Crystal structure, magnetic susceptibility and electrical conductivity of the uranium silicide carbides U3Si2C2 and U20Si16C3
Abstract
The carbides U3Si2C2 and U20Si16C3 have been prepared in well crystallized form from the elemental components by arc-melting and subsequent annealing slightly below the melting point in a high-frequency furnace. The crystal structure of U3Si2C2, previously reported with the composition ‘U3Si2C3’, was determined from single-crystal X-ray data and refined to a residual of R= 0.026 for 236 structure factors and 12 variable parameters. It is of a new type with tetragonal symmetry: I4/mmm, a= 357.35(5) pm, c= 1888.2(3) pm, V= 0.2411 nm3, Z= 2. The uranium and silicon atoms form a distorted body-centred cubic arrangement. The carbon atoms are on octahedral interstitial sites with five uranium and one silicon neighbours. The Si—C bond distance of 193(2) pm corresponds to a single bond. The compound may therefore be written as (3U4+)12+(2SiC6–)12–. The crystal structure of U20Si16C3 is confirmed by a refinement from single-crystal diffractometer data: P6/mmm, a= 1037.7(2) pm, c= 800.5(1) pm, Z= 1, R= 0.036 for 199 F values and 24 variables. Magnetic susceptibility measurements indicate spin–glass behaviour and a strongly temperature-dependent paramagnetism for U3Si2C2 and U20Si16C3, respectively. Both compounds are metallic conductors.