Predicted stable high-pressure phases of copper-nitrogen compounds

The nitrogen-rich compounds are promising candidates for high-energy-density applications, owing to the large difference in the bonding energy between triple and single/double nitrogen bonds. The exploration of stable copper-nitrogen (Cu-N) compounds with high-energy-density has been challenging for a long time. Recently, through a combination of high temperatures and pressures, a new copper diazenide compound (P63/mmc-CuN2) has been synthesized (Binnset al2019J. Phys. Chem. Lett.101109-1114). But the pressure-composition phase diagram of Cu-N compounds at different temperatures is still highly unclear. Here, by combining first-principles calculations with crystal structure prediction method, the Cu-N compounds with different stoichiometric ratios were searched within the pressure range of 0-150 GPa. Four Cu-N compounds are predicted to be thermodynamically stable at high pressures,Pnnm-CuN2, two CuN3compounds with theP-1 space group (named as I-CuN3and II-CuN3) andP21/m-CuN5containing cyclo-N5-. Finite temperature effects (vibrational energies) play a key role in stabilizing experimentally synthesizedP63/mmc-CuN2at ∼55 GPa, compared to our predictedPnnm-CuN2. These new Cu-N compounds show great promise for potential applications as high-energy-density materials with the energy densities of 1.57-2.74 kJ g-1.