Structure of Hexagonal Boron Nitride

Crystal Structure. Hexagonal boron nitride is composed of series of stacked parallel layer planes shown schematically in Fig. 12.2. In this figure, the circles showing the position of the nitrogen and boron atoms do not represent the actual size of these atoms. In fact, each atom contacts its neighbors. 

Within the layer plane, eadr nitrogen atom is bonded to three boron atoms (and vice versa) with a short bond length (0.1446 tun), forming a series of continuous hexagons in what can be considered as an essentially 

In contrast, the bond between the planes of h-BN is very weak and even weaker than that of graphite. It is readily broken and layers can be cleaved with a knife like an onion skin. However this bond is electronically different from that in graphite. In graphite, it stems from the hybridized fourth valence electron which is paired with another delocalized electron of the adjacent plane by a weak van der Wools bond (pi bond). The high electrical conductivity of graphite is attributed to these delocalized electrons. 

In h-BN, no free electrons are available since the corresponding p orbitals in the boron atom are vacant and those of the nitrogen atom are occupied by two electrons. Overlap to form pi bonds is not possible. In addition, the relatively large difference in electronegativity between nitrogen (3.0) and boron (2.0) imparts greater localization of pi electrons than in gr hite. As a result, boron nitride is an electrical insulator. OH3] 

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FIGURE 14.27 (a) The structure of hexagonal boron nitride, BN, resembles that of graphite, consisting of flat planes of hexagons of alternating B and N atoms (in place of C atoms but, as shown for two adjacent layers in part (b), the planes are stacked differently, with each B atom directly over an N atom and vice-versa (compare with Fig. 14.29). Note that (to make them distinguishable) the B atoms in the top layer are red and the N atoms blue. 

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