Configurational Disorder, Strong Anharmonicity, and Coupled Host Dynamics Lead to Superionic Transport in Li₃YCl₆ (LYC)

Abstract

In superionic crystals, liquid‐like ionic diffusivities often come hand‐in‐hand with ultra‐low thermal conductivity and soft vibrational dynamics. However, generalized relationships between ion transport and vibrational dynamics remain elusive due to the diversity of superionic materials and complex underlying mechanisms. Here, we examine links between vibrational dynamics and ion transport in close‐packed lithium halide ion conductor Li3YCl6 (LYC) using a suite of atomistic first‐principles methods. We show that configurational disorder, lattice anharmonicity, and coupled host‐mobile ion vibrational dynamics together induce a transition to the superionic state. Statistical correlations between ionic hops and activation of the distribution of vibrational modes are found. However, typical phenomena associated with superionic conductors such as selective breakdown of zone‐boundary soft phonons, or long wavelength transverse acoustic modes as in the concept, are not present. Instead, anharmonic zone‐boundary modes aiding Li diffusion are found to broaden and soften selectively but persist across the superionic transition. These anharmonic modes couple Li ion motion with the vibrations of the flexible close‐packed anion framework, which remains stable and facilitates ionic hopping. our results provide insights into how configurational disorder and soft‐yet‐resilient vibrational modes enable ionic hopping, particularly in three‐dimensional close‐packed crystals.

Elucidating Anisotropic Ionic Diffusion Mechanism in Li3YCl6 with Molecular Dynamics Simulations

“…Li 3 YCl 6 with P 3̅ m 1 space group contains three types of Y sites, including a fully occupied 1 a positions M1 and two partially occupied 2 d positions M2 and M3 in (002) and (001) planes, respectively (Figure a). , Here we adopt the notation from previous studies. , The M2 and M3 sites exhibit different degrees of disorder due to various synthesis conditions. Experimentally, the M3 site is usually more occupied (∼77%) than the M2 site (∼23%) .…”