Origin of Glass-like Thermal Conductivity in Crystalline TlAgTe

TL;DR

This study uses advanced first-principles calculations to uncover the microscopic mechanisms behind the glass-like, ultralow thermal conductivity in crystalline TlAgTe, highlighting the roles of localized phonon modes and anharmonic lattice dynamics.

Contribution

It provides a comprehensive theoretical explanation for the low and glass-like thermal conductivity in TlAgTe, integrating phonon localization, anharmonic effects, and transition from particle-like to wave-like phonon behavior.

Findings

Localized phonon modes from Tl rattling vibrations suppress thermal conductivity.

Strong four-phonon scattering enhances anharmonicity and reduces phonon transport.

Transition from particle-like to wave-like phonon behavior occurs above 40 cm$^{-1}$.

Abstract

Ordered crystalline compounds exhibiting ultralow and glass-like thermal conductivity are both fundamentally and technologically important, where phonon quasi-particles dominate their heat transport. Understanding the microscopic mechanisms that govern such unusual transport behavior is necessary to unravel the complex interplay of crystal structure, phonons, and collective excitations of these quasi-particles. Here, we use state-of-the-art first-principles calculations based on quantum density functional theory to investigate the origin of experimentally measured unusually low and glassy thermal conductivity in semiconducting TlAgTe. Utilizing a unifying framework of anharmonic lattice dynamics theory that combine phonon self-energy induced frequency renormalization, particle-like Peierls ($\kappa_{l}^{P}$) and wave-like coherent ($\kappa_{l}^{C}$) thermal transport contributions including three and four-phonon scattering channels, we successfully explain the experimental results both in terms of magnitude and temperature dependence. Our analysis reveals that TlAgTe exhibits several localized phonon modes arising from concerted rattling-like vibration of Tl atoms, which show strong temperature dependence and enhanced four-phonon scattering rates that are dominated by Umklapp processes, suppressing $\kappa_{l}^{P}$ to ultralow values. The ensuing strong anharmonicity induced by local structural distortions, lone-pair electrons, and rattling-like vibrations of the heavy cations lead to a transition from particle-like behavior to wave-like tunneling characteristics of the phonon modes above 40 cm$^{-1}$, contributing significantly to $\kappa_{l}^{C}$ which increases with temperature. Our analysis uncovers important structure-property relationship, which may be used in designing of novel materials with tunable thermal conductivity.