x 0.6 [17,18]. In order to simulate the experimental behaviour more realistically, we have also checked the effects of Ga substitutions in equivalent calculations performed for the crystal structure of the highly diluted LaMn 0.1 Ga 0.9 O 3 sample [17]. ...
We report on experimental oxygen K-edge x-ray absorption near edge structure (XANES) spectra of the LaFe(1 - x)Ga(x)O(3) and LaMn(1 - x)Ga(x)O(3) series. Transition metal substitution by the 3d full shell Ga atom is mainly reflected in a systematic decrease of the pre-edge structures in the XANES spectra of the two series. This result shows that the associated states originate from the hybridization of oxygen 2p and unoccupied Fe (or Mn) 3d states. In order to gain insight into the states associated with the pre-edge spectral features, we have performed ab initio theoretical calculations based on multiple scattering theory. Simulations with variable cluster size and composition around the absorber oxygen in the LaFeO(3) and LaMnO(3) crystal structures were carried out. We find that the low-energy pre-peak is reproduced once the absorbing oxygen and the two nearest neighbour Fe (or Mn) ions are considered in the cluster. Conversely, higher energy pre-peaks only arise when the full oxygen coordination geometry around the two metal sites is taken into account, implying that their energy distance is a reflection of the strength of the oxygen ligand field. Substitutions of the two nearest neighbours by Ga atoms in the cluster of calculation lead to changes in the theoretical spectra that reasonably agree with the evolution of the pre-peaks in the experimental XANES spectra of both the series.
... This suggested that the Mn clusters have to have a minimum size in order to show the tetragonal distortion typical of Jahn-Teller Mn 3+ ions. 28 The persistence of this type of distortion in the whole TbMn 1−x Sc x O 3 series adds a new factor not previously considered, the degree of the anion octahedra tilting. This is the main difference between the TbMn 1−x Sc x O 3 and the LaMn 1−x Ga x O 3 series. ...
... Second, the octahedron shows the same distortion and finally, this distortion conserves its orientation in the lattice. This behavior contrasts with the one observed in the LaMn 1−x Ga x O 3 series, where the tetragonal distortion of the MnO 6 octahedron decreases with the Ga substitution25,28 and the appearing of ferromagnetism concurs with the presence of regular MnO 6 octahedra. The stability of the tetragonal distorted MnO 6 octahedron in TbMn 1−x Sc x O 3 prevents the formation of a ferromagnetic ground state for x = 0.5 and suggests a close relationship between local structure around Mn 3+ and the magnetic behavior in the diluted samples. ...
The new TbMn1−xScxO3 series has been synthesized and the structural properties have been characterized by x-ray diffraction, neutron diffraction, and x-ray absorption spectroscopy. The whole series belongs to the family of ABO3 perovskites. All of the samples are isostructural to the parent compound TbMnO3, crystallizing in an orthorhombic cell with space-group Pbnm symmetry. The homovalent substitution of Mn3+ with the larger Sc3+ ion leads to an increase in both the unit-cell volume and the BO6 octahedra tilting. The analysis of neutron and x-ray diffraction patterns suggest a continuous evolution from a Jahn-Teller distorted BO6 octahedron in TbMnO3 into a nearly regular one in TbScO3. However, x-ray absorption measurements at the Mn K edge reveal that the local geometry around the Mn3+ cation remains distorted in the series even for high values of x. This result is in opposition to previous findings in similar compounds such as LaMn1−x(Ga/Sc)xO3 and suggests that the strong orthorhombic distortion of the unit cell due to the small Tb3+ cation size favors the stability of the distorted MnO6 octahedron in diluted systems. Long-range magnetic ordering is not found for x≥0.3 samples and there is not sign of ferromagnetism for x=0.5 in opposite to the behavior observed in the La-based compounds. Since Sc substitution induces a minor perturbation on the local structure of the Mn sublattice but magnetic ordering of both Mn and Tb sublattices disappear for a small degree of substitution, we conclude that the involved magnetic interactions should have a strong directional anisotropy and the Tb magnetic ordering strongly depends on the existence of a long-range coherent Mn ordering.
In order to realize the effect of Mn doping on dielectric behavior and optical band gap (Eg) of Lanthanum Gallate; pure and Mn doped LaGaO3 samples were prepared. The phase purity of these samples was confirmed by x-ray diffraction experiments. It was found that all samples possess orthorhombic structure. Room temperature (RT) dielectric response and Eg of LaGa1−xMnxO3 (LGMO) with, x = 0.0 to 0.9, has been recorded using impedance analyzer (LCR meter) and diffuse reflectance spectroscopy respectively. It has been observed that at RT, pure LaGaO3 exhibits moderate dielectric constant (ε′ ≈ 25) with very low dielectric loss (tan δ). Interestingly, the value of RT ε′ significantly increases with Mn doping whereas tan δ remains relatively low upto x = 0.4 and increases abruptly x > 0.4. The observed increase in ε′ and tan δ with Mn doping has been understood in terms of structural coherency and systematic decrease in Eg. A shift in ε′-anomaly (tan δ-peak) with increasing Mn percentage and low frequency large ε′ were also observed in Mn doped LGO. Both of these effects have been understood in terms of coexisting Mn³⁺ and Mn⁴⁺ ions. The LGMO samples (x ≤ 0.4) showing high value of ε′ and very low tan δ can be suitable candidates for capacitive applications.
