Perovskite Synthesis and Analysis Using Structure Prediction Diagnostic Software

A software program, SPuDS (Structure Prediction Diagnostic Software) was developed to predict the crystal structures of perovskites, including those distorted by tilting of symmetric and Jahn-Teller distorted octahedra. Only the composition and oxidation states of the ions were required as inputs. Rigid octahedra were maintained in electronically symmetric octahedra, while distortions of the octahedral bond lengths were utilized in Jahn-Teller distorted compositions. SPuDS calculates the optimal structure in Glazer tilt systems most often observed experimentally and for tilt systems with interesting multiple A-cation environments. Structure optimization occurs by distorting the structure to minimize the global instability index. Location of the A-site cation is chosen to maximize the symmetry of its coordination environment.

SPuDS has been employed in a number of useful applications, including use of predicted structural information to estimate physical properties of both hypothetical compositions and those materials for which accurate structural data is unavailable, as a guide for exploratory synthetic efforts, as a starting model for Rietveld refinements in the course of structurally characterizing materials, and for extracting the effects of octahedral tilting distortions from other structural distortion mechanisms. Structural predictions were made for a variety of compositions with single and multiple octahedral cations and compared with a previously determined structures to illustrate the accuracy of this approach.

Synthesis and structural refinement from x-ray and neutron powder diffraction of Ca₂MnMO₆ (M = Nb, Sb, Ru) and Sr₂MnMO₆ (M=Nb, Sb) is reported. Various types of cooperative Jahn-Teller distortions (CJTD) that occur in ternary (AMX₃) and double (A₂MM′O₆) perovskites are reviewed. Interplay between CJTD’s, octahedral tilting and cation order is systematically examined in A₂MM′O₆ perovskites, where M is an active Jahn-Teller (J-T) ion (Mn³⁺ or Cu²⁺) and M′ is an octahedrally symmetric cation.

Perovskites with a⁺a⁺a⁺ octahedral tilting and a single octahedral cation exhibit interesting dielectric properties. SPuDS was also used to examine the prospects for synthesizing new compounds with multiple A-cation coordination geometries. Perovskites with octahedral tilting (a⁺a⁺a⁺) coupled with A-cation and rock-salt octahedral cation ordering were predicted to exist. High-pressure high-temperature synthesis, characterization of the structures and dielectric properties of CaCu₃Ga₂Sb₂O₁₂, CaCu₃Cr₂Sb₂O₁₂, CaCu₃Ga₂Nb₂O₁₂ and CaCu₃Ga₂Ta₂O₁₂ is reported.