The Properties Of Aromatic Hydrocarbon Materials Under High Pressure Conditions

High pressure technique plays an important role in modern science and technology, with impressive development and broad applications in many fields likes physics, chemistry, materials and earth science. Due to the unique structures and novel properties, aromatic hydrocarbons have attracted much attention recently, aromatic hydrocarbons has been regarded as good building block for constructing various new function materials, for which the structures and properties are closely related to the interaction and bonding types of the constructed units. On the other hand, as another effect other than temperature, pressure exhibits its strength on changing properties of materials to a large extent. High pressure can change the distances between molecules or atoms, and then affect their interactions and bonding, which results in the variation of its structure and composition (crystal structure, molecular structure, the alignment of atoms), and further a series of changes happen, such as the energy band structure, the combination, orbital configuration and density of states of electrons. For those reasons above, investigations of physical properties under high pressure is of great importance and thus highlighted.With the booming of experiments, theoretical work also has made great development. For example, density functional theory (DFT) calculations and simulations can not only be used to explain and understand experimental observations, but also give some interested theoretical predictions. In this article, the high Raman scattering measurements are performed on naphthalene and anthracene by using diamond anvil cells. Meanwhile, we mainly use the first-principles method to study and calculation of the structural and electronic properties of aromatic hydrocarbons. The original and innovational research results including:(1) High-pressure Raman scattering measurements are performed on naphthalene by using diamond anvil cells at room temperature. Our studies provide an investigation of the pressure-related change in molecular vibrations and rules out the possible existence of phase transformation in naphthalene under hydrostatic pressure. Our results also demonstrate that almost all the modes shift toward higher frequencies and some peaks are broadened with increasing pressure. When pressure is increased up to 11.1 GPa, the intensities of the vibrational modes drastically decrease accompanied with the increasing fluorescence background. With increasing pressure up to 17.1 GPa, some of peaks nearly vanish except for the 105 cm-1,150 cm-1,195 cm-1,231 cm-1,765 cm-1,779 cm-1,1029 cm-1,1402 cm-1, and 1458 cm-1 could be observed. According to the previously studies, only four peaks of the lattice modes at 105 cm-1,150 cm-1,195 cm-1, and 231 cm-1 can be observed at ambient pressure in our measurement. And the four peaks at 105 cm-1,195 cm-1, and 1029 cm-1 were first observed. Additionally, within our data the lattice modes exhibit more drastic changes than intramolecular modes, which are due to there being greater intermolecular distortions than intramolecular under applied pressure.The pressure effect on the geometrical and electronic structures of crystalline anthracene is calculated up to 30 GPa by performing density functional calculations. All the cell parameters decrease with increasing pressure and the cell parameter a shows large change for the applied pressure. The lattice parameters have the strongest pressure dependence with △a= 1.65 A, whereas b and c show a pressure dependence of △b= 0.73 A and △c= 0.79 A up to 30 GPa, respectively. The monoclinic angle increased by △β= 4.6° in the same pressure region. The nonuniform pressure dependence of the lattice parameters may imply that the sample undergoes anisotropic compression with pressure. At ambient pressure, all the shortest intermolecular C-C are larger than this value. When increasing pressure, these distances all fall below the van der Waals radii. The C-C bond lengths have the pressure dependence with △c-c= 18 mA, but the decrease in intermolecular C-C and C-H distances turn out to be 739-618 mA between 0 and 30 GPa. Comparing the pressure effect on the shortest intermolecular distances and on the bond lengths we confirm the expected result that the intermolecular interactions are more sensitive to pressure than the intramolecular interactions. As mentioned above, the observed anisotropic high-pressure of the lattice parameters is consistent with the compression mechanism found previously for a narrower pressure range of 0-5 GPa, which includes rotation of the neighboring molecular within the herringbone pattern relative to each other, so that they become more parallel.(3) The high-pressure vibrational properties of anthracene are investigated by using Raman scattering techniques in diamond anvil cells up to 7.1 GPa at room temperature. Our data over a wide frequency range do not support the existence of the possible phase transition at 7.1 GPa as observed or reported previously, rather than the molecular deformation instead. Only four peaks of the lattice modes at 28 cm-1,34 cm-1,60 cm-1, and 115 cm-1 can be observed at ambient pressure in our measurement. At 1.4 GPa, the peak at 115 cm-1 associated with lattice mode splits into two modes. Whilst the skeletal deform mode originally at 467 cm-1 disappears. Further increase of pressure up to 5.5 GPa shows a decrease in intensity associated with the lattice modes to the point where they are lost in the increasing fluorescence background. Additionally, in frequencies range from 900 to 1600 cm-1, no apparent discontinuity, disappearance, or splitting were observed in the C-C stretching vibration modes under pressure, implying the stability of carbon bonding below 6.8 GPa, again supporting the stability of the phase. Moreover, within our data the lattice modes exhibit more drastic changes than intramolecular modes, which are due to there being greater intermolecular distortions than intramolecular under applied pressure.(4) The band structure and the partial density of state reduce of naphthalene, anthracene and perylene under high pressure conditions are calculated by using the first-principle method. Moreover, a pressure induced decrease of the band gap is observed. Our results indicate that the valence and conduction bands near the Fermi level mainly come from C 2p and the conduction band minimum is mainly composed of H 1s and C 2p states. However, the valence band maximum is mainly composed of H 1s state.