Synthesis, crystallographic and theoretical studies of the new Zintl phases Ba(2)Cd(2)Pn(3) (Pn = As, Sb), and the solid solutions (Ba1-xSrx)(2)Cd2Sb3 and Ba2Cd2(Sb1-xAsx)(3)

Two new Zintl compounds Ba2Cd2As3 and Ba2Cd2Sb3 have been synthesized and structurally characterized. They crystallize in a novel monoclinic structure type with the space group C2/m (no. 12), featuring polyanionic layers made of CdPn(4) tetrahedra (Pn = As, Sb) and homoatomic Pn-Pn bonds. The topological relationships between the structure of Ba2Cd2Sb3 and those of BaCd2Sb2 (CaAl2Si2 type) and Ba3Cd2Sb4 ( own type) are discussed as well. Based on electronic structures calculations, carried out by the density-functional method, and resistivity measurements, pure Ba2Cd2As3 is shown to be a small-gap semiconductor and pure Ba2Cd2Sb3 to be a poor metal. The structures of the title compounds are amenable to doping on both cation and pnicogen sites, which could enable fine-tuning the transport properties, and make them promising materials for thermoelectric applications.