Composition-dependent bonding and hardness of γ-aluminum oxynitride: A first-principles investigation

Spinel phase aluminum oxynitride solid solution (γ-alon, with formula of Al(8+x)/3O4−xNx) exists in the narrow Al2O3-rich region of Al2O3-AlN systems. The first-principles calculations were developed to investigate the composition-dependent bonding and hardness of γ-alon. Six supercell model for Al(8+x)/3O4−xNx (x = 0, 0.25, 0.44, 0.63, 0.81, and 1) was constructed to perform our calculations with high accuracy. It was found that the lattice constant increases with increasing composition of nitrogen in γ-alon. The bond lengths of AlIV–O, AlVI–O, AlIV–N, and AlVI–N all increase with the expansion of crystal structure. The well-known Mulliken overlap populations were calculated to estimate the bonding and hardness. As the content of nitrogen substitute increases, the Al–N bonds present more covalent characteristic, while the Al–O bonds present more ionic characteristic. The AlIV–N is the hardest bond in γ-alon. The theoretical hardness of γ-alon could be slightly enhanced from 17.16 GPa to 17.97 GPa by incr...