Structure of γ-Bi2Sn2O7 by high temperature powder neutron diffraction

Abstract

Abstract The compound Bi2Sn2O7 shows three polymorphic forms between room temperature and 1400 K. As a first step towards the structural characterization of this material in the present study the crystal structure of the high-symmetry γ-phase stable above 899 K has been investigated by high temperature neutron diffraction of samples prepared by solid state reactions. At 996 K this modification possesses the following parameters: cubic, space group Fd[unk]m, a = 10.7225(7) Å, V = 1232.8(2) Å3, Z = 8, D calc = 8.27 g cm−3. A Rietveld analysis was performed using the structural model of pyrochlore for this phase, where the Bi3+ and Sn4+ cations occupy the Wyckoff-positions 16(c) and 16(d), respectively. The oxygen atoms O1 and O2 are distributed to the positions 48(f) and 8(a). The refinement calculations confirm this approach. The octahedral SnO16-network shows the lowest degree of distortion within the group of A2Sn2O7 pyrochlores. It is suggested that the comparatively high isotropic displacement factor of the Bi3+ cation indicates that the phase transition in this compound occurring at 899 K is primarily due to displacements of the bismuth atoms.