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.% c arbon and a model calculation of the anisotropic distortion of a singl e 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 o n boron carbide using structural data of other authors, partly confirm ed 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 t he 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 imm ediately correlated with the changes in the concentrations of structur al elements.