Interstitial oxide ion migration in scheelite-type electrolytes: a combined neutron diffraction and computational study

W-doped LnNbO(4+d) compounds (Ln = La, Nd) have been recently proposed as novel interstitial-based oxide ion conductors. In this study, a powerful combination of neutron diffraction, ab initio calculations and classical molecular dynamic simulations is used in a complementary way to investigate the high temperature behavior of this system, identify possible oxygen interstitial positions and investigate the mechanism of oxygen diffusion at a high temperature. The results indicate that both systems (LaNb0.84W0.16O4.08; NdNb0.84W0.16O4.08) undergo a phase transition from a monoclinic to a tetragonal phase as a function of temperature. In the scheelite phase more than one interstitial site is potentially accessible based on the calculated energy differences. The accessibility of such interstitial positions poses the basis for an efficient oxygen diffusion migration that is predicted to be roughly isotropic and based on a knock-on mechanism.