Theoretical Studies On Crystal Structure And Physical Properties Of Magnesium Azides Under High Pressure

The rapid development of high-pressure opened up a new dimension for the research of condensed matter physics.The design and synthesis of the novel super-hard and nitrogin-containing high-energy materials at high-pressure is one of the most attractive topics in current condensed matter physics and material science In this thesis,we carried out swarm intelligence structure searching technique in conjunction with first-principles calculations to investigated the the high-pressure properties of new magenium azides.In the pressure range 0 to 100 GPa,two pressure-induced structural transfor-mations are unraveled,following the phase sequence of ambient-pressure Cmmm?a-phase?? C₂/m??-phase?? P-1??-phase?.In ?-and ?-phase,the bonding styles of N atoms are both dumbbell-like.For the ?-phase,it is observed as six-membered rings.The structure transitions are accompanied with abrupt volume collapses.The predicted of ?-and ?-phases are metals with conduction band and valence band near the Fermi level,while the ?-phase is a wide direct-gap semiconductor with a band gap of 2.033 eV.The three phases are all mechanically stable by the calculation of elastic con-stants.More over,the hardness calculation shows that a-phase has the biggest Vicker's hardness of 60 GPa.The N-N bond lengths of six-membered rings in ?-phase are 1.352 A,1.357 A and 1.358A,respectively,which are longer than that of double bond but shorter than that of single bond.The decomposition of 7-phase is expected to be highly exothermic with approximately 3.99 KJ/g.So this phase is promising in high energy density application.Based on the research of MgN₃ in high pressure,we find MgN₃ possess very diverse and interesting structures and exhibits useful physical properties which is a useful reference for research of new materials in high pressure.