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, MgN, MgN, MgN, and MgN) were theoretically predicted at high pressures. The predicted MgN compounds contain a rich variety of polynitrogen forms ranging from charged molecules (one-dimensional bent molecules N, planar triangle N to benzene-like rings N) to extended polymeric chains (N). To the best of our knowledge, this is the first time that stable bent molecules N, planar triangle N, and polymeric chains (N) were predicted in alkaline-earth metal polynitrides. The decomposition of P1[combining macron]-MgN and P1[combining macron]-MgN are expected to be highly exothermic, releasing an energy of approximately 2.83 kJ g and 2.01 kJ g, respectively. Furthermore, P1[combining macron]-MgN 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.