Soft-mode dynamics in the ferroelectric phase transition of GeTe

Chen Wang,Yanzhong Pei,Zezhu Zeng,Jie Ma,Jan Embs,Yue Chen,Jiangtao Wu

doi:10.1038/s41524-021-00588-4

Abstract

GeTe that exhibits a strong anharmonicity and a ferroelectric phase transition between the rhombohedral and cubic structures has emerged as one of the leading thermoelectric materials. Herein, combining molecular dynamics simulations and inelastic neutron scattering measurements, the lattice dynamics in GeTe have been investigated to reveal the soft-mode mechanisms across the phase transition. We have constructed a first-principles-based machine-learning interatomic potential, which successfully captures the dynamical ferroelectric phase transition of GeTe by adopting the neural network technique. Although the low-energy acoustic phonons remain relatively unaffected at elevated temperatures, the high-energy optical, and longitudinal acoustic phonons demonstrate strong renormalizations as evidenced from the vibrational phonon spectra, which are attributed to the large anharmonicity accompanying the phase transition. Furthermore, our results reveal a nonmonotonic temperature dependence of the soft-modes beyond the perturbative regime. The insight provided by this work into the soft-modes may pave the way for further phonon engineering of GeTe and the related thermoelectrics.

Highlights

Thermoelectric materials have spurred a considerable technological interest to alleviate the energy demands for their capability to directly convert thermal gradient to electrical energy and vice versa[1,2,3]
Phonon scattering can be enhanced by various defects or nanostructures[5], single-crystalline semiconductors that possess strong intrinsic lattice anharmonicity, while simultaneously maintaining favorable electronic transport properties have emerged as promising candidates for high-performance thermoelectric applications[6,7]
The thermodynamic and vibrational phonon properties of GeTe have been investigated within the perturbation theory, a comprehensive understanding of its lattice dynamics is still hindered by the challenges in handling the lattice instability and high-order anharmonicity across the phase transition[13,14,15]

Summary

INTRODUCTION

Thermoelectric materials have spurred a considerable technological interest to alleviate the energy demands for their capability to directly convert thermal gradient to electrical energy and vice versa[1,2,3]. The thermodynamic and vibrational phonon properties of GeTe have been investigated within the perturbation theory, a comprehensive understanding of its lattice dynamics is still hindered by the challenges in handling the lattice instability and high-order anharmonicity across the phase transition[13,14,15]. The vanish of rhombohedral lattice distortion above Tc observed in the neutron diffraction experiment[11], as well as the concurrent softening of the A1 and E phonon modes with increasing temperature measured for α-GeTe in Raman scattering experiments[12,22], suggest a displacive nature underlying the GeTe ferroelectric phase transition. A thorough investigation of temperaturedependent phonon spectra with a nonperturbative treatment of the anharmonic effects; is of great importance to better understand the vibrational properties of GeTe, as well as the nature of the ferroelectric phase transition

Wang et al 2

RESULTS

DISCUSSION

METHODS