Mixed oxide ion–proton conductivity and the ionic migration mechanism in isolated tetrahedral LaVO4 by acceptor doping†

Solid-state oxide ion and proton conductors are garnering significant attention due to their high ionic conductivity and potential applications in a range of electrochemical devices, including solid oxide fuel cells and gas sensors. In this study, we report the influence of partial substitution of La³⁺ in isolated tetrahedral LaVO₄ ceramics with 0.01 mol of alkaline-earth metals Ca²⁺, Sr²⁺ and Ba²⁺ on the phase stability and electrical properties. It was found that acceptor doping effectively enhances mixed oxide ion and proton conductivities, with Sr²⁺ substitution yielding the highest conductivity, achieving ∼10⁻³ S cm⁻¹ at 900 °C under a wet O₂ atmosphere. DFT calculations and ab initio molecular dynamics simulations revealed that protons preferentially form hydrogen bonds with the lattice oxygen near the dopants and migrate through a continuous process of hopping and rotation between inter- and intra-tetrahedral VO₄ groups. Additionally, the existence of oxygen vacancies facilitates the formation of V₂O₇ dimers through sharing corners with adjacent isolated VO₄ tetrahedra, enabling ion exchange through a synergistic mechanism involving V₂O₇ dimer breaking and reforming. This research highlights the critical role of the deformation and rotational flexibility of isolated tetrahedral units in facilitating oxide ion and proton transport, underscoring the potential for developing mixed oxide ion and proton conductors in oxygen vacancy-deficient oxides with tetrahedral-based structures.