Lattice hardening due to vacancy diffusion in (GeTe)mSb2Te3 alloys

Abstract

GeTe-Sb2Te3 alloys have been widely studied for use in rewritable media, and in recent years, they have emerged as excellent thermoelectric materials, with reports of zT>2 for Ge-rich compositions. GeTe-Sb2Te3 alloys exhibit a solid-state phase transition from a layered structure with rhombohedral symmetry to a cubic rocksalt structure, which plays an important role in their thermoelectric behavior. Here, we investigate the impact of the phase transition on the thermal expansion and elastic moduli of (GeTe)17Sb2Te3 using high-temperature X-ray diffraction and resonant ultrasound spectroscopy. The high-temperature elastic moduli of GeTe, Sb2Te3, and Bi2Te3 were also measured for comparison. While it is typical for materials to soften with increasing temperature due to thermal expansion, our study reveals anomalous hardening of the elastic moduli in (GeTe)17Sb2Te3 at temperatures below the phase transition, followed by further hardening at the transition temperature. In contrast, the elastic moduli of GeTe, Sb2Te3, and Bi2Te3 soften with increasing temperature. We attribute the anomalous hardening of (GeTe)17Sb2Te3 to the gradual vacancy diffusion accompanying the transition from a layered to a cubic structure. The stiffening elastic moduli lead to increasing speed of sound, which impacts the lattice thermal conductivity by flattening the temperature dependence.

Physical Insights on the Lattice Softening Driven Mid‐Temperature Range Thermoelectrics of Ti/Zr‐Inserted SnTe—An Outlook Beyond the Horizons of Conventional Phonon Scattering and Excavation of Heikes’ Equation for Estimating Carrier Properties

“…[101] In stark contrast, there are also compounds with high shear and Young's moduli, such as (GeTe) 17 Sb 2 Te 3 , which were reported for lattice "hardening" effects. [102] Mössbauer spectroscopy, which can enlighten more on the correlation between elastic moduli, the speed of sound, and the Debye temperature, [103] makes a strong case for potential future work. [56] κ latt = Aν s T -1 .…”

Positive Effect of Ge Vacancies on Facilitating Band Convergence and Suppressing Bipolar Transport in GeTe‐Based Alloys for High Thermoelectric Performance

“…[ 55 ] On the other hand, high density of planar Ge‐vacancy arrays have been frequently observed in GeTe‐based materials by alloying with Cd‐Bi, [ 46 ] Bi 2 Te 3 , [ 57,58 ] and Sb 2 Te 3 , [ 37,50,59 ] which have been demonstrated as an efficient way for reducing κ lat . Other effects of Ge vacancies such as phase transition [ 60 ] and lattice hardening [ 61 ] have also been reported in the GeTe‐based materials. Thus, the Ge‐vacancy engineering is significant for enhancing the ZT of GeTe‐based alloys, [ 19,28 ] and more effects of Ge vacancies besides the tuning of carrier density are still worth exploring in GeTe‐based alloys for further enhancing their thermoelectric performance.…”

Influence of the stacking sequence on layered-chalcogenide properties: first principles investigation of Pb2Bi2Te5

“…Indeed by using different types of stacks as well as various combinations, these homologous series behave as compounds generating devices. Many n(A IV C V I )m(B V 2 C V I 3 ) compounds have been already investigated as potential thermoelectric materials [3][4][5][6] , phase change memories for optical data storage 6,7 and topological insulators (TIs) [8][9][10][11][12][13][14][15][16] .…”