The atomic order of the pnictogen and chalcogen atoms in equiatomic ternary compounds TPnCh (T = Ni, Pd; Pn = P, As, Sb; Ch = S, Se, Te)

The crystal structures of the compounds NiAsS, NiSbS, PdAsS, PdAsSe, PdSbSe, and PdSbTe were refined from single-crystal X-ray diffractometer data. In these cubic compounds the pnictogen and chalcogen atoms show an ordered distribution corresponding to the ullmannite (NiSbS) type structure (space group P2(1)3, Z = 4). By analogy this can be assumed for NiPS and NiAsSe, although for these compounds the structure refinements did not make it possible to distinguish between a disordered distribution of the pnictogen and chalcogen atoms (corresponding to the pyrite structure) or an ordered distribution (corresponding to ullmannite or cobaltite). All atomic positions are fully occupied within the error limits of +/- 2%. A refinement from X-ray powder data established the ullmannite structure also for NiSbSe. The PdPS type structure of PdPSe was refined from single-crystal data. No equiatomic ternary compounds were observed in the ternary systems Ni-P-Se, Ni-P-Te, Ni-As-Te, Ni-Sb-Te, Pd-P-Te, and Pd-As-Te. A refinement from single-crystal data showed Ni0.766(5)Sb0.50(3)Te0.50(3) to crystallize with a defect-NiAs type structure. Extended solid solutions were observed for the pseudobinary system NiP2-NiSe2, where a slowly cooled equiatomic sample contained NiP1.86(4)Se0.14(4) and NiP0.24(4)Se1.76(4) as limiting compositions, as shown by structure refinements from X-ray powder data. Chemical bonding in these compounds is briefly discussed. (C) 2001 Academic Press.