Crystal structures and physical properties of the ternary carbides ScT1–xC2(T = Fe, CO, Ni)

Abstract

The crystal structures of the title compounds, originally determined for ScCoC₂, have been refined from single-crystal X-ray data to residuals of R= 0.037 (154 structure factors), R= 0. 031 (433 F values) and R= 0.049 (161 F values) each with 12 variable parameters, for the Fe, Co and Ni compounds, respectively. The C–C distances in the carbon pairs are 138.1(9), 139.7(3) and 135(2) pm. They are thus close to the double-bond distance of 134 pm in alkenes. The refinement of the occupancy parameters showed that the iron, cobalt and nickel positions are occupied to only 88, 88 and 83% in the crystals used for the structure refinements. In general, interatomic distances in the three compounds are very similar. This is rationalized by the counterbalancing effects of the electron counts and electronegativity of the transition-metal component. The structure consists of two-dimensional infinite layers of composition ScC₂ which have a CaC₂-like structure. These layers are separated by sheets of the transition elements, possibly with two-dimensional metallic conductivity. Compact polycrystalline samples of the three compounds are very weakly paramagnetic and show semiconducting behaviour.