Crystal Structure and Properties of Novel Quaternary Actinoid Boron Carbides U2ScB6C3 and Th2ScB6C3

The crystal structure of a novel quatenary compound Th2ScB6C3 was derived by combined Rietveld-Patterson methods from room temperature X-ray powder diffractometiy. Th2ScB6C3 crystallises with a unique structure type (space group P6/mmm) with a = 0.660296(7), c = 0.358421(4) nm. Metal atoms form a planar hexagonal net of thorium atoms centered by smaller scandium atoms. Non-metal atoms are found in planar Kagome nets 6B.(6B+3C)2 at z = 1/2. The non-metal layer corresponds to the one in the ThB2C-type and forms a link to the simple structure of the AlB2-type which has recently become the focus of intense interest due to discovery of superconductivity (Tc = 39 K) in MgB2. Physical properties (magnetization, specific heat, resistivity) of Th2ScB6C3 indicate metallic characteristics. Low temperature measurements reveal no evidence for a superconductive transition in this new quatenary compound. The magnetization of isostructural U2ScB6C3 is also investigated and a ferromagnetic transition is observed at Tc = 56 K.