-Type Double Doping and the Diamond-like Morphology Shift of the Zintl Phase Thermoelectric Materials: The CaASbGe (A = Na, Li; 0.06(3) ≤ ≤ 0.17(5), 0.19(1) ≤ ≤ 0.55(1), 0.13(1) ≤ ≤ 0.22(1)) System.

Five ternary and quaternary Zintl phases in the solid-solution CaASbGe (A = Na, Li; 0.06(3) ≤ ≤ 0.17(5), 0.19(1) ≤ ≤ 0.55(1), 0.13(1) ≤ ≤ 0.22(1)) system have been successfully synthesized by both of the arc-melting and the molten Pb metal-flux reactions. The crystal structure of these title compounds was characterized by powder and single-crystal X-ray diffractions analyses, and all title compounds crystallized in the HoGe-type phase in the tetragonal space group 4/ ( = 4, Pearson code 84). The complex crystal structure can be described as an assembly of 1) three kinds of cationic polyhedra centered by three different Sb and 2) the cage-shaped anionic frameworks built through the connection of two types of Sb. The newly substituted -type double dopants of the cationic (Na and Li) and anionic (Ge) elements displayed particular site preferences, which were successfully explained by either the size-factor criterion based on the atomic size or the electronic-factor criterion based on the electronegativity of an element. Quite interestingly, as the reaction conditions were changed, the morphology shift of single crystals in CaNaSbGe occurred from a cubic-shaped to a hummocky-type, to a hopper-type, and eventually to an octahedral-shaped crystal, just like the Yakutian kimberlite diamonds. Moreover, we firmly believe that the inclusion of the -type Ge dopant for Sb was crucial to trigger this type of morphology shift and complete the octahedral-shaped morphology in the overall crystal-growth mechanism. The theoretical calculations using a DFT method rationalized the observed site preference of Na and the electronic effect of the -type Ge dopants. The Seebeck coefficient measurements for CaLiSbGe indicated that some portions of electron charge carriers were effectively eliminated by the -type double dopants using Li and Ge.