Soft Phonon Modes Lead to Suppressed Thermal Conductivity in Ag-Based Chalcopyrites Under High Pressure

Abstract

Pressure is a powerful way to modulate physical properties. Understanding the pressure evolution of thermal transport properties of thermoelectric materials has great significance in the efficient design and optimization of thermoelectric performance. In this work, based on first-principles calculations and phonon Boltzmann transport theory, we find that the lattice thermal conductivities of Ag-based chalcopyrites AgXY2 (X = Al, Ga, In; Y = S, Se, Te) are dramatically suppressed by applying pressure. The inherent distorted tetrahedral configuration together with the highly delocalized p-orbital electrons promotes the formation of metavalent bonding, and the half-filled bonds lead to the soft low-frequency optical phonons. With the increase of pressure, the softening of acoustic and low-frequency optical phonons induces enhanced anharmonicity and scattering channels. Such strong acoustic-optical phonon coupling results in larger phonon scattering rates and thus lowers the lattice thermal conductivity. The findings not only help unveil the underlying physical mechanisms for the anomalous thermal transport behaviors under high pressure, but also pave the way for the pressure tunning of high-performance Ag-based thermoelectric materials.

Keywords: Thermal Conductivity, Phonon, High pressure, Soft mode