Ab initio molecular dynamics simulations of oxide-ion disorder in the δ-Bi2O3

We study in detail the structural, dynamical, and electronic properties of the beta and delta phases of Bi2O3 using Born-Oppenheimer molecular dynamics together with extensive lattice static simulations at the level of gradient-corrected density-functional theory. The short Bi-O bonds of similar to 2.1-2.2 A degrees and the broad peak in the O-Bi-O angular distribution function at similar to 70 degrees of delta-Bi2O3 are in good agreement with those from neutron diffraction, locally resembling the distorted structures of many fully ordered oxides of bismuth under ambient conditions. This places some doubt on structural models where the oxygen vacancies are either distributed at random over a set of ideal anion lattice sites or preferentially aligned in pairs at these positions. The irregular local structure of delta-Bi2O3 is intimately connected to the pronounced electron density around bismuth, providing evidence for the presence of a sterochemically active lone pair. The dominant influence of the anion 2p orbital on this asymmetry mirrors recent findings for many ordered post-transition-metal oxides and contrasts that of the conventional ns(2) lone-pair model. The markedly curved diffusion trajectories and an unusually high occurrence of short residence times show that the oxygen diffusion is strongly influenced by the local distortions of the immobile lattice. In contrast, we find little evidence for collective diffusion of oxygens.