Ionic bonding, boron nitrides

Under high pressure and temperature, boron nitride can be converted to a cubic form. The cubic form of (BN) is known as borazon, and it has a structure similar to that of diamond. Its hardness is similar to that of diamond, and it is stable to higher temperatures. The extreme hardness results from the fact that the B-N bonds possess not only the covalent strength comparable to C-C bonds, but also some ionic stabilization due to the difference in electronegativity between B and N. 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

While metallic solids are deposited by reactions that involve metallic intermediates and ionic solids result from ionic reactions, the solids with covalent bonds grow by means of radical surface reactions. Examples of such materials are diamond, amorphous diamond-like carbon, silicon, and silicon carbide. Diamond and diamond-like carbon can be deposited if hydrocarbon and hydrogen radicals are available at the growing surface. Silicon carbide and boron nitride growth has also been modeled in terms of radical reactions at the surface. 

Bk ring structure of a-boron nitride. Unlike graphite, the layers sit on top of each other are bonded together by a combination of ionic and covalent bonds to make a very strong and inert structure. 

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