Insights into the Crystal Structure and Thermoelectric Properties of the Zintl Phase Ca9Cd3+x–yMx+ySb9 (M = Cu, Zn) System

Four quaternary Zintl phase solid-solutions in the Ca₉Cd_3+x–yM_x+ySb₉ (M = Cu, Zn) system were successfully synthesized, and their crystal structures were characterized by using both powder X-ray and single-crystal X-ray diffractions. All title compounds adopted the Ca₉Zn_3.1In_0.9Sb₉-type phase, featuring three-dimensional anionic frameworks composed of tetrahedral [MSb₄] and tricoordinated [MSb₃] moieties interconnected via vertex-sharing. Detailed structural analyses revealed that various types of partial and mixed occupations existed at the M sites, and the Zn2 site in Ca₉Cd_2.65(1)Zn_1.84(2)Sb₉ exhibited a trigonal pyramid geometry, distinct from the simple planar geometry observed in parental Ca₉Zn_3.1In_0.9Sb₉. The density of states (DOS) analyses revealed successful p-type doping in the Cu-containing compound, indicated by a shift in the Fermi level and an increase in the hole carrier concentration. Crystal orbital Hamilton population curves displayed optimized interatomic interactions between neighboring anionic elements, maintaining structural stability despite a slight increase in the DOS level. Thermoelectric property measurements were conducted for the first time on the Ca₉Zn_3.1In_0.9Sb₉-type phase. The results demonstrated that the Zn-containing compounds exhibited higher Seebeck coefficients and lower thermal conductivities, resulting in larger ZT values compared to the Cu-containing compounds. The highest ZT value of 0.70 at 775 K was observed for Ca₉Cd_4.05Zn_0.45Sb₉.