Elusive β-Zn8Sb7: A New Zinc Antimonide Thermoelectric.

Zn8Sb7 was theoretically predicted to exhibit superior thermoelectric properties; however a crystalline phase with a similar composition was only stabilized in the form of nanoparticles. We report a new metastable compound, β-Zn8Sb7, which was synthesized in the form of bulk polycrystalline powder via high-temperature solid-state annealing followed by quenching. Single crystal X-ray diffraction indicates that β-Zn8Sb7 crystallizes in a new structure type (noncentrosymmetric orthorhombic space group Pmn21 (no. 31) with unit cell parameters of a = 15.029(1) Å, b = 7.7310(5) Å, c = 12.7431(9) Å, which is different from the nanoparticulate phase. According to differential scanning calorimetry, the β-Zn8Sb7 phase melts incongruently at 825(5) K. β-Zn8Sb7 is a p-type semiconductor with high Seebeck thermopower and low thermal conductivity stemming from the complex crystal structure. β-Zn8Sb7 exhibits a promising thermoelectric figure-of-merit, zT, of 0.33 at 400 K, which is comparable to the state-of-the-art thermoelectric materials based on binary zinc antimonides.