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

Zn-Sb compounds (e.g., ZnSb, beta-Zn13Sb10) are known to have intriguing thermoelectric properties, but studies of the Zn3Sb2 composition are largely absent. In this work, alpha-Zn3Sb2 was synthesized and studied via temperature-dependent synchrotron powder diffraction. The alpha-Zn3Sb2 phase undergoes a phase transformation to the beta form at 425 degrees C, which is stable until melting at 590 degrees C. Rapid quenching was successful in stabilizing the a phase at room temperature, although all attempts to quench beta-Zn3Sb2 were unsuccessful. The structure of alpha-Zn3Sb2 was solved using single crystal diffraction techniques and verified through Rietveld refinement of the powder data. alpha-Zn3Sb2 adopts a large hexagonal cell (R (3) over bar space group, a = 15.212(2), c = 74.83(2) angstrom) containing a well-defined framework of isolated Sb3- anions but highly disordered Zn2+ cations. Dense ingots of both the alpha-Zn3Sb2 and beta Zn13Sb10 phases were formed and used to characterize and compare the low temperature thermoelectric properties. Resistivity and Seebeck coefficient measurements on alpha-Zn3Sb2 are consistent with a small-gap, degenerately doped, p-type semiconductor. The temperature-dependent lattice thermal conductivity of alpha-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 alpha-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 Umldapp scattering seen in beta-Zn13Sb10. Using the Debye-Callaway interpretation of the lattice thermal conductivity, we use the differences between alpha-Zn3Sb2 and beta-Zn13Sb10 to illustrate the potential significance of cation/anion disorder in the Zn-Sb system.