ON THE INSERTION OF CARBON-ATOMS IN B-12 ICOSAHEDRA AND THE STRUCTURAL ANISOTROPY OF BETA-RHOMBOHEDRAL BORON AND BORON-CARBIDE

X-Ray diffraction of beta-rhombohedral boron doped with up to 1 at.% carbon and a model calculation of the anisotropic distortion of a single icosahedron were carried out. The carbon atoms substitute for boron, but only at polar sites of the B-12 icosahedra. Compared with a boron atom, the carbon atom is shifted by 0.115 angstrom (6.5%) towards the centre of the icosahedron. The application of the model calculation on boron carbide using structural data of other authors, partly confirmed by our own measurements, shows that the distortion of the unit cell of boron carbide parallel to c exceeds that of the icosahedron by far, while the distortions are similar for the a axis. These are double the corresponding distortion in beta-rhombohedral boron. The effect of elementary cells of boron carbide without chains on the distortion of the structure is evident; the effect of an electron-phonon interaction seems possible. The anisotropy of the structural modifications is immediately correlated with the changes in the concentrations of structural elements.