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Cu2Se and Cu2S are excellent model systems of superionic conductors withlarge diffusion coefficients that have been reported to exhibit differentsolid-liquid-like Cu-ion diffusion. In this paper, we clarify the atomicdynamics of these compounds with temperature-dependent ab-initio moleculardynamics (AIMD) simulations and inelastic neutron scattering (INS) experiments.Using the dynamical structure factor and Van-Hove correlation function, weinterrogate the jump-time, hopping length distribution and associated diffusioncoefficients. In cubic-Cu2Se at 500 K, we find solid-like diffusion withCu-jump lengths matching well the first-neighbour Cu-Cu distance of ~3 {\AA} inthe crystal, and clearly defined optic phonons involving Cu-vibrations. Above700 K, the jump-length distribution becomes a broad maximum cantered around 4{\AA}, spanning the first and second neighbour lattice distances, and aconcurrent broadening of the Cu-phonon density of states. Further, above 900 K,the Cu-diffusion becomes close to liquid-like, with distributions of Cu-atomscontinuously connecting crystal sites, while the vibrational modes involving Cumotions are highly damped, though still not fully over-damped as in a liquid.At low temperatures, the solid-like diffusion is consistent with previous X-raydiffraction and quasielastic neutron scattering experiments, while thehigher-temperature observation of the liquid-like diffusion is in agreementwith previous AIMD simulations. We also report AIMD simulations in Cu2S in thehexagonal and cubic superionic phases, and observe similar solid andliquid-like diffusion at low- and high-temperatures, respectively. Thecalculated ionic-conductivity is in fair agreement with reported experimentalvalues.

Solid-like to Liquid-like Behavior of Cu Diffusion in Superionic Cu2X  (X=S, Se): An Inelastic Neutron Scattering and Ab-Initio Molecular Dynamics  Investigation