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

Đorđe Dangić,Olle Hellman,Stephen Fahy,Ivana Savić

doi:10.1038/s41524-021-00523-7

Đorđe Dangić, Olle Hellman + Show 2 more

Open Access

https://doi.org/10.1038/s41524-021-00523-7

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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 κ. However, the κ 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 κ is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a method of calculating κ based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates κ near the phase transition. Our findings elucidate the influence of structural phase transitions on κ and provide guidance for design of better thermoelectric materials.

Highlights

Reducing the lattice thermal conductivity κ is one of the most successful ways of improving the efficiency of thermoelectric materials[1,2,3,4,5,6,7]
At temperatures higher than 600–700 K, germanium telluride (GeTe) transforms to the rocksalt structure, losing its ferroelectric nature[41,42,43,44]
600–700 K, the GeTe structure can be described by the following set of lattice vectors r1 1⁄4 aðb; 0; cpÞ;ffiffi r2

Summary

INTRODUCTION

Reducing the lattice thermal conductivity κ is one of the most successful ways of improving the efficiency of thermoelectric materials[1,2,3,4,5,6,7]. Many of the best thermoelectric materials have intrinsically low lattice thermal conductivity This is usually related to the large anharmonicity of phonon modes, which can stem from weak bonding, as in van der Waals materials[2,5,8,9,10,11], or rattling modes[12,13,14,15,16,17]. We implement a method of calculating κ that includes these non-Lorentzian lineshapes of the phonon power spectra near the phase transition. This approach further increases κ at the phase transition, which can be attributed to further softening of the phonon frequencies due to phonon–phonon interaction

RESULTS AND DISCUSSION

DISCUSSION

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