Study of crystal structure and phase transition studies in perovskite-type oxides using powder-diffraction techniques and symmetry-mode analysis.

251 p.In this work, six families of Perovskite materials have been studied: Na0.5K0.5NbO3, La2MMnO6, Sr2MSbO6 (M=Ln, Y, In), Ca2MSbO6 (M = Ln, Y, In) Sr2M1-xM¿xTeO6 (M, M¿= Co, Mn, Ni, Fe, Mg), and NaLnMM¿O6 (Ln = La, Nd, Pr; M=Co, Mn, Mg; M¿= W, Te). The study was focused on the Synthesis, on the Crystal Structure Analysis, at room temperature and to the search of the phase transitions at low- and/or high-temperatures. The materials have been analyzed using X-ray, Synchrotron and Neutron Powder Diffraction, by the Rietveld method. The presence of light elements, the fact that the most accessible structural determination technique, X-rays, does not discriminate between some elements, the not-so-easy-access to best suited high-resolution techniques, has lead us, to think on a more efficient workflow for the refinements. The Bilbao Crystallographic Server, with AMPLIMODES, and related Utilities, has facilitated the path to elaborate the proposed structural analysis workflow, less expensive, more autonomous and independent and less time-consuming. It is based on a special parametrization in the refinements of some degrees of freedom. It has two levels of parametrization a) preferable directions in the irreps spanned space are present and b) preferable directions in the multidimensional spaces associated with some irreps are present. We have complemented the study with the aid of energy calculations. The structures are relaxed, to find where the minimum sits in the space spanned by the irreps. One of the novelties of the analysis is that we seek the minima taking into account explicitly the components of the X5+ irrep transforming distortion, in the P21/n space group, and not its global effect. Outstanding results of this systematic minimization are: in all the materials analyzed the third component of the X5+ transforming distortion is zero and the set of mode-amplitudes that "experimentally" nullify, and that we do not include in the final refinements, show zero values in the final minima. The experimental structure coincides with the relaxed one obtained theoretically. The application of the proposed refinement-process workflow has proven to be extendable and coherent, predicting and efficient. The medium- and low-resolution data, X-ray powder diffraction data, refined using this workflow can be used to obtain a trustable indirect calculation of a physical property. The comparison amongst the structures of related materials resulting from this workflow is more accurate and more confident

Crystallographic at non-ambient conditions and physical properties of the synthesized double-perovskites Sr2(Co1-xFex)TeO6

Polycrystalline double perovskite-type Sr2(Co1-xFex)TeO6 with various stoichiometric compositions (x = 0, 0.25, 0.5, 0.75, and 1) were synthesized by solid-state reactions in air. The crystal structures and phase transitions of this series at different temperature intervals were determined by X-ray powder diffraction, and from the obtained data the crystal structures were refined. It has been proven that for the compositions x = 0.25, 0.50, and 0.75 the phases crystallize at room temperature in the monoclinic space group I2/m. Down to 100 K, depending on the composition, these structures experience a phase transition from I2/m to P21/n. At high temperatures up to 1100 K their crystal structures show two further phase transitions. The first one is a first-order phase transition, from monoclinic I2/m to tetragonal I4/m, followed by a second-order phase transition to cubic Fm3 @#x0305;m. Therefore, the phase transition sequence of this series detected in a temperature range from 100 K to 1100 K is: P21/n → I2/m → I4/m → Fm3 @#x0305;m. The temperature-dependent vibrational features of the octahedral sites were investigated by Raman spectroscopy, which furthermore complements the XRD results. A decrease of the phase-transition temperature with increasing iron content has been observed for these compounds. This fact is explained by the progressive diminishing of the distortion of the double-perovskite structure in this series. By means of room-temperature Mössbauer spectroscopy, the presence of two iron sites is confirmed. The two different transition metal cations Co and Fe on the B sites give the opportunity to explore their effect on the optical band-gap

Structural and magnetic properties of frustrated GaxMn(3-x)O4(1.2 ≤ x ≤ 1.6) spinels

