CRYSTAL-STRUCTURE AND PHYSICAL-PROPERTIES OF THE CARBIDES UAL3C3 AND YBAL3C3

UAl3C3 - previously described with the tentative composition UAl5C4 - was prepared from the elemental components by arc melting. The new compound YbAl3C3 was obtained best in a lithium flux, which subsequently was dissolved in ethanol. Both carbides are isotypic with ScAl3C3. Their hexagonal lattice constants are a = 339.88(5) pm, c = 1711.3(2) pm and a = 338.9(1) pm, c = 1739.4(3) pm for the ytterbium and uranium compound respectively. The crystal structure of UAl3C3 was refined from single-crystal X-ray data to a residual of R = 0.039 for 231 structure factors and 12 variable parameters. Mass spectroscopy analyses of the hydrolyses products of YbAl3C3 and UAl3C3 with 2N hydrochloric acid essentially show only methane. Magnetic susceptibility measurements with a SQUID magnetometer indicate antiferromagnetism with Neel temperatures T(N) = 8 +/- 1 K (YbAl3C3) and T(N) = 13 +/- 1 K (UAl3C3). At lower temperatures the uranium compound shows metamagnetic behaviour. Electrical conductivity measurements of a single crystal of YbAl3C3 indicate semiconductivity.