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 (P6(3)/mmc-CuN2) has been synthesized (Binns et al 2019 J. Phys. Chem. Lett. 10 1109-1114). 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 CuN3 compounds with the P-1 space group (named as I-CuN3 and II-CuN3) and P2(1)/m-CuN5 containing cyclo-N-5 (-). Finite temperature effects (vibrational energies) play a key role in stabilizing experimentally synthesized P6(3)/mmc-CuN2 at similar to 55 GPa, compared to our predicted Pnnm-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).

Automated summary

The nitrogen-rich compounds are promising candidates for high-energy-density applications. A new copper diazenide compound (CuN2) has been synthesized through high-temperature and high-pressure conditions, but the pressure-composition phase diagram of these compounds at different temperatures is still unclear. By combining first-principles calculations with crystal structure prediction method, we searched for stable Cu-N compounds within the pressure range of 0-150 GPa and identified four thermodynamically stable compounds, showing great promise as high-energy-density materials.