Insights into the Crystal Structure and Thermoelectric Properties of the Zintl Phase Ca₉Cd_3+x–yM_x+ySb₉ (M = Cu, Zn) System

Four quaternary Zintl phase solid-solutions in the Ca 9 Cd 3+ x–y M x+y Sb 9 (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 Ca 9 Zn 3.1 In 0.9 Sb 9 -type phase, featuring three-dimensional anionic frameworks composed of tetrahedral [MSb 4 ] and tricoordinated [MSb 3 ] 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 Ca 9 Cd 2.65(1) Zn 1.84(2) Sb 9 exhibited a trigonal pyramid geometry, distinct from the simple planar geometry observed in parental Ca 9 Zn 3.1 In 0.9 Sb 9 . 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 Ca 9 Zn 3.1 In 0.9 Sb 9 -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 Ca 9 Cd 4.05 Zn 0.45 Sb 9 .