The Two Major Structures of Boron Nitride

The structural analogy with the carbon allotropes can be extended to the properties of the two borrm-nitride allotropes. For instance, hexagonal boron nitride (h-BN) is soft and lubricious like grsqihite, and cubic boron nitride (c-BN), like diamond, is extremely hard. 

No boron-nitrogen phase diagram is presendy available but a tentative pressure-temperature diagram is shown in Fig. 12.1.1 1 This diagram suggests that c-BN (widi the zincblende structure) is stable below 225°C while h-BN is stable above 225 C and melts at approximately 3000°C. A second cubic structure (with the wurtzite structure) is stable only at pressures 11 GPa. 

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