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.
[1] R S Roth, J. Res. Natl. Bur. Std 56, 17-25, 1956.
[2] R D Shannon, J D Bierlein, G A Jones, A W Sleight, J. Phys. Chem. Solids 41, 117-122, 1980.
[3] R J Jones, K S Knight, J. Chem. Soc. Dalton, 1997, 2551-2555
[4] B J Kennedy, Ismunander, M M Elcombe, Mater. Sci. Forum 278-281, 762-767, 1998.
[5] I R Evans, J A K Howard, J S O Evans. J. Mater. Chem. 13, 2098-2103, 2003.