Understanding the Origin of Glass-like Thermal Conductivity in Good Solid Electrolytes: The Case of Li ₃ YCl ₆ (LYC)

Solid electrolytes feature an ordered framework interlaced with a disordered sublattice of mobile ions, placing them structurally between crystals and glasses. While their high ionic conductivity is well studied, the origin of their glass-like thermal conductivity remains poorly understood. Here, we investigate the halide solid electrolyte Li 3 YCl 6 (LYC) using harmonic lattice dynamics, spectral energy density analysis, and extended machine-learned molecular dynamics (MLMD). We identify a boson peak-like vibrational excess near 1 THz, arising from quasi-localized, strongly anharmonic modes following glassy ω 4 scaling. Thermal transport is governed by diffuson-like carriers, as confirmed by Green–Kubo and Wigner transport calculations. Acoustic phonons near the Brillouin zone boundary become ill-defined by 300 K, yet ionic residence times remain much longer than phonon lifetimes, indicating that intrinsic anharmonicity, not ion hopping, causes this breakdown. These results show that structural complexity and strong anharmonicity drive glass-like thermal transport in LYC and may underlie similar behavior in other complex crystalline systems.