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 di stribution corresponding to the ullmannite (NiSbS) type structure (space gr oup P2(1)3, Z = 4). By analogy this can be assumed for NiPS and NiAsSe, alt hough for these compounds the structure refinements did not make it possibl e to distinguish between a disordered distribution of the pnictogen and cha lcogen atoms (corresponding to the pyrite structure) or an ordered distribu tion (corresponding to ullmannite or cobaltite). All atomic positions are f ully occupied within the error limits of +/- 2%. A refinement from X-ray po wder data established the ullmannite structure also for NiSbSe. The PdPS ty pe structure of PdPSe was refined from single-crystal data. No equiatomic t ernary 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 da ta showed Ni0.766(5)Sb0.50(3)Te0.50(3) to crystallize with a defect-NiAs ty pe structure. Extended solid solutions were observed for the pseudobinary s ystem 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 com pounds is briefly discussed. (C) 2001 Academic Press.