Crystal structure, electronic structure and thermoelectric properties of β- and γ-Zn4Sb3 thermoelectrics: a (3 + 1)-dimensional superspace group approach

Abstract

Thermoelectric (TE) materials are promising candidates for solving today's energy problem owing to their ability to directly convert waste heat into electricity via the Seebeck effect. One of the most efficient TE materials known is Zn₄Sb₃. To understand its high efficiency, a novel composite crystal structure model for β- and γ-phases of Zn₄Sb₃ is constructed using a (3 + 1)-dimensional ((3 + 1)-D) superspace group approach. This (3 + 1)-D model is expressed as [Zn_3+δSb][Sb]_p with the superspace group of Rm(00γ)0s. The [Zn_3+δSb] and [Sb] subsystems have same a- and b-axis lengths but a different c-axis length. The (3 + 1)-D model contains four atomic sites: a Zn(1) normal site, an interstitial Zn (Zn_i) site and an Sb(1) site in the [Zn_3+δSb] subsystem and an Sb(2) site in the [Sb] subsystem, which is different from a conventional 3D model containing additional Zn_i sites. The crystal structures of β- and γ-Zn₄Sb₃ are investigated via powder and synchrotron X-ray diffraction (XRD) measurements. The XRD patterns are well analysed by the (3 + 1)-D model. The occupancies of Zn(1), Sb(1) and Sb(2) sites are 100%, whereas the Zn_i occupancy changes depending on the heating time during the preparation of β-Zn₄Sb₃. Moreover, electronic density of states (DOS) of β-Zn₄Sb₃ with and without Zn_i is calculated based on the (3 + 1)-D model, demonstrating a close relationship between the DOS and the Zn_i occupancy. The calculated TE properties, such as Seebeck coefficient, electrical conductivity and power factor, also depend on the Zn_i occupancy.