Oxygen Dynamics in Transition Metal-Doped Bismuth Oxides

A detailed quasielastic neutron scattering (QENS) study provides direct insights into the mechanism of ionic conduction in the type II transition metal-doped bismuth oxides Bi22W5O48, Bi22Nb5O45.5, and Bi22W2.5Nb2.5O46.75. Quantitative analysis of the QENS broadening reveals an oscillating behavior similar to that observed recently for pure fluorite-type δ-Bi2O3, which can be adequately modeled by a coherent adaption of the Chudley-Elliott jump diffusion model. In conjunction with detailed ab initio molecular dynamics simulations, this shows that oxygen diffuses through these compounds almost exclusively within fluorite-type regions via an isotropic, liquid-like mechanism. The average oxygen jump length is slightly shorter than half the length of the fluorite-type subcell; this is attributed to the flexible coordination environments around Bi, in contrast to the more regular coordination environments around the transition-metal dopants, which localize vacancies and prevent the remaining oxygen atoms from contributing to the overall oxygen diffusivity.