Synthesis, Crystal Structure, and Properties of Mg_(x)B_(50)C_(8) or Mg_(X)(B_12)4(CBC)_2(C_2)_2 (x= 2.4-4)

摘要

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) A, b= 5.6514(9) A, c= 9.6021(13) A, 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 vari ous compositions and colors (Mg_(2.4)B_(50)C_8, brown; Mg_4B_(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_4C. 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.