Tuning of the Ionic Conductivity of Ba7Nb4MoO20 by Pressure: A Neutron Diffraction and Atomistic Modelling Study

Abstract

Ba7Nb4MoO20 is a hexagonal perovskite derivative that exhibits both high oxide-ion and proton conductivity. The high oxide-ion conductivity results from the presence of disordered flexible MOx (x = 3, 4, 5) units within the palmierite-like layer. The high proton conductivity arises from the dynamic and rotational flexibility of the MOx units that results in delocalisation of the proton defects, high proton mobility and low energy diffusion pathways. Herein, using a combination of neutron diffraction experiments and atomistic modelling simulations, we demonstrate that both the crystal structure and ion transport of Ba7Nb4MoO20 can be tuned by applying pressure. Applying pressure results in a reduction in the fraction of MO4 tetrahedra within the P-L layer with a concomitant increase in the fraction of MO6 octahedra. Density functional theory and molecular dynamics simulations using a newly developed machine-learned forcefield reveal a significant decrease in oxide-ion transport with increasing pressure whilst proton transport is comparatively unaffected.