Electron-phonon coupling and thermal transport in the thermoelectric compound $\mathrm{Mo_3Sb_{7-x}Te_x}$

TL;DR

This study investigates how tellurium substitution in Mo3Sb7-xTex affects phonon properties, electron-phonon interactions, and thermal conductivity, revealing that alloying unexpectedly increases lattice thermal conductivity due to suppressed electron-phonon coupling.

Contribution

It provides a detailed first-principles analysis of the impact of Te substitution on phonon behavior and electron-phonon coupling in Mo3Sb7-xTex, highlighting mechanisms influencing thermal transport.

Findings

Te substitution stiffens interatomic force-constants.

Suppression of electron-phonon coupling increases phonon group velocities.

Alloying leads to increased lattice thermal conductivity.

Abstract

Phonon properties of $\mathrm{Mo_3Sb_{7-x}Te_x}$ ($x=0,1.5, 1.7$), a potential high-temperature thermoelectric material, have been studied with inelastic neutron and x-ray scattering, and with first-principles simulations. The substitution of Te for Sb leads to pronounced changes in the electronic structure, local bonding, phonon density of states (DOS), dispersions, and phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near the top of the valence band, resulting in a strong suppression of electron-phonon screening, and a large overall stiffening of interatomic force-constants. The suppression in electron-phonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassing the effects of alloy-disorder scattering, and resulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes non-uniformly around different atoms, leading to variable perturbation strengths for different optical phonon branches. The respective roles of changes in phonon group velocities and phonon lifetimes on the lattice thermal conductivity are quantified. Our results highlight the importance of the electron-phonon coupling on phonon mean-free-paths in this compound, and also estimates the contributions from boundary scattering, umklapp scattering, and point-defect scattering.