The Rare Earth Osmium CarbidesLn5Os3C4−x(Ln=La–Nd, Sm) with a New Type of Filled Mn3Si3Structure

The five compoundsLn5Os3C4−x(Ln=La–Nd, Sm) have been prepared by arc-melting cold-pressed pellets of the elemental components and subsequent annealing. They crystallize with a new hexagonal structure type (P63/mcm,Z=2), which has been determined from single-crystal X-ray data ofLn5Os3C3.25(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 Mn5Si3structure. The carbon position of the Mo5Si3C1−xstructure is partially filled. The three additional carbon atoms of the formula La5Os3C4−xoccupy distorted octahedral La4Os2voids. 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.