Diffusion of protons and sodium ions in silicophosphate glasses: insight based on first-principles molecular dynamic simulations

Abstract

We propose a microscopic diffusion mechanism of protons and Na⁺ ions in phosphate glasses using first-principles molecular dynamic simulations. Protons hop and are chemisorbed onto non-bridging oxygen (NBO) of nearby PO₄ tetrahedra through hydrogen bonds. The subsequent behavior depends on the morphology of the PO₄ tetrahedra (Qⁿ_P values). When a proton is adsorbed onto the NBO of a Q³_P unit, it is desorbed on a short time scale of within 10 fs and re-adsorbed onto the NBO that was previously adsorbed. However, when a proton is adsorbed onto the NBO of a Q²_P unit, another proton coordinated before adsorption is desorbed in a chain, resulting in the diffusion of protons. When a Na⁺ ion is present in the vicinity, the adsorption of a proton onto a Q²_P unit leads to a decrease in the electrostatic interaction between Na⁺ and O⁻ ions and induces the diffusion of Na⁺ ions. We conclude that the difference in the morphology of PO₄ tetrahedra greatly affects the diffusion of protons and Na⁺ ions.