**2.3 Wurtzite boron nitride**

Metastable-phase stacking sequence produces additional sp3 - and sp2 -bonded phase forms in polytype crystal structures. For example, the stacking relationship between wurtzite (w-BN) and sp3 -bonded cubic phases of boron nitride is analogous to that between cubic and h-BN. W-BN consists of two layers (0002 planes) structurally identical to the (111) plan c-BN. These ABC stacking arrangements yield additional metastable rhombohedral boron nitride (r-BN) crystal phase [42]. The atomic planer arrangement in w-BN and r-BN is shown in **Figure 3**.

### **2.4 Octahedral model: BN fullerenes**

Ultrathin layer BN (typically <3 nm) forms fullerenes under in situ electron irradiation. The BN fullerenes exhibit B/N stoichiometry of ~1. Also, fullerenes revealed unusual polyhedral electron microscope images based on microscope projection, i.e. *Nested* BN fullerenes and *single* cell fullerenes square-like and rectangle-like outlines, respectively [43]. The structure of BN fullerene is shown in **Figure 4** with the help of the ball and stick model. In **Figure 4**, the black ball represents B and the white ball represents N.

*Boron Nitride Fabrication Techniques and Physical Properties DOI: http://dx.doi.org/10.5772/intechopen.106675*

#### **Figure 3.**

*(a) Sp3 -bonded-phase wBN (lattice constant a = 2.5505A and c = 4.210A) (b) Sp2-bonded-phase rBN (lattice constant a = 2.5042A and c = 9.99A) [42].*

**Figure 4.** *3D octahedral B76N76 molecule in [111] and [211] orientation [43].*

### **2.5 Amorphous BN**

BN exhibits amorphous nature when it reacts with cesium metal. After heat treatment, the amorphous nature is transformed into turbostratic nature (tubular and corpuscles morphology) with a diameter of 3 μm for 50 to 100 μm. The researchers observed that the interlayer distance was 3.5A [44].
