Novel Phase of AlN₄ as a Possible Superhard Material

Nitrides have attracted great attention due to their outstanding applications as superconductors, high-energy materials, and hard materials. Utilizing particle swarm optimization algorithm on crystal structure prediction in combination with first-principle calculations, a novel stable stoichiometry, AlN4 with space group P31c, is thermodynamically stable above 123 GPa and remains metastable at ambient conditions. Structurally, AlN4 is intriguing with the appearance of N8 with distorted double triangular conical columns and Al atoms sharing the same lattice hP4 with sodium. The metastable of AlN4 stems from the inherent stability of three-dimensional covalent bonded nets from N8 and Al–N8. The new aluminum nitride with hardness of 45.7 GPa by Gao’s model is a potential hard material. Interestingly, the band gap of AlN4 is becoming wider with increasing pressure. Our work reveals a new form of N cluster and also provides a key perspective toward the understanding of novel chemical bonding in nitrogen-rich...