The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe

TL;DR

This study explains the unexpected increase in lattice thermal conductivity of GeTe at its ferroelectric phase transition, highlighting the roles of negative thermal expansion and phonon lifetime changes, using advanced first-principles calculations.

Contribution

It introduces a novel Green-Kubo based method for calculating thermal conductivity, revealing limitations of the Boltzmann approach near phase transitions in thermoelectric materials.

Findings

Thermal conductivity increases near the phase transition due to phonon lifetime enhancements.

Negative thermal expansion in the rhombohedral phase boosts phonon group velocities.

The Green-Kubo method captures effects missed by the Boltzmann transport equation.

Abstract

The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity $\kappa$. However, the $\kappa$ of GeTe increases at the ferroelectric phase transition near $700$ K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in $\kappa$ is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Negative thermal expansion increases phonon group velocities, which counteracts enhanced anharmonicity of phonon modes and boosts $\kappa$ close to the phase transition in the rhombohedral phase. A drastic decrease in the anharmonic force constants in the cubic phase increases the phonon lifetimes and $\kappa$. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a novel method of calculating $\kappa$ based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates $\kappa$ near the phase transition. Our findings elucidate the influence of structural phase transitions on $\kappa$ and provide guidance for design of better thermoelectric materials.