Single crystal growth and characterization of new Zintl phase Ca9Zn3.1In0.9Sb9

Complex Zintl phases have yielded a large variety of promising new thermoelectric materials. In this study we report the discovery of the new Zintl phase Ca9(Zn1–xInx)4Sb9 (x ​~ ​0.9), needle-like crystals of which were serendipitously obtained from an In- and Sb-rich flux. Although its composition is reminiscent of Ca9Zn4+xSb9, an excellent thermoelectric material with zT ​> ​1, the substitution of In on the Zn site leads to the formation of an entirely new structure type. Single crystal X-ray diffraction revealed a structure characterized by TSb4 tetrahedra (T ​= ​statistically disordered Zn and In atoms) and ZnSb3 triangular units, which share common corners to form [T4Sb9]18− polyanions. The average structure was found to have hexagonal symmetry. The valence electron count in this heavily-disordered structure appears to follows the Zintl-Klemm rules, suggesting semiconducting behavior. Single crystal electrical conductivity and Seebeck coefficient measurements support this conclusion, suggesting that the as-grown crystals are degenerate p-type semiconductors.