Nanovoids in thermoelectric β-Zn4Sb3: A possibility for nanoengineering via Zn diffusion

Abstract The binary Zn4Sb3 phase is a promising material for thermoelectric applications due to its extraordinarily low thermal conductivity. The present study attempts to rationalize this property by investigating its nanostructure. β-Zn4Sb3 samples with varying Zn content were thus synthesized, quenched from the melt and annealed at 350 °C. Their nanostructure was observed using transmission electron microscopy. The samples presented in this work have Zn:Sb atomic ratios of 1.30 and 1.33 and exhibit a single phase of β-Zn4Sb3 in X-ray diffraction studies. Transmission electron microscopy (TEM) observations revealed nanoporous morphologies for both compositions, with a random distribution of the voids. Both samples contain voids, while the sample with Zn:Sb = 1.33 also consists of Zn nanoinclusions. Density functional theory calculations were carried out to study the stability of atomistic β-Zn4Sb3 models with varying Zn content. This was used to suggest a mechanism for the formation of Zn nanoparticles and subsequently nanovoids. The calculations imply that the Zn solubility in the Zn4Sb3 matrix increases with decreasing temperature. Combined with a high Zn diffusivity, this work explains how Zn could leave the nanoprecipitates during annealing, resulting in the voids seen by TEM.