UCr4C4 with filled MoNi4 type structure

The title compound was prepared by arc melting coldpressed pellets of the elemental components with subsequent annealing at both 800 oC or 1100 oC. UCr4C4 crystallizes tetragonal, space group I4/m, a = 0.79363(4)nm, c = 0.30754(3)nm, V = 0.19470nm3 with Z = 2 formula units per cell. The structure was determined from single-crystal X-ray data and refined to a residual of R = 0.027 for 16 variable parameters and 279 strucute factors. The positions of the metal atoms correspond to those of the MoNi4 type structure. The carbon atoms occupy octahedral vois formed by four chromium and two adjacent uranium atoms. Chemical bonding in UCr4C4 and in other interstitial compounds is briefly discussed. The average valence electron number of the metal atoms is usually greater for the unfilled (host) structure than for the corresponding filled structure. (Author)