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

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