Structure And Physical Properties Of Barium Nitrogen Crystal Under High Pressure

High-pressure physics is a science that explores the structure and physical properties of matter under high pressure,high-pressure technology is one of the most important techniques for designing novel materials.In this paper,the crystals of BaN5 are predicted by using the CALYPSO structure exploration technique com-bined with the pseudo-potential plane wave method that based on the first-principles of density functional theory.The physical properties and crystal structures are sys-tematically studied.In the pressure range of 0-100 GPa,with the increase of pressure,the BaN5 crystal undergoes structural phase transition at the pressure of 34 GPa,which is converted from the ?-phase(the space group of P1)to the ?-phase(the space group of Cmm2).During the phase transition,the bonding form of the nitrogen atoms changes significantly,Two nitrogen atoms form a dumbbell-shaped nitrogen pair(N2),and three nitrogen atoms form a chain of N3 are found in a-phase and?-phase contain an exceptionally high content of the five-ring shaped of N5.The phase transition of BaN5 crystals occurs and the structure collapses,the collapse ratio is 5.61%,due to the abrupt volume during the phase transition,so the phase transition belongs to the first order phase transition.Phonon calculation carried out no imaginary modes in the a and ?-phase of the BaN5,suggesting that it is dynamically stable at studied pressure region.Analysis of the electronic properties of the predicted crystal structure shows that there is a weakly band gap occurred at the fermi level in the a-phase.After the HSE hybrid functional modification,the band gap of the ?-phase is 0.46 eV,which shows a narrow band-gap semiconductor.The valence band and the conduction band of the ?-phase overlap at the Fermi level,and the ?-phase exhibits metallic.In the a-phase,the bond order between N-N is 2 and 3,and the bond order between N-N in ?-phase are 1.284,1.491 and 1.369,which indicates that the N-N bond in?-phase is more easily dissociated,releases more energy,and,?-phase is high energy-density material.The hardness calculations show that the Vickers hardness in the?-phase and ?-phase are 0.816 GPa and 3.934 GPa respectively,so a-phase and ?-phase are not suitable for the hardness material.