Oxide-ion disorder within the high temperature delta phase of Bi2O3

The delta phase of Bi(2)O(3), 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 Bi(3+) within delta-Bi(2)O(3) is shown to be highly irregular, resembling the asymmetric "lone-pair" coordination found within many (fully ordered) oxides of Bi(3+) under ambient conditions. The asymmetric electron density around the Bi(3+) plays a central role in promoting the extreme anion disorder within delta-Bi(2)O(3), 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 O(2-) vacancies are aligned in pairs in 100, 110, and 111 directions is discussed in light of these conclusions.