Crystal Structure of the Distorted Pyrochlore α-Bi2Sn2O7

Institute of Materials and Engineering Science, Australian Nuclear Science and Technology Organisation* Department of Chemistry, University of Durham, UK** ISIS Facility, Rutherford Appleton Laboratory, UK***
â—‹Christopher J Howard* Ivana R Evans** John S O Evans** Kevin S Knight***

Bismuth stannate, Bi2Sn2O7, was recognised as a distorted pyrochlore some fifty years ago[1], but a satisfactory crystal structure solution has hitherto proved elusive. At temperatures above about 700C it was thought to have the ideal pyrochlore structure, at lower temperatures a face-centred cubic structure on a doubled (21.4) unit cell, which at room temperature showed a tetragonal distortion to a large (15.08 by 21.5 ) body-centred tetragonal structure[2]. There were various reports in the literature on the higher temperature phases [3,4], but refinements of the room temperature structure in the preferred tetragonal space group I-4c2, were never satisfactory. Recently, direct space methods were applied to the problem [5]. The structure was solved on the accepted 15.08 by 21.5 tetragonal cell, but turned out to be monoclinic in Pc, with 176 atoms in the asymmetric unit. A problem evident in this solution, however, is the large number of space group allowed reflections that are not observed. The solution presented here is based on a successful indexing of the X-ray and neutron diffraction patterns on a smaller C-centred monoclinic cell, implying only 22 atoms in the asymmetric unit. The higher temperature phases need to be re-examined in the light of this work.

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