Features of the High-Temperature Structural Evolution of GeTe Thermoelectric Probed by Neutron and Synchrotron Powder Diffraction

Javier Gainza,María Teresa Fernández-Díaz,José Antonio Alonso,Norbert Marcel Nemes,José Luis Martínez,Federico Serrano-Sánchez

doi:10.3390/met10010048

Abstract

Among other chalcogenide thermoelectric materials, GeTe and derivative alloys are good candidates for intermediate temperature applications, as a replacement for toxic PbTe. We have prepared pure polycrystalline GeTe by using arc-melting, and investigated its structural evolution by using neutron powder diffraction (NPD) and synchrotron X-ray diffraction (SXRD), as well as its correlation with the thermal variation of the Seebeck coefficient. Besides a significant Ge deficiency (~7% Ge vacancies), the thermal evolution of the unit-cell volume and Ge-Te bond lengths in the rhombohedral phase (space group R3m), below 700 K, show unexpected anomalies involving the abrupt Ge-Te bond lengthening accompanied by increased Te thermal displacements. Above 700 K, the sample is cubic (space group Fm-3m) and shows considerably larger displacement parameters for Ge than for Te, as a consequence of the random distribution of the lone pair lobes of Ge2+. The Seebeck coefficient, reaching 120 μV K−1 at 775 K, shows a shoulder in the 500–570 K region that can be correlated to the structural anomaly, modifying the electron-phonon scattering in this temperature range.

Highlights

Group IV–VI materials and their alloys display an extensive set of functionalities of scientific and technological interest [1,2]
Owing to their excellent electrical transport properties, the thermal transport of Germanium Telluride (GeTe) materials have been optimized to show a high-thermoelectric figure of merit, evaluated as zT = Sσ κ T, where T is the absolute temperature, S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the total thermal conductivity [6,7,8,9]
An endothermic peak at 699 K was observed during the heating run corresponding to the structural phase transition, while the cooling run presents an exothermic peak at 683 K, showing a significant hysteretic effect suggesting a first-order transition

Summary

Introduction

Group IV–VI materials and their alloys display an extensive set of functionalities of scientific and technological interest [1,2]. The parent GeTe compound presents high-carrier mobilities and concentration [10,11], which are tuned by dopants such as In, Bi, and Sb, to reduce the carrier concentration to an optimum value in order to obtain high power factors and to reduce the lattice thermal conductivity by enhancing phonon scattering mechanisms [12,13,14]. Pristine α-GeTe presents a rhombohedral R3m (No 160) structure, which is described as a distortion of the rocksalt NaCl (PbTe) structure with pseudocubic α angle ≈88.3◦. GeTe undergoes a structural phase transition to a cubic Fm-3m structure along with a ferroelectric to paraelectric transition at Tc ~ 705 K [18] This was described as a displacive second-order transition, as the x parameter deviation and α angle (in the rhombohedral setting) show a continuous variation for which the different lengths of the Ge-Te bonds gradually become equal. We observed an unreported anomaly in the thermal evolution of the rhombohedral phase concerning the unit-cell volume, bonding distances, and displacement parameters, which can be correlated to the thermoelectric power in the temperature range 500–570 K

Experimental Section

Results and Discussion

Conclusions