Exploring structural changes and distortions in quaternary perovskites and defect pyrochlores using powder diffraction techniques.

摘要

The perovskite structure with 1:1 M-site cation ordering (or double perovskite; A₂MM′X₆) is a well known and extensively studied structure type in solid state chemistry. The ideal double perovskite has cubic symmetry, but many are distorted from the ideal structure. Structural distortions seen in perovskites are caused by electronic factors ( i.e., Jahn-Teller ions), M-cation displacement from the center of the MX₆ octahedra (i.e., cations with a stereoactive lone pair of electrons), and most commonly, octahedral tilting. Tilting of the octahedra within the perovskite structure leads to lowering of its symmetry. A major factor that influences the degree of octahedral tilting in a given compound is the nature of the cuboctahedral A-cation.{09}Perovskites with Ba 2+ as the A-site cation typically exhibit cubic symmetry and those with Ca2+ have orthorhombic or monoclinic symmetry. Changes (or lack of) in space group symmetry for A = Ba2+ and Ca 2+ are often easily seen in the X-ray powder diffraction data (XRPD) of their respective compounds. Yet, compounds with A = Sr2+ are prone to subtle octahedral tilting distortions that are not readily seen in XRPD, so many are mistakenly assigned to the incorrect space group. In this study, nine double perovskites with A = Sr2+ (M 3+ = Al, Sc, Cr, Mn, Fe, Co, Ga, Y; M5+ = Nb, Ta, Sb), were examined using Rietveld refinements of XRPD and neutron powder diffraction data (NPD) in order to appropriately discern their respective crystallographic symmetry. The approach taken for determining appropriate possible space groups, the reliability of peak splitting seen in the XRPD data for determining space group symmetry, the extent of M-site cation ordering, and the degree of octahedral tilting seen in this family of compounds will be discussed. The principles used to determine the appropriate space group symmetry for the nine compounds examined by NPD was applied to the XRPD data of sixteen additional double perovskites.; The perovskite CaCu₃Ti₄O₁₂ has received significant attention recently due to its giant dielectric constant (ϵ = 1.0 × 104). Such high dielectric constants are seen in ferroelectric or relaxor materials, both of which show sharp maxima in ϵ as a function of temperature. This is not the case CaCu₃Ti ₄O₁₂, whose dielectric constant shows little temperature dependence between 100 and 600 K. Variable temperature studies of the crystal structure clearly showed that no distortion of the crystal structure occurs down to 35 K. This structural evidence has led to several possible mechanisms for the dielectric properties of this material. (Abstract shortened by UMI.)