Binary Boron-Rich Borides of Magnesium: Single-Crystal Investigations and Properties of MgB7 and the New Boride Mg5B44

Single crystals of dark-red MgB7 were grown from the elements in a Cu-melt. The crystal structure (Pearson symbol oI64; space group Imma; a = 10.478(2) Å, b = 5.977(1) Å, c = 8.125(2) Å, 2842 reflns, 48 params, R1(F) = 0.018, R2(I) = 0.034) consists of a hexagonal-primitive packing of B12-icosahedra and B2-units in trigonal-prismatic voids. According to the UV−vis spectra and band structure calculations MgB7 is semiconducting with an optical gap of 1.9 eV. The long B−B distance of 2.278 Å within the B2-unit can be seen as a weak bonding interaction. The new Mg5B44 occurs beside the well-known MgB12 as a byproduct. Small fragments of the black crystals are dark-yellow and transparent. The crystal structure (Pearson symbol tP196, space group P41212, a = 10.380(2) Å, c = 14.391(3) Å, 4080 reflns, 251 params, R1(F) = 0.025, R2(I) = 0.037) is closely related to tetragonal boron-II (t-B192). It consists of B12-icosahedra and B19+1-units. With a charge of −6 for the B19+1-units and a Mg-content of 20 Mg-atoms per unit cell the observed Mg content in Mg5B44 is quite close to the expected value derived from simple electron counting rules. All compositions were confirmed by EDXS. The microhardness was measured on single crystals for MgB7 (HV = 2125, HK = 2004) and MgB12 (HV = 2360, HK = 2459).