Narrow Results

In this work, a detailed study of the structural, electronic and absorption properties of crystalline 2,6-dimethyl-4-(diphenylmethylene)-2,5-cyclohexadienone with $\alpha$ form ($\alpha$-DDCD) in the pressure range of 0–250GPa is performed by density-functional theory (DFT) calculations. The particular analysis of the variation tendencies of the lattice constants, bond lengths and bond angles under different pressures shows that there occur complex transformations in $\alpha$-DDCD under compression. In addition, it can be see that the $b$-direction is much stiffer than the $a$- and $c$-axes in the structure of $\alpha$-DDCD, suggesting the compressible crystal of $\alpha$-DDCD has anisotropy. Then, by analyzing the bandgap and density of states (DOS) of $\alpha$-DDCD, it is found that the crystal undergoes a phase transformation from semiconductor to metal at 90GPa and it becomes more sensitive under compression. Besides, in the pressure range 110–170GPa, repeated transformations between metal and semiconductor occur four times, suggesting the structural instability of $\alpha$-DDCD in this pressure range. Finally, the relatively high optical activity with the pressure increases of $\alpha$-DDCD is seen from the absorption spectra, and two obvious structural transformations are also observed at 130GPa and 140GPa, respectively.

In this work, a detailed study of the structural, electronic and optical absorption properties of crystalline benzoic acid in the pressure range of 0–300GPa is performed by density functional theory (DFT) calculations. We found that occur complex transformations in benzoic acid under compression occurs, by analyzing the variation tendencies of the lattice constants, bond lengths and bond angles under different pressures. In the pressure range 0–280GPa, repeated formations and disconnections of hydrogen bonds between H1(P1) atom and O1(P1), O2(P4-$x$-$y$-$z$) atoms occur several times, and a new eight-atom ring (benzoic acid dimer) forms at 100GPa and 280GPa. Then, by analyzing the band gap and density of states (DOS) of benzoic acid, it is found that the crystal undergoes a phase transformation from insulator to semiconductor at 240GPa and it even becomes metal phase at 280GPa. In addition, the relatively high optical activity with the pressure increases of benzoic acid is seen from the absorption spectra, and three obvious structural transformations are also observed at 110, 240 and 290GPa, respectively.

In this paper, the structural, electronic and absorption properties of 2,2′-iminobis (acetamide oxime) (IBO) under pressure of 0–300GPa are calculated by the density functional theory (DFT) calculations. Analysis of the variation trend of lattice constant, bond length and bond angle of IBO under compression conditions, shows there are complex transformations under different pressure. In addition, it is found that the structure of IBO in the $a$-axis is stiffer than that along the $b$- and $c$-axes, which indicates that the crystal has anisotropic compressibility. By analyzing the band structure and the density of states of IBO, it is seen that at 120GPa, the electronic structure of IBO changes into metallic system, and becomes more sensitive under compression conditions. The transition between metal and semiconductor occurs again at 150Gpa. Finally, at 180GPa, the crystal transforms into metal again. The three obvious phase transitions indicate that the structure of IBO becomes more unstable with the increase of pressure. The absorption spectra show that with the increase of pressure, the optical activity of IBO crystal grows higher, and three obvious structural transitions are, respectively, observed at 120, 150 and 180GPa.

In this work, the structural, electronic and absorption properties of 2-methyl-2H-naphtho-[1,8-de]triazine in the pressure ranges of 0–250GPa are studied in detail (hereinafter referred to as 2-methyl crystal). Density functional theory (DFT) is used to calculate the lattice constants, bond lengths and bond angles of 2-methyl under different pressures. The results show that the crystals undergo complex transformations under compression, and the major structural transformations occur at pressures of 90GPa and 210GPa with repeated formations and disconnections. In addition, the $a$- and $c$-directions of the 2-methyl are stiffer than the $b$-direction, which indicates that the compressibility of the crystal is anisotropic. From the specific analysis of the bandgaps of 2-methyl, we can know that the crystal is converted from semiconductor to metal at 90GPa. The absorption spectrum of the crystal also indicates that 2-methyl has a relatively high optical activity with the increasing pressure.

In this paper, the structural, electronic and optical absorption properties of $m$-aminobenzoic acid crystals (hereinafter referred to as $m$-amino) in the pressure range of 0–300GPa are calculated by density functional theory (DFT). The changing trend of the lattice constant of $m$-amino under different pressures is analyzed. We find that the crystal undergoes complex transformation. Furthermore, it can be seen that the structure of $m$-amino along the $b$-axis is stiffer than that along the $a$-axis and $c$-axis, suggesting that the crystal has anisotropic compressibility. Through the analysis of the band gap and density of states of $m$-amino, it is found that the electronic properties of $m$-amino are transformed from semiconductor phase to metal phase at 100GPa, then jump into the semiconductor phase and maintain the metal phase again in the pressure range of 150–250GPa. Repeated phase transitions indicate that the structure of $m$-amino becomes more unstable as the pressure increases. Besides, from the absorption spectra, the optical activity of $m$-amino is relatively high with the increase of pressure, and two obvious structural transitions are observed at 70 and 270GPa, respectively.

In this paper, density functional theory (DFT) is used to study the structure, electron and absorption properties of 6-Amino crystal in the pressure range of 0–300GPa. The variations of the lattice constants, bond lengths and bond angles show that they undergo complex transformations under different pressures. More narrowly, the covalent bonds of the benzene ring and the uracil ring in the planar intramolecular structure are broken, and then the new covalent bonds between the adjacent intermolecular structure are reshaped at about 80, 100 and 160GPa. The analysis results of band gap and DOS of 6-Amino indicate that the electronic properties of 6-Amino repeatedly transform from the semiconductor system into the metal phase at 80, 180 and 260GPa. The absorption spectra show two important structural changes at 100 and 180GPa, and present the high optical activity with the change of pressure.