The mechanism of oxide ion conductivity in bismuth rhenium oxide, Bi28Re2O49

We have carried out a combined experimental and computational study of oxide ion conductor Bi28Re2O49, with the aim of elucidating the conductivity mechanisms and pathways in this material. Single crystals of Bi28Re2O49 were grown from melt and the structure was investigated for the first time by single crystal X-ray diffraction. The structural model obtained is consistent with the Re atoms in Bi28Re2O49 being both four- and six-coordinate, in a 3:1 ratio, in agreement with previous EXAFS and IR spectroscopy studies. The thermal displacement parameters of the oxygen atoms bonded to Re suggest substantial disorder of the Re coordination polyhedra. Ab-initio molecular dynamics simulations were performed to probe the oxide ion migration pathways in Bi28Re2O49 and the roles of the BiO and ReO sublattices. The key conclusion is that the ability of Re to support variable coordination environments is vitally important in Bi28Re2O49; it provides a mechanism for ‘self-doping’ of the structure, i.e. the creation of O2 − vacancies in the fluorite-like BiO sublattice by exchange of O atoms with the ReO sublattice, and the subsequent increase of the average coordination number of Re. All three crystallographically unique oxygen sites in the BiO sublattice play roles in the ionic migration processes, by facilitating the O2 − exchange between the ReOx groups and by contributing to the O2 − diffusion via the vacancy-hopping mechanisms.