International audienceWe report a systematic study of the structural and magnetic properties of frustrated compounds of GaxMn(3−x)O4 (1.2 ≤ x ≤ 1.6) prepared by solid-state reaction. Using Rietveld refinement of X-ray diffraction patterns and O'Neill-Navrotsky model, we demonstrate that the system GaxMn(3−x)O4 (1.2 ≤ x ≤ 1.6) is an inverse spinel with low inversion parameter, in which Ga3+ replaces Mn3+ cations located in B-sites. The inverse magnetic susceptibility, the shape of ZFC/FC magnetization curves at low temperatures, the existence of hysteresis in all compounds, the frustration parameter and the spontaneous magnetization analysis show that the compounds with x = 1.2–1.4 exhibit a non-collinear ferrimagnetic order and the compounds with x = 1.5–1.6 exhibit a frustrated non-collinear ferrimagnetic order. Spin wave stiffness parameters were determined for each composition using the fitting results of spontaneous magnetization curves. It is demonstrated that for the compounds x = 1.2–1.4 with a non-frustrated ferrimagnetic order, the change of spontaneous magnetization Ms(T) obeys to Bloch's law (T3/2). For x = 1.5–1.6, the compounds exhibit a frustrated ferrimagnetic order, and the Ms(T) shows a deviation from Bloch's law

Effects of iron substitution and anti-site disorder on crystal structures, vibrational, optical and magnetic properties of double perovskites Sr2(Fe1-xNix)TeO6

The double-perovskite series, Sr2(Fe1−xNix)TeO6 (x = 0, 0.25, 0.50, 0.75, and 1) has been synthesized in polycrystalline form by solid-state reaction at 1300 K in air. Their crystal structures were probed by powder X-ray diffraction at room temperature. Rietveld analysis revealed that all samples crystallize in the monoclinic space group I2/m. The double-perovskite structures ideally contain two alternating types of octahedra (Fe/Ni)2dO6 and (Te)2aO6, tilted in the system (a−a−c0). However, the refinements have shown a complex distribution of all three cations over the two available octahedral sites; 2d (½, ½, 0) and 2a (0, 0, 0). Raman spectroscopy further complements the obtained results, by revealing a tiny increase of the wavenumber of some Raman modes when Fe is substituted by Ni. The optical characteristics of the series were determined by fitting diffuse reflectance UV/Vis spectra enabling the optical band gaps to be derived from Tauc method and derivation of absorption spectra fitting (DASF) techniques. Analyses of the obtained 57Fe Mössbauer hyperfine parameters at room temperature of samples with compositions x = 0, 0.25, 0.50 and 0.75 reveal the presence of Fe3+ in high-spin state with an anti-site disorder of Fe–Ni–Te cations in distorted octahedral environments (site 2d and 2a). The results show that significant correlations exist between the crystal structures and physical properties of double perovskites containing B site transition elements of different charge and size. Temperature-dependent magnetic susceptibility data show magnetic transitions below 40(1) K (38(1) K, 31(1) K, 25(1) K, 20(1) K, and 35(1) K for x = 0, 0.25, 0.50, 0.75, and 1, respectively. A divergence between FC and ZFC curves for all compositions has been observed. The results show that the ground states of the doped materials might be spin glasses or magnetically ordered

Comparative Study of Sb2O3 (Sb2O5) and Ta2O5 Doping Effects with TeO2 on Electrical Properties of delta-Bi2O3

23rd Conference on Applied Crystallography, Krynica Zdroj, POLAND, SEP 20-24, 2015International audienceIn this study, Sb2O3 (Sb2O5) and Ta2O5 are used as co-dopants with TeO2 to stabilize the delta phase of bismuth oxide (delta-Bi2O3). Some compositions with formula (1 x) BiO1.5-(x /4) Sb2Te2O9 and (1 - x) BiO1.5-(x/4) Ta2Te2O9 (x = 0.1, 0.2, 0.3, 0.6, and 0.9) have been synthesized by solid state reaction at 850 degrees C and characterized by powder X-ray diffraction. The Bi0.9Sb0.05Te0.05O1.575, Bi0.9Ta0.05Te0.05O1.575 and Bi0.8Ta0.1Te0.1O1.65 retain a cubic fluorite structure of delta-Bi2O3 phase. The electric properties were studied by impedance spectroscopy. All samples were evaluated by calculating conductivities and activation energies. Various impedance model including constant phase element and the Warburg impedances have been used to interpret the Nyquist representations of electrical analyses

