Profiling the off-center atomic displacements in CuCl at finite temperatures with a deep-learning potential

Abstract

Cuprous halides ($\mathrm{Cu}X$, $X=\mathrm{Cl}$, Br, or I) have been extensively investigated in the literature, but there still exist debates on whether the ground-state structures of $\mathrm{Cu}X$ are zinc blende. By performing molecular dynamics simulations at finite temperatures with a newly developed accurate deep-learning potential for CuCl, we find that, in the absence of accurate exchange interactions, there exist collective off-center displacements of Cu, leading to the formation of large complex Cu cluster structures. However, these cluster structures are unstable once exchange interactions are properly included. Nevertheless, due to strong anharmonicity, thermal fluctuations could also lead to sizable off-center Cu displacements, resulting in instantaneous small Cu clusters. Still, these cluster configurations are not stable or metastable in the ground state. We thus unambiguously demonstrate that, although anharmonic off-center displacements of Cu are present in CuCl at finite temperatures, zinc blende is still the thermodynamically most favorable phase. These insights are critical to the understanding of liquidlike anharmonic behavior of Cu atoms in various Cu-containing compounds, which impacts a number of important properties such as thermal conductivity, ferroelectricity, and superconductivity.