An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi₂Sn₂O₇

Abstract

The exploitable properties of many materials are intimately linked to symmetry-lowering structural phase transitions. We present an automated and exhaustive symmetry-mode method for systematically exploring and solving such structures which will be widely applicable to a range of functional materials. We exemplify the method with an investigation of the Bi2Sn2O7 pyrochlore, which has been shown to undergo transitions from a parent γ cubic phase to β and α structures on cooling. The results include the first reliable structural model for β-Bi2Sn2O7 (orthorhombic Aba2, a = 7.571833(8), b = 21.41262(2), and c = 15.132459(14) Å) and a much simpler description of α-Bi2Sn2O7 (monoclinic Cc, a = 13.15493(6), b = 7.54118(4), and c = 15.07672(7) Å, β = 125.0120(3)°) than has been presented previously. We use the symmetry-mode basis to describe the phase transition in terms of coupled rotations of the Bi2O' anti-cristobalite framework, which allow Bi atoms to adopt low-symmetry coordination environments favored by lone-pair cations.

Structural Disorder in (Bi, M)2(Fe, Mn, Bi)2O6+x (M = Na or K) Pyrochlores Seen from Reverse Monte Carlo Analysis of Neutron Total Scattering

“…[42][43][44][45][46][47][48] More recently, a symmetry-based approach showed how a monoclinic cell was required to model correctly the much-debated α-structure of pyrochlore Bi 2 Sn 2 O 7 . 32 However, for our materials we find no evidence in the diffraction data for such superstructures (extra weak Bragg peaks in the data are due only to a small level of α-Bi 2 O 3 impurity, which has negligible contribution to the total PDF, Supporting…”

The impact of anion ordering on octahedra distortion and phase transitions in SrTaO2N and BaTaO2N

“…Describing complex structures in terms of distortion modes has a significant advantage in that, under some circumstances, it can reduce the number of structural parameters and this can increase the reliability of the refined structural model (an application of Occam's razor to crystallographic analysis). It has previously been demonstrated that the structure analysis based on the symmetry-mode refinements is an effective and powerful method, and has been successfully used in the analysis of complex oxides including WO 3 and Bi 2 Sn 2 O 7 (Kerman et al, 2012;Lewis et al, 2016), although this approach does not appear to have been used in systems with mixed anions. Thus, a structure analysis of SrTaO 2 N and BaTaO 2 N based on the distortion refinement is helpful for more reliable structure determination of them.…”

Generation of Basis Vectors for Magnetic Structures and Displacement Modes

“…Their direct application in the refinement of magnetic structures was originally developed in SARAh [1] and is also available in Fullprof [2] and for commensurate systems in TOPAS [3]. More recent interest has focused upon developing the application of this technique to displacive phase transitions [4][5][6][7][8][9].…”