Bonding Properties of Aluminum Nitride at High Pressure.

Exploring the bonding properties and polymerization mechanism of stable polymeric nitrogen phases is the main goal of our high-pressure study. The pressure versus composition phase diagram of the Al-N system is established. In addition to the known Fm3̅m phase of AlN, a notable monoclinic phase with N66- anion polymeric nitrogen chains for AlN3 in the pressure range from 43 to 85 GPa is predicted. Its energy density is up to 2.75 kJ·g-1, and the weight ratio of nitrogen is nearly 61%, which make it potentially interesting for the industrial applications as a high energy density material. The high-pressure studies of atomic and electronic structures in this predicted phase reveal that the formation of N66- anion is driven by the sp2 hybridization of nitrogen atoms. The resonance effect between alternating π-bonds and σ-bonds in polymeric nitrogen chains are all responsible for the structural stability. Because of the electrons transfer from aluminums to polymeric nitrogen chains, there is a pseudogap in the electronic structures of AlN3. The N_p electrons form π-type chemical bonds with the neighboring atoms, resulting in the delocalization of π electrons and charge transfer in polymeric nitrogen chains. Furthermore, disparities of charge density distribution between nitrogen atoms in polymeric nitrogen chains are the principal reason for the metallicity.