Periodic DFT Studies Of High-pressure Behavior For Molecular Energetic Crystals

Periodic density functional theory(DFT)method was performed to study the crystal,molecular and electronic structures of molecular energetic crystals nitroguanidine(NQ),4-amino-3,5-dinitropyrazole(LLM-116)and 4-amino-3,7-dinitro-[1,2,4]triazolo[5,1-c][1,2,4]triazine(TTX)under hydrostatic pressure.The effects of different pressures on the structures and properties were analyzed.The main contents of the dissertation are as follows:1.The structure of NQ crystal was optimized by GGA-PBE-G06 functional at 0~200GPa.NQ crystal undergoes three significant structural transitions at 40,80 and 125 GPa.The total compressibilities of NQ crystal in a,b and c directions are 23%,14.9%and 35.3%,respectively.The compressibility order is c>a>b.At 40 GPa,N1–H1···O4,N3–H4···O9,N13–H14···O1 and N23–H23···N3 are formed by intermolecular hydrogen bonds.The hydrogen bond length of N10–H10···N2 is shortened to 1.499?,and the hydrogen bond is converted to covalent bond.At 80 GPa,the intermolecular hydrogen bonds of N1–H1···N10,N1–H2···O11 and N3–H3···N21 are formed.The intermolecular hydrogen bond lengths of N10–H9···O3 and N11–H12···O1 were reduced to 1.493?and 1.484?respectively,and the hydrogen bonds were converted to covalent bonds.At 125 GPa,the intermolecular hydrogen bond N11–H12···N4 is formed.With the increase of pressure,the band fluctuation amplitude of NQ crystal increases.The band gap decreases gradually.The density of state moves to the low energy region,and the stability of NQ crystal decreases.2.The structure of LLM-116 crystal was optimized in the range of 0~150 GPa by using GGA-PBE-G06 functional.In the whole range of pressure,the compressibilities of LLM-116crystal in three directions of a,b and c are 14.9%,30.1%and 26.9%.The compressibility order of LLM-116 crystal in three directions is b>c>a.The cell volume decreases from645.883?~3to 281.220?~3,and the compressibility of LLM-116 crystal is 56.5%.The LLM-116 crystal undergoes three transitions at 5,35 and 145 GPa.At 5 GPa,intramolecular hydrogen bonds of N4–H3···O3 and N4–H2···O1 and intermolecular hydrogen bonds of N4–H3···O10 are formed.At 35 GPa,the hydrogen bond in N6–H4···O2 is shortened to 1.50?,and the H4–O2 covalent bond is formed.At 145 GPa,the distance between H3 and O10increased rapidly from 1.50?to 1.595?,and the covalent bond of H3–O10 breaks.The delocalization of electrons increases with the increase of pressure.The greater the pressure,the easier it is for electrons to transition from the valence band to the conduction band.3.The structure of TTX crystal was optimized by GGA-PBE-G06 functional at 0~100GPa.The cell parameters a,b and c decreased with the increase of pressure.The compressiones rate are 28.88%,27.98%and 15.53%in the three directions of a,b and c.The cell volume was compressed from 2557.3?~3to 1099.7?~3with a compression rate of 57%.TTX crystal undergoes three significant structural transitions at 5,61 and 93 GPa.At 5 GPa,N2–H2···O5,N15–H4···O3,N23–H5···O7 and N23–H6···O8 intermolecular hydrogen bonds were formed,and N16-O8-H6-N23-H5-O7 six-atom rings were formed.At 61 GPa,the intermolecular hydrogen bond of N2–H2···O5 is converted to the covalent bond of H2–O5.At 93 GPa,the distance between H2 and O5 increases rapidly from 1.429 to 2.287,the covalent bond of H2-O5 breaks,and the intramolecular hydrogen bond of N2–H2···N7 forms.The covalent bond length of H3-N15 on amino group increases from 1.058 to 1.572,the covalent bond breaks.The band gap decreases with the increase of pressure and decreases to 0eV at 93 GPa,indicating that the conductivity is enhanced.When the pressure is greater than93 GPa,the band gap value increases rapidly to 0.388 eV,and the stability of the system is enhanced.