Abstract
The rhombohedral elementary cell of the complex boron carbide structure is composed of B12 or B11C icosahedra and CBC, CBB or B□B (□, vacancy) linear arrangements, whose shares vary depending on the actual chemical compound. The evaluation of the IR phonon spectra of isotopically pure boron carbide yields the quantitative concentrations of these components within the homogeneity range. The structure formula of B4.3C at the carbon-rich limit of the homogeneity range is (B11C) (CBC)0.91 (B□B)0.09 (□, vacancy); and the actual structure formula of B13C2 is (B12)0.5(B11C)0.5(CBC)0.65(CBB)0.16 (B□B)0.19, and deviates fundamentally from (B12)CBC, predicted by theory to be the energetically most favourable structure of boron carbide. In reality, it is the most distorted structure in the homogeneity range. The spectra of natBxC make it evident that boron isotopes are not randomly distributed in the structure. However, doping with 2% silicon brings about a random distribution.
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