Graphite and hexagonal boron-nitride have the same interlayer distance. Why?

Graphite and hexagonal boron nitride (h-BN) are two prominent members of the family of layered materials possessing a hexagonal lattice structure. While graphite has nonpolar homonuclear C-C intralayer bonds, h-BN presents highly polar B-N bonds resulting in different optimal stacking modes of the two materials in the bulk form. Furthermore, the static polarizabilities of the constituent atoms considerably differ from each other, suggesting large differences in the dispersive component of the interlayer bonding. Despite these major differences, both materials present practically identical interlayer distances. To understand this finding, a comparative study of the nature of the interlayer bonding in both materials is presented. A full lattice sum of the interactions between the partially charged atomic centers in h-BN results in vanishingly small contributions to the interlayer binding energy. Higher order electrostatic multipoles, exchange, and short-range correlation Kohn-Sham contributions are found to be very similar in both materials and to almost completely cancel out by the kinetic energy term, which partly represents the effects of Pauli repulsions, at physically relevant interlayer distances, resulting in a marginal effective contribution to the interlayer binding. Further analysis of the dispersive energy term reveals that despite the large differences in the individual atomic polarizabilities, the heteroatomic B-N C ₆ coefficient is very similar to the homoatomic C-C coefficient in the hexagonal bulk form, resulting in very similar dispersive contribution to the interlayer binding. The overall binding energy curves of both materials are thus very similar, predicting practically the same interlayer distance and very similar binding energies. The conclusions drawn here regarding the role of electrostatic interactions between partially charged atomic centers for the interlayer binding of h-BN are of a general nature and are expected to hold true for many other polar layered systems.