Local Structure, Dynamics, and the Mechanisms of Oxide Ionic Conduction in Bi₂₆Mo₁₀O₆₉

We report the results of a computational and experimental study into the stabilized fluorite-type δ-Bi₂O₃-related phase Bi₂₆Mo₁₀O₆₉ aimed at clarifying the local and average structure, for which two distinct models have previously been proposed, and the oxide ionic diffusion mechanism, for which three distinct models have previously been proposed. Concerning the structure, we propose a new model in which some molybdenum atoms have higher coordination numbers than 4; that is, some MoO₅ trigonal bipyramids coexist with MoO₄ tetrahedra. This accounts for the additional oxygen required to achieve the nominal composition (a tetrahedron-only model gives Bi₂₆Mo₁₀O₆₈) without invoking a previously proposed unbonded interstitial site, which we found to be energetically unfavorable. All these MoO_<i>x</i> units are rotationally disordered above a first-order transition at 310 °C, corresponding to a first-order increase in conductivity. Concerning oxide ionic diffusion above that transition temperature, we found excellent agreement between the results of ab initio molecular dynamics simulations and quasielastic neutron scattering experiments. Our results indicate a mechanism related to that proposed by Holmes et al. (<i>Chem. Mater.</i> <b>2008</b>, <i>20</i>, 3638), with the role previously assigned to partially occupied interstitial oxygen sites played instead by transient but stable MoO₅ trigonal bipyramids and with more relaxed requirements in terms of the orientation and timing of the diffusive jumps