Alkaline-earth metal (Mg) polynitrides at high pressure as possible high-energy materials

The high-pressure structural evolutionary behaviors of magnesium polynitrides were studied up to 100 GPa using first-principles calculations. Using the unbiased structure searching method, five stable chemical stoichiometries of magnesium polynitrides (MgN, Mg₂N₃, MgN₂, MgN₃, and MgN₄) were theoretically predicted at high pressures. The predicted MgN_x compounds contain a rich variety of polynitrogen forms ranging from charged molecules (one-dimensional bent molecules N₃, planar triangle N₄ to benzene-like rings N₆) to extended polymeric chains (N_∞). To the best of our knowledge, this is the first time that stable bent molecules N₃, planar triangle N₄, and polymeric chains (N_∞) were predicted in alkaline-earth metal polynitrides. The decomposition of P1[combining macron]-MgN₃ and P1[combining macron]-MgN₄ are expected to be highly exothermic, releasing an energy of approximately 2.83 kJ g^-1 and 2.01 kJ g^-1, respectively. Furthermore, P1[combining macron]-MgN₄ can be synthesized at several GPa. The results of the present study suggest that it is possible to obtain energetic polynitrogen in main-group nitrides under high pressure.