Crystal structure, electronic structure and thermoelectric properties of β- and γ-Zn4Sb3 thermoelectrics: a (3 + 1)-dimensional superspace group approach†
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 [Zn3+δSb][Sb]p with the superspace group of Rm(00γ)0s. The [Zn3+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 (Zni) site and an Sb(1) site in the [Zn3+δSb] subsystem and an Sb(2) site in the [Sb] subsystem, which is different from a conventional 3D model containing additional Zni 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 Zni 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 Zni is calculated based on the (3 + 1)-D model, demonstrating a close relationship between the DOS and the Zni occupancy. The calculated TE properties, such as Seebeck coefficient, electrical conductivity and power factor, also depend on the Zni occupancy.