Atomistic origin of glass-like Zn4Sb3 thermal conductivity

Abstract

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...

Inorganic Crystals with Glass‐Like and Ultralow Thermal Conductivities†

“…These interstitial Zn atoms display large and anisotropic mean square atomic displacement parameters, indicative of substantial dynamic and/or static disorder. Experimental and theoretical studies suggest a very high mobility of such atoms in these materials, which may even diffuse throughout the structure . Such mobile, “liquid‐like” behavior of one of the atomic species in a crystal, sometimes referred to as “sublattice melting,” reflects substantially intrinsic dynamic disorder to which the glass‐like thermal conductivity of Zn 4 Sb 3 (mobile Zn ions), Cu 2 Se (mobile Cu ions), and other related materials has been attributed .…”

Thermal conductivity of intercalation, conversion, and alloying lithium-ion battery electrode materials as function of their state of charge

“…For example, amorphous Sb2Te3 shows Λ = 0.23 W m -1 K -1 [48] and disordered Zn4Sb3 shows Λ = 0.6 W m -1 K -1 at 300 K, [49] due to the large phonon anharmonicity induced loose coupling of atoms with different masses. [50] A first-principles calculation predicts the suppression of ΛL in the lithiated state (crystalline Li3Sb, ΛL = 2.2 W m -1 K -1 ). [51] Further decrease in measured Λ below 1 m -1 K -1 can be attributed to the loss of crystallinity.…”

Atomistic mechanisms governing structural stability change of zinc antimony thermoelectrics

“…[16] Our previous simulation results showed that the diffusion of Zn atoms above 425K brings remarkable anharmonicity to the system vibration, and therefore is responsible for the low thermal conductivity of Zn 4 Sb 3 in the order of those found in amorphous solids. [17] However, the physical reasons behind the structural stability change remained as a open question, despite this is another key to its excellent thermoelectric performance at moderate temperature.…”

“…In our previous studies, we found that this volume change is triggered by a diffusion-like behavior of interstitial Zn atoms, this produces remarkable phonon anharmonicity responsible for the Zn 4 Sb 3 low thermal conductivity. [17] Here we instead focus our attention on a different aspect, including a structural transition after this avalanche of atomic migration. The energy curve in Fig.1(a) reveals the nature of this process, in which a group of loosely bonded atoms adapts to a different crystallographic structure in their search for a more energetically favorable arrangement.…”

“…The simulations were performed in the isothermal-isobaric ensemble (NPT) with the Nosé-Hoover thermostat helping the system reach thermal equilibrium at different temperatures before any statistical analysis was performed. Atomistic interactions were described by a pairwise potential that has been successfully applied to study mechanical [22] and phononic behaviors of Zn 4 Sb 3 [17]. Benchmark runs were performed to reproduce the characteristic β to α phase transition around 250K, which manifests itself as a sharp step in the potential energy during a cooling process.…”