Incoherent Bi off-centering in Bi2Ti2O7 and Bi2Ru2O7: Insulator versus metal

TL;DR

This study compares incoherent Bi3+ off-centering in Bi2Ti2O7 and Bi2Ru2O7, revealing how electronic properties influence lone-pair activity and structural disorder in these pyrochlore oxides.

Contribution

It provides detailed analysis of Bi3+ off-centering in both compounds using neutron scattering and shows how conduction electrons affect lone-pair behavior.

Findings

Bi3+ off-centering is incoherent in both compounds.

Off-centering magnitude differs between the two materials.

Electronic conduction influences lone-pair activity and structural disorder.

Abstract

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.