Boron nitride cubic polymorph

Boron nitride (BN) is a versatile ceramic that is found and used in various crystallographic and geometrical forms. Although there are chemical similarities, the conditions for the synthesis and fabrication of each form (hexagonal, cubic, polymorphs of BN, fibers, two-dimensional forms and composites) have different requirements and are... 

Heating the layered form of BN at rj2000 K and >50 kbar pressure in the presence of catalytic amounts of Li3N or MgsN2 converts it to a more dense polymorph, cubic-BN, with the zinc blende structure (see Section 6.11). Table 13.2 shows that the B—N bond distance in cubic-BN is similar to those in R3N-BR3 adducts and longer than in the layered form of boron nitride this further supports the existence of 7r-bonding within the layers of the latter. 

Structurally, the cubic form of BN resembles diamond (Figure 6.19) and the two materials are almost equally hard. Crystalline cubic BN is called borazon and is used as an abrasive. A third polymorph of boron nitride with a wurtzite-type structure is formed by compression of the layered form at 12 kbar. 

BORON NITRIDE. BN. A highly refractory material with physical and chemical properties similar to carbon. Graphite-like (g-BN), wurzite (w-BN) and zinc blende (z-BN) are known polymorphs of BN corresponding to the graphite (hexagonal) and diamond (cubic) structures. Transformation of g-BN to w-BN occurs at pressures above 12 GPa at relatively low temperature (230°C). 

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