Synthesis, Structure, and Thermoelectric Properties of α-Zn₃Sb₂ and Comparison to β-Zn₁₃Sb₁₀

Zn-Sb compounds (e.g., ZnSb, β-Zn₁₃Sb₁₀) are known to have intriguing thermoelectric properties, but studies of the Zn₃Sb₂ composition are largely absent. In this work, α-Zn₃Sb₂ was synthesized and studied via temperature-dependent synchrotron powder diffraction. The α-Zn₃Sb₂ phase undergoes a phase transformation to the β form at 425 °C, which is stable until melting at 590 °C. Rapid quenching was successful in stabilizing the α phase at room temperature, although all attempts to quench β-Zn₃Sb₂ were unsuccessful. The structure of α-Zn₃Sb₂ was solved using single crystal diffraction techniques and verified through Rietveld refinement of the powder data. α-Zn₃Sb₂ adopts a large hexagonal cell (R 3-space group, a = 15.212(2), c = 74.83(2) Å) containing a well-defined framework of isolated Sb^3- anions but highly disordered Zn²⁺ cations. Dense ingots of both the α-Zn₃Sb₂ and β-Zn₁₃Sb₁₀ phases were formed and used to characterize and compare the low temperature thermoelectric properties. Resistivity and Seebeck coefficient measurements on α-Zn₃Sb₂ are consistent with a small-gap, degenerately doped, p-type semiconductor. The temperature-dependent lattice thermal conductivity of α-Zn₃Sb₂ is unusual, resembling that of an amorphous material. Consistent with the extreme degree of Zn disorder observed in the structural analysis, phonon scattering in α-Zn₃Sb₂ appears to be completely dominated by point-defect scattering over all temperatures below 350 K. This contrasts with the typical balance between point-defect scattering and Umklapp scattering seen in β-Zn₁₃Sb₁₀. Using the Debye-Callaway interpretation of the lattice thermal conductivity, we use the differences between α-Zn₃Sb₂ and β-Zn₁₃Sb₁₀ to illustrate the potential significance of cation/anion disorder in the Zn-Sb system.