Low-Energy Dynamics in Superionic Argyrodite Single Crystals

We aim at studying the ion diffusion and lattice dynamics of Cu7PS6 argyrodite by single crystal samples. The key motivations of the proposed research are as follows: (1) Cu argyrodite Cu7PS6 exhibit very large ionic conductivities, while the key atomistic mechanism of fast ion diffusion is poorly understood. (2) Our group recently points out lattice dynamics plays an important role in the fast diffusion process by combining INS and QENS measurements. Yet, our previous experiments are limited to powder samples, preventing us from directly assessing our simulations of phonon damping and coherent QENS associated with correlated ionic motions. Recently, single crystal of Cu7PS6 is successfully grown by a Bridgman method. Here, we propose to measure the temperature-dependent S(Q,E) on Cu7PS6 single crystal, focusing on low-E phonons and quasi-elastic response (Cu ion diffusion). We plan to probe the strong Q-dependence that modulates the phonon overdamping and the QENS signal to probe the concerted collective hopping behaviors. The proposed measurements will be analyzed with our ab-initio simulations and will be used to benchmark machine-learned neural-net force-fields for accelerated MD simulations. This project will advance our understanding of the origin of superionicity by providing real-space insights into the Q-dependent coherent QENS and selective phonon damping, and also identify the key atomistic processes enabling fast ion diffusion.