β-Zn4Sb3 has one of the highest ZT reported for binary compounds, but its practical applications have been hindered by a reported poor stability. Here we report the fabrication of nearly dense single-phase β-Zn4Sb3 and a study of its thermoelectric transport coefficients across a wide temperature range. Around 425 K we find an abrupt decrease of its thermal conductivity. Past this point, Zn atoms can migrate from crystalline sites to interstitial positions; β-Zn4Sb3 becomes metastable and gradually decomposes into Zn(hcp) and ZnSb. However, above 565 K it recovers its stability; in fact, the damage caused by decomposition can be repaired completely. This is key to its excellent thermoelectric performance at high temperature: the maximum ZT reaches 1.4. Molecular dynamics simulations are used to shed light on the microscopic behavior of the material.
Zinc antimony stands out among thermoelectrics because of its very low lattice thermal conductivity, close to the amorphous limit. Understanding the physical reason behind such an unusual crystal property is of fundamental interest for the design of new thermoelectric materials. In this work we report the results of atomistic computer simulations on experimentally determined β−Zn 4 Sb 3 structures. We find a remarkably anharmonic behavior of Zn atoms that could be responsible for the low thermal conductivity of Zn 4 Sb 3 : their movement, better explained as diffusive, does not contribute to thermal conduction. Moreover, phonon transport is impeded by a lack of coupling between Zn and Sb atoms in crystalline positions.As one of the best thermoelectric materials at moderate temperature, 1 Zn 4 Sb 3 's remarkable performance (as gauged by its high dimensionless thermoelectric figure of merit, ZT ) is mainly derived from its glass-like lattice thermal conductivity, below 1.0 W m −1 K −1 . Such a low thermal conductivity approaches the theoretical lower bound known as the amorphous limit, introduced by Slack 2 and refined by Cahill and coworkers. 3 . The idea behind this theoretically minimal thermal conductivity is a physically plausible lower bound to phonon mean free paths, 4 such as half a wavelength. Crystalline compounds with thermal conductivities close to 5 or even below 6 this theoretical limit are few and usually display interesting physics.A straight path to the design of efficient thermoelectrics may lie in the extremely poor thermal conduction of Zn 4 Sb 3 . Thus, many attempts at understanding this feature have been made by placing Zn 4 Sb 3 under microscopes. 7-10 For instance, using single-crystal Xray and powder-synchrotron-radiation diffraction, Snyder and coworkers have found that the hexagonal unit cell of Zn 4 Sb 3 containing at least three interstitial Zn atoms follows the rules of valence compounds, and that Zn occupancy at the strongly bonded crystal site reaches only about 90%. 1 This has provided significant insight for explaining the unusual chemical and physical properties of this material. Specifically, interstitial atoms are suspected to be an extremely effective mechanism for reducing thermal conductivity by introducing disorder. 11,12 Despite many attempts, the precise mechanism behind the low lattice thermal conductivity of Zn 4 Sb 3 is still a puzzle, due to the lack of atomic-scale information and the challenges posed by the study of dynamical effects in large systems through ab-initio methods. For instance, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations and density functional theory (DFT) calculations have provided evidence of Zn diffusion and nanovoid formation in β−Zn 4 Sb 3 , 13 but their connection to thermal conduction remains an elusive essential part. More generally, self-diffusion of ions in crystals has long been observed in experiments; 14 its influence on conduction by phonons is also not well understood. Here we performed mole...
The structural stability of thermoelectric materials is a subject of growing importance for their energy harvesting applications. Here we study the microscopic mechanisms governing the structural stability change of zinc antimony at its working temperature, using molecular dynamics combined with experimental measurements of the electrical and thermal conductivity. Our results show that the temperature-dependence of the thermal and electrical transport coefficients is strongly correlated with a structural transition. This is found to be associated with a relaxation process, in which a group of Zn atoms migrated between interstitial sites. This atom migration gradually leads to a stabilizing structural transition of the crystal framework, then results in a more stable crystal structure of β−Zn 4 Sb 3 at high temperature. * Electronic address: jaredlin@163.com (for questions on the experimental part) † Electronic address: zwangzhao@gmail.com 1 arXiv:1611.00894v1 [cond-mat.mtrl-sci]
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