Synthesis, Crystal Structure, and Properties of Mg_xB₅₀C₈ or Mg_x(B₁₂)₄(CBC)₂(C₂)₂ (x = 2.4−4)

Single crystals of a new magnesium boride carbide Mg_xB₅₀C₈ (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_∼3B₅₀C₈ 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_∼3B₅₀C₈ (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 B₁₂ icosahedra. The icosahedra are interconnected by four exohedral B−B bonds to layers. The layers are connected to a three-dimensional covalent network by C₂ 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_2.4B₅₀C₈, brown; Mg₄B₅₀C₈, black). The vibrational spectra show the modes of B₁₂ icosahedra and C₂ 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 B₄C. Optical spectra reveal a band gap of 2.7 eV for Mg_∼3B₅₀C₈, in agreement to the observed color. This justifies an ionic description, and the formula can be written as (Mg²⁺)₃(B₁₂²⁻)₄(CBC⁺)₂(C₂)₂.