Structural and bonding trends in tin pyrochlore oxides

The structures of a number of pyrochlore stannates of the type Ln(2)Sn(2)O(7), with Ln = Y, La, Pr, Nd, Tb-Lu and Ln = Sm, Eu, and Gd, have been refined by Rietveld analysis from 1.4925 Angstrom neutron powder and 1.0 Angstrom X-ray powder diffraction data, respectively. The single variable oxygen positional parameter is observed to decrease systematically with the increasing lattice parameter, causing periodic shifts in both the Sn-O and Sn-O-Sn interactions. The Ln temperature factors are highly anisotropic, and this is explained in terms of the stereochemistry of the Ln atoms. Bond valence sum calculations of the materials have been performed to explore changes in the bonding resulting from the structural changes. (C) 1997 Academic Press.