Central and noncentral forces on the lattice dynamics of boron-rich solids - The bonding nature of icosahedral boron solids

The distinct feature of the chemical bonding of boron solids is three-center or icosahedral bonding. The consequences of this peculiar bonding on the lattice dynamics are discussed in terms of the central and noncentral forces, while the recent development in this field is reviewed. The character of noncentral force is very small for the intraicosahedral bonding, Many features of the optical spectra of alpha-boron can be understood without the help of large noncentral forces of the intraicosahedral bond. The intericosahedral bonding is basically a covalent bond, but is less stable than the usual covalent bond based on the sp(3) hybridization. The balance between these forces has a significant role on the determination of the crystal structure of boron carbide. The icosahedral character further decreases in boron carbide. Roles of the noncentral forces become important for this crystal. A discussion of the mechanical stability provides a reasonable account for the following features of the a-boron modifications: the resistance for shear strains, the deviation of the rhombohedral site from the ideal lattice position, and the relaxation for a strain along the c-axis. A variety of boron modifications exhibit various types of distortions of icosahedra. Such distortions are not the cause of the different crystal modifications, but are the consequence of the external influences. (C) 1997 Academic Press.