A Novel Boron-Rich Scandium Borocarbide; Sc4.5-xB57-y+zC3.5-z(x=0.27, y=1.1, z=0.2)

A novel ternary boron-rich scandium borocarbide Sc4.5-xB57-y+zC3.5-z (x=0.27, y=1.1, z=0.2) was found. Single crystals were obtained by the floating zone method by adding a small amount of Si. Single-crystal structure analysis revealed that the compound has an orthorhombic structure with lattice constants of a=1.73040(6), b=1.60738(6) and c= 1.44829(6) nm and space group Pbam (No. 55). The crystal composition ScB13.3C0.78Si0.008 calculated from the structure analysis agreed with the measured composition of ScB12.9C0.72Si0.004. The orthorhombic crystal structure is a new structure type of boron-rich borides and there are six structurally independent B12 icosahedra I1 --I6, one B8/B9 polyhedron and nine bridging sites all which interconnect each other to form a three-dimensional boron framework. The main structural feature of the boron framework structure can be understood as a layer structure where two kinds of boron icosahedron network layer L1 and L2 stack each other along the c-axis. There are seven structurally independent Sc sites in the open spaces between the boron icosahedron network layers.