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

Abstract

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.