| Nitrogen-rich energetic crystals,as a new energetic material,are widely used in both national military,aerospace industry and civilian applications and attract great attention.A shock wave passes through an energetic crystal during the detonation and generates a pressure of 50 GPa.Under this extreme condition,the nitrogen-rich energetic crystal would undergo structure transformations,phase transitions or decomposition.Therefore,the study of high-pressure behavior for energetic nitrogen-rich crystals is helpful to analyze the transformation of molecular configurations and intermolecular interactions,and contributes to explore the stability of energetic nitrogen-rich crystals.In this study,the density functional theory(DFT)periodic calculations was used to investigate the high pressure behavior of nitrogen-rich energetic crystal 4-amino-3,7-dinitrotriazolo-[5,l,c][l,2,4]triazine(DPX-26)and co-crystal designed with 3,4-diaminofurazan(DAF)and 4-amino-3,5-dinitropyrazole(ADNP).The major contents of this study include two parts.1.The LDA/CA-PZ function was performed to study the crystal structure,molecular structure and electronic structure of crystalline DPX-26 from 0 to 130 GPa.The calculation of lattice parameters(a,b,c and ?)and total energy indicates that the compressibility of DPX-26 crystal is anisotropic and structure along c direction is much stiffer than the other two directions,meanwhile,total energy of DPX-26 crystal increases with the increasing pressure and the stability of meta-stable states gradually weakened.The bond lengths and torsion angles were used to analyze structural transformations in DPX-26 crystal and transformation between intermolecular,intramolecular hydrogen bonding and covalent interactions.The results showed that DPX-26 crystals undergo four distinct structural transitions at 81,82,92 and 123 GPa,respectively.At 81 GPa,the planarity of molecules is severely destroyed in DPX-26 crystal;at 82 GPa,the triazole ring cleavages,meanwhile,covalent interaction between N4 and H6 atoms converts to hydrogen bonding;at 92 GPa,the triazole ring forms again;at 123 GPa,the hydrogen bond H2·N6 transforms into covalent bond.The specific changes of hydrogen bonds with hydrostatic pressure were further studied by topological analysis.On the basis of the analysis of band gaps and density of states,it is shown that DPX-26 crystal becomes more and more unstable under high compression and the electronic delocalization increases with the increasing pressure.2.The GGA/PW91-OBS function were used to study the crystal structure,molecular structure and electronic structure of energetic co-crystal DAF/ADNP under pressure ranging from 0 to 100 GPa.With the pressure increasing,lattice constants(a,b,c and ?)and total energy change gradually which indicate that the compressibility of DAF/ADNP co-crystal along a-axis,b-axis and c-axis is anisotropic and the structure along a direction is much stiffer,in addition,with the increasing total energy,the stability of meta-stable states gradually weakened.Configuration changes of DAF/ADNP co-crystal,transformations between intermolecular(intramolecular)hydrogen bonds and covalent bond and cyclization were studied by analyzing the interatomic distances,bond lengths and torsion angles.It is shown that three obvious structural transformations occurred at 4,8,and 23 GPa,respectively.At 5 GPa,the hydrogen bonding between N1 and H5 turns into covalent interaction;at 16 GPa,the nine-membered ring distorted obviously;at 80 GPa,the relative position of molecules changed significantly in crystalline DAF/ADNP.The topological parameters and hydrogen-bond energy were used to study the intermolecular and intramolecular hydrogen bonds.Hirshfeld surface and 2D Fingerprint plots was used to visually analyze intermolecular interactions in co-crystal DAF/ADNP.According to the results of the density of states and bad gaps,the electrons delocalization increases as pressure increases and co-crystal DAF/ADNP becomes unstable under high compression. |
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