TERNARY ALCALINE EARTH, LANTHANOID, AND ACTINOID TRANSITION METAL CARBIDES. W. Jeitschko, Anorganisch-Chemisches Institut, Universität Münster, Wilhelm-Klemm-Str. 8, D-48149 Munster, Germany

Some 500 ternary carbides have been prepared during the past ten years. They crystallize with almost 50 different structure types. In many of these the carbon atoms are isolated from each other, even though the carbon content may be high, e.g. in Ho₂Cr₂C₃, UW₄C₄, or U₅Re₃C₈. In others the carbon atoms form pairs as, for instance, in Gd₃Mn₂C₆, La_3.67FeC₆, GdRuC₂, Sc₃RuC₄, Er₈Rh₅C₁₂, and in the many carbides with CeNiC₂-, CeCoC₂-, and UCoC₂-type structures. In Dy₁₀Mn₁₃C₁₈, La₁₂Re₅C₁₅, Er₇Ru₂C₁₁, Ca₄Ni₃C₅, Yb₄Ni₂C₅, Th₄Ni₃C₅, and U₂NiC₃ carbon pairs are found together with isolated carbon atoms. Sc₅Re₂C₇ contains C₃ units derived from propadiene. The C-C bond lengths usually correspond to double bonds, however, the hydrolyses usually result in saturated and unsaturated hydrocarbons ranging from methane to the various isomers of hexane and hexene. The positions of the metal atoms in the carbides UCr₄C₄, LaMn₁₁C₂, Pr₂Mn₁₇C₃, Tb₂Mn₁₇C₃, Pr₂ReC₂, and La₅O₃C₄ correspond to those of the binary structure types MoNi₄, BaCd₁₁, Th₂Zn₁₇, Th₂Ni₁₇, Co₂Si, and Mn₅Si₃. Most of these carbides are metallic conductors, others (LaRhC₂, CeRhC₂, Sc₅Re₂C₇, and ScCrC₂) are semiconductors; Y₂FeC₄ is superconducting below 3.6 K. The tetragonal structure of YCoC is most simple with no variable positional parameter. It contains a linear [-Y-C-]_n^-3n polyanion. In the carbides Pr₂ReC₂, Gd₁₂Ru_7,5C₂₀, Th₂NiC₂, and Th₃Ni₅C₅ the 18-electron rule seems to be obeyed for the transition metal atoms. The structures may be classified by the extension of the transition metal-carbon polyanions, which may be isolated from each other, or they form infinite chains, nets, and three dimensional networks.