Synthesis, Crystal and Electronic Structures of the Pnictides AE3TrPn3 (AE = Sr, Ba; Tr = Al, Ga; Pn = P, As).

The new ternary arsenides AE₃TrAs₃ (AE = Sr, Ba; Tr = Al, Ga) and their phosphide analogs Sr₃GaP₃ and Ba₃AlP₃ have been prepared by reactions of the respective elements at high temperatures. Single-crystal X-ray diffraction studies reveal that Sr₃AlAs₃ and Ba₃AlAs₃ adopt the Ba₃AlSb₃-type structure (Pearson symbol oC56, space group Cmce, Z = 8). This structure is also realized for Sr₃GaP₃ and Ba₃AlP₃. The compounds Sr₃GaAs₃ and Ba₃GaAs₃ crystallize with the Ba₃GaSb₃-type structure (Pearson symbol oP56, space group Pnma, Z = 8). Both structures are made up of isolated pairs of edge-shared ALPN4 and GaPn4 tetrahedra (Pn = pnictogen, i.e., P or As), separated by the alkaline-earth Sr²⁺ and Ba²⁺ cations. In both cases, there are no homoatomic bonds, hence, regardless of the slightly different atomic arrangements, both structures can be rationalized as valence-precise [AE²⁺]₃[TR³⁺][Pn^3-]₃, or rather [AE²⁺]₆[Tr²⁺]Tr₂Pn₆]^12-, i.e., as Zintl phases. [ABSTRACT FROM AUTHOR]