Tuning of the ionic conductivity of Ba₇Nb₄MoO₂₀ by pressure: a neutron diffraction and atomistic modelling study

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 MO_x (x = 3, 4, 5) units within the palmierite-like layer. The high proton conductivity arises from the dynamic and rotational flexibility of the MO_x units that results in disorder 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.