Thermal stability and electrical transport properties of Ge/Sn-codoped single crystalline β-Zn₄Sb₃ prepared by the Sn-flux method

This study prepares a group of single crystalline β-Zn₄Sb₃ with Ge and Sn codoped by the Sn-flux method according to the nominal stoichiometric ratios of Zn_4.4Sb₃Ge_xSn₃ (x=0-0.15). The prepared samples possess a metallic luster surface with perfect appearance and large crystal sizes. The microscopic cracks or defects are invisible in the samples from the back-scattered electron image. Except for the heavily Ge-doped sample of x=0.15, all the samples are single phase with space group R3c. The thermal analysis results show that the samples doped with Ge exhibit an excellent thermal stability. Compared with the polycrystalline Ge-substituted β-Zn₄Sb₃, the present single crystals have higher carrier mobility, and hence the electrical conductivity is improved, which reaches 7.48×10⁴ S·m^-1 at room temperature for the x=0.1 sample. The change of Ge and Sn contents does not improve the Seebeck coefficient significantly. Benefiting from the increased electrical conductivity, the sample with x=0.075 gets the highest power factor of 1.45×10^-3 W·m^-1·K^-2 at 543 K.