Structural Design and Physical Properties of Gallium Nitrides under High Pressures

Polymeric nitrogen at high pressures has been proven to be a promising candidate for the design and discovery of high-energy-density materials as well as hard materials. Here, we report predicted high-pressure structures and physical properties of gallium nitrides using an effective unbiased structure searching method combined with first-principles calculations. Several unconventional stoichiometries of GaN2, GaN5, and GaN8 compounds were predicted to be stable at a moderate pressure. The results reveal that GaN5 with a N content of 83.3%, which is thermodynamically stable at 62.3 GPa by squeezing solid GaN and N2 gas, is likely to be recovered as a metastable multifunctional material at ambient pressure. Given a computed energy density of ∼3.85 kJ/g, a Vicker's hardness of ∼30 GPa, and an indirect band gap of ∼2.1 eV, GaN5 is a potentially functional material with high-energy-density, hard, and photovoltaic properties. The present results open a new avenue to synthesize multifunctional polynitrogen compounds.