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.