Competition between the ionic and covalent character in the series of boron compounds BP, BAs, and BSb

We present first-principles calculations of the bonding properties for the series of boron compounds BP, BAs, and BSb. The plane-wave pseudopotential approach to density functional theory in the local density approximation ha s been used to calculate the equilibrium properties, i.e., the ground-state energy, the lattice constant, the bulk modulus, its pressure derivative, a nd the ionicity factor. The valence electron density is used to study the m odification of the bonding with respect to different pressures. The calcula ted electronic, charge densities present an anomalous behaviour which can b e characterized by reversing the standard assignments for the anion and cat ion in these compounds. The competition between the ionic and the covalent character in these materials is discussed in relation to the charge transfe r. Estimates of the ionicity and its pressure derivative for the series of boron compounds are presented. The distribution of the valence charge densi ty suggests that the bonding in these materials is less ionic than in other zinc-blende compounds.