Prediction of structure of Co-N and Nb-N crystals at high pressure.

 The new structures of Co-N and Nb-N crystals are predicted at high and ambient pressures using evolutionary algorithms and DFT calculations. Thermo-dynamical stability of the structures is evaluated using the construction of a convex hull. Calculated phonon dispersion provides evidence for the dynamic stability of the crystals. Calculation of vibrational spectra of the predicted structures was used for comparison with experimentally found structures Co-N [1] and Nb-N. The predicted structure CoN5 with C2/c space group and polymeric N chains is in excellent agreement with synthesized solids. Crystals of CoN₂ and NbN₂ are found to be thermodynamically stable at high pressure, but dynamically unstable at 0 K. Calculated elastic constant of the nitrides were used for understanding of mechanical stability of the predicted crystals. The calculated energy density of the predicted crystals is about 40-50% of that of TNT. The estimates of the energy density are made assuming a certain decomposition path to the structure at ambient pressure. Typically, this structure is found as the result of relaxation of the high-pressure phase at ambient pressure. Here the structure at ambient pressure was also found in the results of the structure search using evolutionary algorithms.

[1] H. Chen, M. Bykov, I. G Batyrev, L. Brüning, E. Bykova, M. F Mahmood et al.,  Chemistry – A European Journal, 30, Issue 32, e202400536 (2024).