American Chemical Society
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Binary Boron-Rich Borides of Magnesium: Single-Crystal Investigations and Properties of MgB7 and the New Boride Mg∼5B44

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journal contribution
posted on 2010-12-06, 00:00 authored by Alexis Pediaditakis, Melanie Schroeder, Vanessa Sagawe, Thilo Ludwig, Harald Hillebrecht
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 Mg∼5B44 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 Mg∼5B44 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).