Anharmonicity and the emergence of diffusive behavior in a lattice-solute model solid-state electrolyte

We have investigated atomic transport properties in a simple two-component solid-state material using molecular dynamics simulations. This model system is composed of a lattice element and a solute. The former is modeled after carbon, which forms a covalent diamond lattice that provides the supporting structure for the solute, which interacts with carbon and with itself via Lennard-Jones potentials. The size of the solute species is systematically varied while maintaining a constant system volume. The change in solute particle size is quantified using an atomic packing fraction, which is used as a mediator variable to find relationships between the internal pressure of the system, its vibrational properties, and the change in diffusivity of the mobile species. The data imply a causal relationship between structural instabilities, reflected in the internal pressure and compressibility curves, the vibrational spectra, and the onset of a diffusive regime manifest in an increase in atomic mobility by orders of magnitude.