Anharmonicity in Thermal Insulators: An Analysis from First Principles

TL;DR

This study classifies materials based on anharmonicity and uses ab initio calculations to identify strong thermal insulators, revealing that defect-driven dynamics dominate in highly anharmonic materials, contrasting with perturbative models.

Contribution

It provides a comprehensive classification of materials' anharmonicity and introduces fully anharmonic ab initio Green-Kubo calculations for thermal conductivity analysis.

Findings

28 materials with thermal conductivity below 10 W/mK

6 materials with ultralow thermal conductivity below 1 W/mK

Strong anharmonicity driven by defect geometries

Abstract

The anharmonicity of atomic motion limits the thermal conductivity in crystalline solids. However, a microscopic understanding of the mechanisms active in strong thermal insulators is lacking. In this letter, we classify 465 experimentally known materials with respect to their anharmonicity and perform fully anharmonic ab initio Green-Kubo calculations for 58 of them, finding 28 thermal insulators with $\kappa < 10$ W/mK including 6 with ultralow $\kappa \lesssim 1$ W/mK. Our analysis reveals that the underlying strong anharmonic dynamics is driven by the exploration of meta-stable intrinsic defect geometries. This is at variance with the frequently applied perturbative approach, in which the dynamics is assumed to evolve around a single stable geometry.