Oxide-Ion Disorder Within the High Temperature delta Phase of Bi2O3

The delta phase of Bi2O3, which adopts an anion-deficient fluorite structure, has the highest known oxide-ion conductivity. Using a combination of neutron powder diffraction and Born-Oppenheimer molecular dynamics, the preferred local anion environment around the Bi3+ within delta-Bi2O3 is shown to be highly irregular, resembling the asymmetric "lone-pair" coordination found within many (fully ordered) oxides of Bi3+ under ambient conditions. The asymmetric electron density around the Bi3+ plays a central role in promoting the extreme anion disorder within delta-Bi2O3, with the ion diffusion facilitated by extensive relaxations of both the surrounding anions and a "soft" cation sublattice. The validity of previously proposed structural models based on a cubic environment in which O2- vacancies are aligned in pairs in , , and directions is discussed in light of these conclusions