Considering the tetragonal crb structures of boron pnictides, it is important to establish the relationship between crystal chemistry and crystal structure due to the expected change in The crb C 8 (Fig. ), made of a single chemical element, shows a purely covalent character with a tetrahedral charge density around the atoms. This is very similar to diamond with the difference that, unlike crb C 8 , the environment is perfectly tetrahedral, i.e. not distorted. In crb BN (Fig. ) with electronegativity difference of the constituents (B-N) = -1, a concentration of charge density around N (grey spheres) is observed, so it can be considered as "boron nitride". For crb BX (X = P, As), (Fig. ) with a small electronegativity difference (B-X) ≈ -0.15, the situation changes with respect to the nitride, and the charge density is almost halfway between B and X. Finally, in BX (X = Sb, Bi) (Fig. ), the boron electronegativity is close to that of Sb and higher than that of Bi. Both compounds are rather borides, i.e. with negatively charged boron, as can be observed with the yellow volumes around boron (green spheres).