A10LaCdSb9 (A=Ca,Yb): A Highly Complex Zintl System and the Thermoelectric Properties

Two new Zintl compounds A₁₀LaCdSb₉ (A=Ca, Yb), namely, Ca_9.81(1)La_0.97(1)Cd_1.23(1)Sb₉ and Yb_9.78(1)La_0.97(1)Cd_1.24(1)Sb₉, have been designed and synthesized by applying the Zintl concept. Although both compounds are isoelectronic with their Ca₁₁InSb₉ and Yb₁₁InSb₉ analogues, they crystallize in a new structure type with the orthorhombic space group Ibam (No.72) and feature very complex anion structures, which are composed of unique [Cd₂Sb₆]^12? clusters, dumbbell‐shaped [Sb₂]^4? dimers, and isolated [Sb]^3? anions. For Yb_9.78(1)La_0.97(1)Cd_1.24(1)Sb₉, an extremely low lattice thermal conductivity of 0.29 W m^?1 K^?1 was observed at 875 K, which almost approaches the lowest reported limit of nonglassy or nonionically conducting bulk materials. According to thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses, both compounds show very good thermal stability and no melting or phase transition processes were found below 1173 K. Although related thermoelectric property studies on Yb_9.78(1)La_0.97(1)Cd_1.24(1)Sb₉ only present a maximum ZT of 0.11 at 920 K, owing to its low Seebeck coefficients, these materials are still very promising for their high temperature stability and low thermal conductivity. Furthermore, as mixed cations exist with different charges, it makes this system very flexible in tuning the related electrical properties.