Periodic DFT Studies Of High Pressure Behavior For Azole-based Energetic Ionic Salt

Density functional theory(DFT)method was performed to study the crystal,geometrical and electronic structures of azole-based energetic ionic salt dihydroxylammonium5,5'-bistetrazole-1,1'-diolate(TKX-50),hydrazinium 5-nitramino-3,4-dinitropyrazolate(HNDP)and hydroxylammonium 3,3'-dinitro-bis-(1,2,4-triazole)-1,1'-diolate(MAD-X1)under hydrostatic pressure.The effects of different pressures on the structure and properties were analyzed.These results may provide useful information in understanding the high pressure behavior of azole-based compounds.The main contents of the dissertation are as follows:1.Four different DFT methods(LDA/CA-PZ,GGA/PW91,GGA/RPBE and GGA/PBE)were applied to relax the experimental TKX-50 crystal at ambient pressure without any constraint,and a proper method was selected based on a comparison of relaxed and experimental crystal parameters.The structures and electronic properties of TKX-50 from 1 to400 GPa were calculated.The variation of the sensitivity under compression was uncovered.The results show that the crystalline TKX-50 undergoes three structural transformations at the pressure of 126,288 and 334 GPa.First of all,at 126 GPa,the positions of the molecules in the crystal is rearranged.At 288 GPa,the planarity of the big conjugated ring is destroyed completely.The last one appears at 334 GPa with the distortion of the tetrazole rings.Overall,the band gap decreases with increasing pressure.Thus,the impact sensitivity of TKX-50 becomes much higher under compression.The results of the density of states show that the electronic delocalization in TKX-50 increases with the increase of pressure.2.According to the comparison of LDA/CA-PZ-,LDA/CA-PZ-OBS-,GGA/PBE-,andGGA/PBE-TS-optimized and experimental values of HNDP crystal,a proper method was obtained.We examined the variations in the structural and electronic properties under hydrostatic pressure of 1-160 GPa.The first structural transformation happens at 6 GPa and makes the positions of the anions and cations in the crystal rearrange.As a result,the N17-H23···N13 hydrogen bond is broken.Next,at 28 GPa,N17-H23···N13 hydrogen bond forms again.Finally,at 110 GPa,the N17-H19 covalent bond is broken and a new bond forms between H22 and O24.An analysis of the band gap and density of states shows that HNDP crystal undergoes an electronic phase transition from a semiconductor to metal and becomes more sensitive under compression.3.To determine the optimal method,the LDA/CA-PZ,GGA/PW91 and GGA/PBE were applied to the computation of crystalline MAD-X1 as a test.On the basis of the relaxed structures,the band structures,density of states,atomic charges and bond populations were calculated in the pressure range of 1-200 GPa.The variation of the crystal,molecular and electronic structure at high pressures were analyzed.At 114 GPa,the O12-H27 covalent bond is broken,forming a new N9-H27 covalent bond.At the same time,O12 is close to H30,forming a O12-H30 covalent bond.As a result,a new seven membered ring of N9-H27-O26-N25-H30-O12-N10 is formed.At 144 GPa,the H30-O12 and O26-H27 covalent bonds are broken and the new seven membered ring is lost at 144 GPa.The bond length of N9-H27 bond decreases suddenly,which indicate that the H27 atom is transferred from O26 to N9.At 150 GPa,H27 goes far away from N9 but gets close to O26,and forms a O26-H27 covalent bond only.At 158 GPa,the O26-H27 covalent bond is broken again,and a new bond forms between O12 and H27.At the same pressure,H30 is also close to O12,forming a covalent bond.As a result,O12 is bonding with both H27 and H30 at this pressure.The analysis to density of states indicates that the electronic delocalization increases with the increasing pressures,which shows that an exerted pressure may increases the impact sensitivity of MAD-X1.