Structural, Chemical, and Physical Properties of Rare-Earth Metal Rhodium Carbides LnRhC(2) (Ln = La, Ce, Pr, Nd, Sm)

The title compounds were prepared by arc melting and subsequent annealing. LaRhC2 and CeRhC2 melt congruently, while the others are formed by peritectic reactions. LaRhC2 and CeRhC2 crystallize with a tetragonal structure of space group P4(3) (and P4(1)), which was determined from single-crystal X-ray data of CeRhC2 (a = 392.45 (9) pm, c = 1526.0 (7) pm; R = 0.037 for 692 structure factors and 26 variable parameters). PrRhC2, NdRhC2, and SmRhC2 crystallize with the orthorhombic CeNiC2 type structure, which was refined from single-crystal X-ray data of SmRhC2 (space group Amm2, a = 358.91 (6) pm, b = 469.29 (7) pm, c = 656.7 (1) pm; R = 0.027 for 373 F values and 12 variables). The compounds contain C-2 pairs with C-C distances of 139 (2) pm (CeRhC2) and 134 (1) pm (SmRhC2), thus indicating double bonds. The hydrolyses with hydrochloric acid yield mixtures of methane, ethane, propane, and the various isomers of butane, pentane, and hexane but little (LaRhC2) or no (SmRhC2) unsaturated hydrocarbons. LaRhC2 is diamagnetic at room temperature, while the carbides LaRhC2 (Ln = Ce, Pr, Nd) show Curie-Weiss behavior with magnetic moments corresponding to those of the trivalent rare-earth metal ions. SmRhC2 is Van Vleck paramagnetic. The samples of LaRhC2 and CeRhC2 are semiconducting, while those of PrRhC2 and NdRhC2 show metallic behavior. The crystal structures and properties of these carbides are discussed.