We report an observation of room-temperature magneto-dielectric (RTMD) effect in LaGa0.7Fe0.3O3+γ compound. The contribution of intrinsic/resistive sources in the presently observed RTMD effect was analyzed by measuring direct-current (dc) magnetoresistance (MR) in four-probe geometry and frequency-dependent MR via impedance spectroscopy (MRIS). Present MRIS analysis reveals that at frequencies corresponding to grain contribution (≥1 × 10(6) Hz for present sample), the observed MD phenomenon is MR-free/intrinsic, whereas at lower probing frequencies (<1 × 10(6) Hz), the observed MD coupling appears to be MR-dominated possibly due to oxygen excess, that is, due to coexistence of Fe(3+) and Fe(4+). The magnetostriction is anticipated as a mechanism responsible for MR-free/intrinsic MD coupling, whereas the MR-dominated part is attributed to hopping charge transport along with Maxwell-Wagner and space charge polarization. The multivalence of Fe ions in LaGa0.7Fe0.3O3+γ was validated through iodometric titration and Fe K-edge X-ray absorption near-edge structure measurements. The excess of oxygen, that is, coexistence of Fe(3+) and Fe(4+), was understood in terms of stability of Fe(4+) by means of "bond-valence-sum" analysis and density functional theory-based first-principles calculations. The cation vacancies at La/Ga site (or at La and Ga both) were proposed as the possible origin of excess oxygen in presently studied compound. Present investigation suggests that, to justify the intrinsic/resistive origin of MD phenomenon, frequency-dependent MR measurements are more useful than measuring only dc MR or comparing the trends of magnetic-field-dependent change in dielectric constant and tan δ. Presently studied Fe-doped LaGaO3 can be a candidate for RTMD applications.
Polycrystalline samples of Mn doped LaGa1-xMnxO3 (LGMO) with 0≤x≤0.2 have been prepared via solid state reaction method. The structural phase purity for all these samples is validated through powder x-ray diffraction experiments carried out at BL-12 on Indus-2 synchrotron radiation source. Room temperature (RT) dielectric measurements have been performed in the absence and presence of magnetic field. A noticeable magneto dielectric (MD) effect i.e. change in the value of dielectric constant due to the application of a low magnetic field is observed in LGMO sample with x=0.2 (LG8M2O). In order to separate the intrinsic and resistive contributions present in observed RT MD effect, magneto resistance impedance spectroscopy (MRIS) has been performed at RT. Present MRIS analysis suggests that at frequencies corresponding to grain contribution (≥105 Hz for present samples), the observed MD phenomenon appears to be an intrinsic property of presently studied samples whereas at lower probing frequencies (<105 Hz) the observed change appears to be MR (considering frequency dependent resistance) dominated possibly due the coexistence of Mn3+ and Mn4+. The coexistence of Mn3+ and Mn4+ is revealed through XANES (Mn K-edge) spectroscopy. Moreover, RT and low temperature magnetization – magnetic field (M-H) along with M-T measurements in FC and ZFC mode have been performed to investigate the state of magnetic orders. The appearance of a narrow M-H loop is indicating presence of some magnetic ordering at RT. Further, ferromagnetic (FM) transition, observed around 36 K and a normal M-H loop with saturated magnetization, recorded at 5K; ensure FM ordering at low temperatures whereas bifurcation in FC-ZFC curves indicates competing FM and antiferromagnetic (AFM) interactions at low temperatures.
This chapter presents a survey of the application of hard x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES) and x-ray scattering (RXS) techniques in the study of the complex electronic phenomena arising in mixed valence 3d transition metal oxides. It then provides an introduction to the relevant structural and electronic data that can be obtained from each of the three related spectroscopic techniques. XAS provides sensitivity to the specific valence state of a given atomic species, while the x-ray diffraction (XRD) part provides the information about the long-range ordering of these electronic states. The chapter also presents the application of these techniques to resolve important questions of archetypical mixed valence oxides such as high Tc superconductors and magnetoresistive manganites. In manganites, the interaction between electrons and lattice vibrations (phonons) is unusually strong and can be tuned by variation of chemical composition, temperature and magnetic field.
We investigated the relative importance of removing the Mn3+ Jahn-Teller distortion in driving ferromagnetism in LaMn1-xScxO3 combining x-ray powder diffraction and x-ray absorption spectroscopy at the Mn and Sc K-edges. By increasing the Sc content, the orthorhombic distortion of the Pbnm cell in LaMnO3 decreases but the unit-cell remains slightly distorted in LaScO3. Besides, the nearly tetragonal-distorted MO6 in LaMnO3 continuously evolves into a nearly regular one in LaScO3. On the other hand, x-ray absorption spectra show that the MnO6 octahedron remains Jahn-Teller distorted and the ScO6 octahedron is nearly regular along the whole series. Moreover, the ordering of the Mn3+ Jahn-Teller distortion is not disrupted in the ab plane for any Sc concentration. This contrasts with the Ga- substituted compounds, where a regular MnO6 is found for x>0.5. However, both LaMn0.5Sc0.5O3 and LaMn0.5Ga0.5O3 show ferromagnetic behavior independently of the presence (or not) of Jahn-Teller distorted Mn3+. Thus, our results point to the Mn-sublattice dilution as the main effect in driving ferromagnetism in these manganites over local structure effects previously proposed by the spin flipping or the vibronic superexchange models.
