A theoretical and experimental study of the distorted pyrochlore Bi ₂Sn₂O₇

The pyrochlore based bismuth stannate, Bi₂Sn₂O ₇, is a material with important applications in catalysis and gas sensing. The thermodynamically stable α phase has 352 atoms in the unit cell and is one of the most complex oxide crystal structures to have been solved by powder diffraction. We have performed a full atomic relaxation and calculated the electronic structure, using gradient corrected density functional theory, with the resulting structure in very good agreement with the previous experimentally determined unit cell. The computed density of states is in excellent agreement with our valence band X-ray photoelectron spectra. The combined results shed new light on the bonding and lone pair activity in this material. A mixture of Bi 6s and O 2p states are found to dominate the top of the valence band while Sn 5s, O 2p and Bi 6p states dominate the bottom of the conduction band. The differing contributions of Sn 5s and Bi 6s states to the valence and conduction bands reflect both differences in atomic binding energies and differences in the strength of the metal s interactions with O 2p. The preference of this material for a distorted structure and its unique catalytic and gas sensing activity are discussed.