Hg0.04Zn3.96Sb3:: Synthesis, crystal structure, phase transition, and thermoelectric properties

The thermoelectric material Zn4Sb3 was mercury doped by introduction of 1 at. % Hg into the synthesis mixture, resulting in Hg0.04Zn3.96Sb3. The doped compound and an undoped reference were characterized by multitemperature short wavelength synchrotron X-ray powder diffraction, SEM/EDX, differential scanning calorimetry (DSC), and physical property measurements. Rietveld refinements suggest that mercury substitution takes place solely on the Zn1 framework site of the disordered room temperature beta-phase crystal structure, while the interstitial positions are mercury-free. The refined composition suggests a doping level of 0.6%. The remaining mercury is present as elemental Hg as evidenced by SEM/EDX analysis, the presence of peaks corresponding to crystalline Hg below the Hg freezing temperature, and the presence of a drop in the resistivity at the superconducting transition temperature of Hg. Rietveld refinements of multitemperature synchrotron X-ray powder diffraction data (180 K < T < 290 K, Delta T = 10 K) reveal a significant change in the phase transition temperatures between undoped and 1% Hg-doped samples. This is corroborated by DSC data that show that the transition enthalpies are very small, about 0.1-0.4 J/g. The largest enthalpy change is observed in the alpha-alpha' transition for the undoped sample, whereas the largest transition enthalpy is found in the alpha'-beta transition for the Hg-doped compound. Complete thermoelectric properties have been measured in the temperature range 2-400 K on dense samples prepared by spark plasma sintering. The Hg doping has a very large effect on the transport properties in the ordered alpha-phase crystal structure but only a minor influence on the properties in the disordered beta-phase. The thermoelectric figure of merit, ZT, is found to be similar to 0.3 for both the undoped and the Hg-doped sample at 300 K.