Synthesis, crystal structure, and properties of Mg(x)B50C8 or Mg(x)(B12)4(CBC)2(C2)2 (x = 2.4-4)

Single crystals of a new magnesium boride carbide Mg(x)B(50)C(8) (x = 2.4-4) were synthesized from the elements in a metallic melt using tantalum ampules. Crystals were characterized by single crystal X-ray diffraction and electron microprobe analysis. The variation of the Mg content results from different reaction conditions. The composition Mg(∼3)B(50)C(8) is by far the most favored. It fulfills the electron counting rules of Wade and Longuet-Higgins and thus explains the light-green to yellow transparent color. The structure of Mg(∼3)B(50)C(8) (C2/m, Z = 1, a = 8.9384(12) Å, b = 5.6514(9) Å, c = 9.6021(13) Å, β = 105.86(1)°) consists of B(12) icosahedra. The icosahedra are interconnected by four exohedral B-B bonds to layers. The layers are connected to a three-dimensional covalent network by C(2) and CBC units and further exohedral B-B bonds. The Mg sites are partially occupied. Different site occupation factors cause the various compositions and colors (Mg(2.4)B(50)C(8), brown; Mg(4)B(50)C(8), black). The vibrational spectra show the modes of B(12) icosahedra and C(2) and CBC units as well. Measurements of the microhardness according to Vickers and Knoop revealed remarkably high values of H(V) = 3286 (32.0 GPa) and H(K) = 3165 (31.5 GPa), which exceed the values of B(4)C. Optical spectra reveal a band gap of 2.7 eV for Mg(∼3)B(50)C(8), in agreement to the observed color. This justifies an ionic description, and the formula can be written as (Mg(2+))(3)(B(12)(2-))(4)(CBC(+))(2)(C(2))(2).