Sodium intercalation/de-intercalation mechanism in Na4MnV(PO4)3 cathode materials

Na4MnV(PO4)3 is a sodium ion conducting material with a NASICON type crystal structure. This phase is not much known as an electrode material. The present work focuses on the sodium ion intercalation/de-intercalation mechanism and charge/discharge behavior of the material. The Na4MnV(PO4)3 is synthesized through a sol-gel process and characterized by XRD, SEM, and XPS. The structural analysis confirms the formation of a phase pure crystalline material with nanometric particle size which adopts a trigonal crystal structure. Galvanostatic intermittent titration technique (GITT) measurements indicate that Na4MnV(PO4)3 is electrochemically active having slanting voltage plateaus. Ex-situ and In-situ XRD analysis, as a function of sodium concentration, indicate that the intercalation/de-intercalation of sodium is associated with a single-phase reaction rather than a biphasic reaction when cycled between 1.5 and 4.5 V. The electrochemical measurements on composite electrodes, Na4MnV(PO4)3/CNTS (1 & 3 wt.%), show promising charge/discharge capacity (?140 mAh/g), good cyclability (100% capacity retention after 40 cycles) and reasonable rate capability. The cyclic voltammetry (CV) and X-ray Photoelectron Spectroscopy (XPS) analyses indicate that the main contributions towards the activity of Na4MnV(PO4)3 can be attributed to the active of Mn2+/Mn3+ and V3+/V4+ redox couple with partial activity of V4+/V5+. The obtained results suggest that Na4MnV(PO4)3 is a promising electrode material which can be achieved better rate performance with long cycling stability and battery performance through engineering of the particle morphology and microstructure. � 2018 Elsevier LtdThe authors acknowledge the financial support from the Center for Advanced Materials (CAM), Qatar University, Doha, Qatar. The authors would also like to thank Mar�a J�uregui and Damien Saurel from XRD platform at CIC Energigune for her help for the in situ-XRD measurements. Appendix AScopu

Dual Substitution Strategy to Enhance Li+ Ionic Conductivity in Li7La3Zr2O12 Solid Electrolyte

Solid state electrolytes could address the current safety concerns of lithium-ion batteries as well as provide higher electrochemical stability and energy density. Among solid electrolyte contenders, garnet-structured Li7La3Zr2O12 appears as a particularly promising material owing to its wide electrochemical stability window; however, its ionic conductivity remains an order of magnitude below that of ubiquitous liquid electrolytes. Here, we present an innovative dual substitution strategy developed to enhance Li-ion mobility in garnet-structured solid electrolytes. A first dopant cation, Ga3+, is introduced on the Li sites to stabilize the fast-conducting cubic phase. Simultaneously, a second cation, Sc3+, is used to partially populate the Zr sites, which consequently increases the concentration of Li ions by charge compensation. This aliovalent dual substitution strategy allows fine-tuning of the number of charge carriers in the cubic Li7La3Zr2O12 according to the resulting stoichiometry, Li7–3x+yGaxLa3Zr2–yScyO12. The coexistence of Ga and Sc cations in the garnet structure is confirmed by a set of simulation and experimental techniques: DFT calculations, XRD, ICP, SEM, STEM, EDS, solid state NMR, and EIS. This thorough characterization highlights a particular cationic distribution in Li6.65Ga0.15La3Zr1.90Sc0.10O12, with preferential Ga3+ occupation of tetrahedral Li24d sites over the distorted octahedral Li96h sites. 7Li NMR reveals a heterogeneous distribution of Li charge carriers with distinct mobilities. This unique Li local structure has a beneficial effect on the transport properties of the garnet, enhancing the ionic conductivity and lowering the activation energy, with values of 1.8 × 10–3 S cm–1 at 300 K and 0.29 eV in the temperature range of 180 to 340 K, respectively