Evolution of Phonon Transport Across Structural Phase Transitions in MgAgSb

TL;DR

This study investigates how structural phase transitions in MgAgSb affect its thermal transport properties, revealing phase-dependent mechanisms and contributions from particle-like and wave-like phonons using first-principles calculations.

Contribution

It provides a comprehensive analysis of the phase-dependent phonon transport mechanisms in MgAgSb, highlighting the interplay between particle-like and wave-like contributions across phases.

Findings

Thermal conductivity increases from $eta$ to $ ext{gamma}$ phase.

Four-phonon scattering suppresses particle-like conductivity in $eta$ and $ ext{gamma}$ phases.

Wave-like conductivity from coherent phonon tunneling is highest in the $ ext{alpha}$ phase.

Abstract

MgAgSb, a promising thermoelectric material, undergoes reversible phase transitions that drastically alter its thermal transport behavior. Using first-principles calculations, we systematically investigate the lattice thermal conductivity ($\kappa_L$) of its three phases: $\alpha$, $\beta$, and $\gamma$, revealing a progressive increase following $\alpha < \beta < \gamma$. This trend originates from distinct scattering mechanisms. Four-phonon scattering substantially suppresses the particle-like conductivity ($\kappa_p$) in the $\beta$ and $\gamma$ phases, while electron-phonon scattering provides a minor additional reduction. In contrast, the wave-like conductivity ($\kappa_c$) from coherent phonon tunneling is highest in the complex $\alpha$ phase, contributing up to 44\% of $\kappa_L$. Notably, the temperature dependence of $\kappa_L$ differs fundamentally between phases: in $\beta$, the weak $\kappa_p$ variation arises from a decreasing Gr\"{u}neisen parameter with temperature; in $\alpha$, the strong rise in $\kappa_c$ with temperature counteracts the decay of $\kappa_p$. Our findings establish a comprehensive picture of thermal transport in MgAgSb, highlighting the phase-dependent interplay between particle-like and wave-like phonon contributions.