  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.
