DISTRIBUTION OF CARBON-ATOMS ON THE BORON-CARBIDE STRUCTURE ELEMENTS

Optical absorption and reflectivity measurements in the spectral range of the 1580 cm-1 phonon, the stretching mode of the central atom of the three-atom chain in the unit cell of boron carbide, were simultaneously fitted by suitable model calculations to determine quantitatively the carbon distribution within the unit cell in the homogeneity range. For example, in B4.3C, which is confirmed to be the carbon-rich limit composition, there are 81% C-B-C and 19% C-B-B chains, and exactly one carbon atom substitutes an icosahedral site. The concentrations of the C-B-C and C-B-B chains change abruptly between 11 and 14 at.% C, while the concentration of the carbon atoms in the icosahedra varies non-monotonically. The reduction in the carbon content towards the boron-rich limit of the homogeneity range is accompanied by an increasing number of unit cells without any chains. This number exceeds even 50% at about 8 at.% C. These results allow us to explain easily the variation in several structure and disorder-dependent physical properties of boron carbide and they disprove certain fundamental model assumptions. The force constants of the C-B and B-B bonds in the chain were determined. At B4.3C they have the same value of 5.6(1) mdyn angstrom-1; at lower carbon contents they differ.