The rare earth osmium carbides Ln(5)Os(3)C(4-x) (Ln=La-Nd, Sm) with a new type of filled Mn3Si3 structure

The five compounds Ln(5)Os(3)C(4-x) (Ln = La-Nd, Sm) have been prepared by are-melting cold-pressed pellets of the elemental components and subsequent annealing. They crystallize with a new hexagonal structure type (P6(3)/mcm, Z = 2), which has been determined from single-crystal X-ray data of Ln(5)Os(3)C(3.25(4)) (a = 919.57(8) pm, c = 673.86(4) pm, R = 0.015 for 293 structure factors and 14 variable parameters) and also refined for Nd5Os3C3.17(7) (a = 897.8(1)pm, c = 667.42(9)pm, R = 0.028, 440 F values, 14 variables). The metal positions correspond to the positions of the Mn5Si3 structure. The carbon position of the Mo5Si3C1-x structure is partially filled. The three additional carbon atoms of the formula La5Os3C4-x OCCUPY distorted octahedral La4Os2 voids. The linear Os-C-Os units of these La4Os2C octahedra have Os-C bond lengths of 191 pm. They are linked via common osmium atoms, thus forming a new kind of zig-zag-shaped polyanion. Using oxidation numbers the compound may be rationalized with the formula (5La(+3))(+15) (Os3C3)(-14) (0.25C(-4))(-1). (C) 1997 Academic Press.