Activating Interstitial Oxygen Conduction in Sr3V2O8 Palmierite Oxides Through Minimal Mo-Substitution

The palmierite oxides are a promising family of ion conductors, and understanding how compositional modification affects their structure and, in turn, their ionic conductivity is pivotal for unlocking high ionic conductivity in these solid electrolytes. Here, we investigate the Sr3V2-xMoxO8+x/2 series (0 ≤ x ≤ 0.4), establishing a solubility limit at x = 0.2, and demonstrating that Mo-substituted compositions exhibit bulk O2– conductivity approximately 2 orders of magnitude higher than the pristine composition at 600 °C. Combined X-ray and neutron diffraction refinement shows little change in the average crystallographic structure, prompting questions about its local structure and dynamics. Using ab initio molecular dynamics, bond-valence site-energy calculations, and 51V and 17O solid-state nuclear magnetic resonance spectroscopy supported by density functional theory calculations, we demonstrate that Mo6+ incorporation generates a new oxygen environment consistent with the formation of a charge-compensating interstitial O3 site. While V5+ remains tetrahedrally coordinated throughout the series, Mo6+ exhibits both MoO4 and distorted trigonal bipyramidal MoO5 coordination polyhedra, with the latter arising from the occupation of O3. This O3 site is proposed to bridge neighboring MOn polyhedra along the ab-plane, shortening the O2– jump distances and shifting the conduction mechanism from a 3D cooperative rotational mechanism to a 2D interstitial-based one.