Further illustration of electronic / crystal structure relationship focusing on the tetrahedral stacking can be provided by the charge density projections. Figs. ) show for the three polymorphs the charge density yellow volume within the BP 3 and PB 3 tetrahedra, skewed toward P as expected from the higher electronegativity of P (χ =2. ) versus B (χ =2.03). Due to high electronegativity of N (χ = 3.04) in the case of polar covalent B-N bonding , the character of B-P bond is expected to be more covalent. For the sake of comparison, Fig. presents the sp 2 -like planar charge density in 2D rh-BP (Fig. ).