College Park, Maryland June 6 - 10 , 2004
TP13: Understanding Octahedral Tilting Distortions in A2M3+M5+O6 Perovskites (M5+ = Ta, Nb, Sb)
P. W. Barnes (The Ohio State University; Argonne National Laboratory), P. M. Woodward (The Ohio State University), P. Karen (University of Oslo, Norway), M. W. Lufaso (The Ohio State University; National Institute of Standards and Technology)
The perovskite structure with 1:1 M-site cation ordering (i.e., double or complex perovskites; 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 complex perovskites are caused by electronic factors (e.g., Jahn-Teller ions), random or partial M-cation ordering, 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. Perovskites with Ba2+ 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 Ca2+ 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, twelve pseudo-symmetric double perovskites with A = Sr2+ (M3+ = Sc, Cr, Mn, Fe, Co, Ga, Y; M5+ = Nb, Ta, Sb), Ba2YNbO6, and Ca2CrTaO6 were examined by Rietveld refinement 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 for these compositions, the extent of M-site cation ordering, the degree of octahedral tilting, and a comparison of the driving forces behind these two distortions in compounds containing transition metal vs. main group metal cations will be discussed.
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