CHARGE AND NUCLEAR DENSITY DISTRIBUTION OF THE PYROCHLORE Y₂Sn₂O₇ BY MEM. C.J. Howard and D.J. Cookson, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia, B.J. Kennedy, School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia, T. Ikeda, M. Takata and M. Sakata, Department of Applied Physics, Nagoya University, Nagoya 464-01, Japan

A Maximum Entropy Method (MEM) has been used to obtain accurate charge and nuclear density distributions in the stoichiometric pyrochlore Y₂Sn₂O₇. The cubic pyrochlore structure with the general formula A₂B₂O₆O^ is formed by a wide variety of ions, and tolerates a high degree of non-stoichiometry on the O^ anion and the A cation sites. The crystal structure can be viewed as two networks, the rigid B₂O₆ framework of vertex sharing BO₆ trigonal anti-prisms and the A₂O^ chains. A great number of materials adopt the structure and these display a large range of physical properties. The aim in this study was to derive the charge and nuclear densities in the typical pyrochlore, Y₂Sn₂O₇, by MEM analysis from X-ray and neutron powder diffraction data, so as to reveal details of the bonding. The synchrotron X-ray data were recorded at the Australian National Beamline Facility at the Photon Factory, Japan, and the neutron data were collected using the high resolution powder diffractometer at ANSTO. The R-factors for the MEM charge and nuclear densities were 4.3% and 4.8%, respectively. The MEM charge density indicates weak covalency in the Y-O(1) bonding, and relatively strong Sn-O covalency. The charge density also shows a slight charge delocalisation between Y and O(2), this being the first experimental confirmation of the theoretically assumed weak electron transfer between the Y₂O and Sn₂O₆ sublattices which partially explains the predisposition of pyrochlores to non-stoichiometry. The MEM nuclear density shows the anisotropic deformation due to thermal motion of atoms, and confirms that the features observed in the charge density analysis are in fact due to electron delocalisation rather than atomic displacements.