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

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 (H_V = 2125, H_K = 2004) and MgB₁₂ (H_V = 2360, H_K = 2459).