Incoherent Bi off-centering in Bi₂Ti₂O ₆O^′ and Bi₂Ru₂O ₆O^′: Insulator versus metal

In the cubic, stoichiometric oxide compounds Bi₂Ti ₂O₆O^′ (also written as Bi ₂Ti₂O₇) and Bi₂Ru₂O ₆O^′ (also written as Bi₂Ru ₂O₇) Bi3⁺ ions on the pyrochlore A site display a propensity to off-center. Unlike Bi₂Ti₂O ₆O^′, Bi₂Ru₂O₆O ^′ is a metal, so it is of interest to ask whether conduction electrons and/or involvement of Bi 6s states at the Fermi energy influence Bi3⁺ displacements. The Bi3⁺ off-centering in Bi ₂Ti₂O₆O^′ has previously been revealed to be incoherent from detailed reverse Monte Carlo analysis of total neutron scattering. Similar analysis of Bi₂Ru₂O ₆O^′ reveals incoherent off-centering as well, but of smaller magnitude and with distinctly different orientational preference. Analysis of the distributions of metal to oxygen distances presented suggests that Bi in both compounds is entirely Bi3⁺. Disorder in Bi ₂Ti₂O₆O^′ has the effect of stabilizing valence while simultaneously satisfying the steric constraint imposed by the presence of the lone pair of electrons. In Bi₂Ru ₂O₆O^′, off-centering is not required to satisfy valence and seems to be driven by the lone pair. Decreased volume of the lone pair may be a result of partial screening by conduction electrons.