3: The structure of cBN 

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... boron atom) of layer A sits directly above the hexagon (or a nitrogen atom) of layer A in that fashion, hence the h BN is often denoted the AA AA . . . stacking sequence (see fig 3.2) [28, 36, 37]. The A layer notation in h BN means that there is a 180 ◦ rotation in the atomic arrangements of the layer with respect to layer A, while graphite exhibits an ABAB. . . stacking sequence where only half the atoms in the basal layer A are sitting directly above the atoms in layer B, alternating in that fashion [30]. The h BN crystal structure has four atoms in its unit cell. X-ray di ff raction data shows that there exist five possible h BN structures of di ff erent space groups. Among the five possible structures, the P6 3 / mmc structure is most commonly accepted one. It is possible for these five structures to be transformed into each other, by translational shearing of one BN basal layer relative to the other layer in the unit cell, or by rotating the BN basal layer around the c -axis of the crystal [27, 38]. The in–plane B — N distance is 0.145nm and the interlayer distance is 0.333nm [36, 37]. The a lattice parameter of h BN is greater than that of graphite whilst the c parameter is smaller; this reflects on the di ff erent bonding environments between the two compounds; Since the C — C bond is pure covalent it is much stronger and therefore shorter than the B — N bond; whilst the B — N bond is partially ionic and partially covalent, the intralayer (covalent bonding) interaction is reduced and the electrostatic interlayer interaction (Van der Waals interaction) strengthens and hence the interlayer distance c of h BN is shorter than that of graphite [39]. As a direct consequence of the nature of bonding and the crystal structure, h BN has been widely used in vacuum technology and in electronics because it is a good electrical insulator that also has a good thermal conductivity and thermal stability. It has applications in the nuclear energy industry and in research as a refractory material. It is as soft as graphite and hence has been used as a lubricant in many industries. It has been used in almost all the cases of c BN synthesis as a starting material or as a critical component of the precursor material. Also known as borazon, sphalerite – BN or BN; c BN crystallizes in the zincblende lattice structure [24]. The structure, shown in fig 3.3, has a cubic unit cell, containing four boron atoms and four nitrogen atoms. The position of the atoms within the unit cell is identical to the diamond crystal structure; both structures consist of two fcc ...