COMPOSITION AND STRUCTURE OF ALUMINUM NITRIDE

Aluminum nitride (AIN) is the only major compound known in the aluminum-nitrogen system (a nitrogen rich phase, AIN9 is also reported). It is formed by the reaction of NH3 and AIH3 and may be a stable phase of the condensed Al-N system. ) 

The structure of aluminum nitride is normally hexagonal close-packed (hep) of the wurtzite (2H) type (hP4) and is shown in Fig. 12.4. The difference between this structure and the zincblende structure of cubic boron nitride shown in Fig. 12.3 should be noted (the so-called chair form vs the boat form). 

The lattice parameters of AIN are = 0.3114 nm and Cg = 0.4986 nm. This structure is similar to that of a-SiC (2H) described in Sec. 3.4 and shown in Fig. 7.5 of Ch. 7. It is essentially isotropic and, like that of c-BN, it can be visualized as a stacking of puckered infinite 111 layers or as two ce-centered interpenetrating cubic sublattices, one consisting entirely of aluminum atoms and the other entirely of nitrogen atoms. 

The less frequent and closely related zincblende cubic structure (cF8) has also been reported (similar to the structure of c-BN shown in Fig. 12.3). The common wurtzite structure is transformed into the zincblende structure at elevated pressure.l l Also recently reported is a cubic (NaCl) phase with a calculated lattice constant of0.3982 nm which is formed at high pressure (16 GPa).[ l 

Each aluminum atom is bonded to four nitrogen atoms in a fourfold coordinated tetrahedral sp ) arrangement. As discussed in Sec. 3.3 above, the bonding is mainly covalent but a large degree of ionicity is retained. The calculated covalent bond energy Eg is 9.42 eV and die ionic bond energy Ep is 4.09 eV.[ l 

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