Incoherent Bi off-centering in Bi2Ti2O7 and Bi2Ru2O7: Insulator versus   metal
  The cubic, stoichiometric oxide compounds Bi2Ti2O6O' (also written Bi2Ti2O7) and Bi2Ru2O6O' (also written Bi2Ru2O7) have in common lone-pair active Bi3+ cations on the pyrochlore A-site with a propensity to off-center. Unlike Bi2Ti2O6O', 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 Bi2Ti2O6O' is incoherent as revealed in detail by reverse Monte Carlo analysis of total neutron scattering. Similar analysis of Bi2Ru2O6O' reveals incoherent off-centering as well, of smaller magnitude and with distinctly different orientational preference. Analysis of total scattering presented here shows that Bi in both compounds is entirely Bi3+. Disorder in Bi2Ti2O6O' has the effect of stabilizing valence while simultaneously providing space for the electronic lone pair. In Bi2Ru2O6O', off-centering is not required to satisfy valence, but is driven by the lone pair which is partially screened by conduction electrons. 
Daniel P. Shoemaker, Ram Seshadri, Makoto Tachibana, Andrew L. Hector
cond-mat.mtrl-sci