Carbides Ln₁₀Ru₁₀C₁₉ (Ln = Y, Gd-Lu): Crystal Structure of their Subcells and the Superstructures of Er₁₀Ru₁₀C₁₉

The nine new rare earth ruthenium carbides (10/10/19) were prepared by arc-melting of the elemental components and subsequent annealing at 1173 K. Guinier powder patterns show these compounds to crystallize with a very pronounced subcell, which was solved from the single-crystal X-ray diffractometer data of Er₁₀Ru₁₀C₁₉, erbium ruthenium carbide: Amm2, Z = 1, R = 0.031 for 1088 structure factors and 39 variable parameters. The single-crystal film data of Er₁₀Ru₁₀C₁₉ reveal several superstructures. Two of these were refined from the single-crystal diffractometer data of multiple domain crystals; one of these in the space group Cm, Z = 2, R = 0.057 for 4848 F values and 147 variables; the other in Amm2, Z = 8, R = 0.078 for 2352 F values and 171 variables. In all of these structures most of the C atoms are paired with C-C distances corresponding to double bonds. Together with the Ru atoms, these C atoms form two-dimensionally infinite layers, which are separated from each other by the Er atoms. In the third dimension the ruthenium-carbon layers are linked by single C atoms and carbon pairs. The subcell shows the superposition of these isolated and paired C atoms, whereas in the idealized superstructures these C atoms are fully ordered and their atomic environments reflect this order. Lu₁₀Ru₁₀C₁₉ is a metallic conductor and Pauli-paramagnetic. The carbides Ln₁₀Ru₁₀C₁₉, with Ln = Gd-Tm, show Curie-Weiss behaviour with magnetic moments corresponding to the free Ln³⁺ ions. The magnetic ordering temperatures are all below 35 K. Chemical bonding in these compounds can be rationalized on the basis of simple concepts assuming the octet and the 18-electron rules to be valid for the C and Ru atoms, respectively.

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