Combined Theoretical And Experimental Studies Of Phase Transition In Polycyclic Aromatic Hydrocarbon Semiconductors Under High Pressure

Recently, alkali-metals or alkali-earth-metals doped polycyclic aromatic hydrocarbons were discovered to be superconductors with the critical transition temperature from 5 K to 33 K. This intrigues the scientists for the electric and magnetic properties of polycyclic aromatic hydrocarbons organic semiconductor of great interest. Further studies showed that the superconducting critical transition temperature of polycyclic aromatic hydrocarbons depends much on pressure. The critical temperature of K3 Picene increased from 18 K at ambient pressure to 33 K at 1.2 GPa. Actually, the first organic superconductor TMTSF was discovered under pressure. Organic superconductor RbCs2C60 with TC of 38 K was also achieved under pressure. Especially in 2015, Drozdov et al. found that H2 S is a new convention superconductor and the superconducting critical transition temperature of H2 S can be improved with increasing pressure. Highest TC of H2 S can be reached up to 203 K at 150 GPa in excess of the highest TC of 164 K of cuprate superconductors. The studies on the physical properties of rich hydrogen compounds under pressure are the research hotspot in the condensed matter physics and materials science. Compared with the chemical doping, the pressure is a pure way to adjust the structure and to find a new state of matter. So it is paramount to explore the phase transition, metallization behavior and superconductivity of polycyclic aromatic hydrocarbons under pressure.In this paper, our research is focused on two typical polycyclic aromatic hydrocarbons:pentacene without any side benzene groups and rubrene with four side benzene groups. As early as in 1964, pentacene were found to show insulator-metal transition at 38 GPa by optical absorption and resistance measurements. Combined theoretical and experimental studies we have investigated the crystalline structure and electronic structure of pentacene and rubrene at high pressure. Firstly, the crystalline structure and electronic structure were calculated up to approximately 40 GPa by employing the first-principle theory. We performed hybrid density functional HSE06 to calculate electronic structure. The results show that pentacene eventually realizes metallization at 40 GPa, which is consistent with the early experimental results. During pressurizing process, pentacene was still a stable structure with five benzenes in the same plane. Analysis of simulated Raman spectrum shows the vibration of pentacene with Ag symmetry. High-pressure synchrotron X-ray diffraction(XRD) and Raman scattering spectra can be used to obtain the pressure induced changes in the structural and vibrational properties of pentacene. There is only a marginal difference betweenexperimental and theoretical results. Variation in intermolecular interactions can be estimated by analyzing the dnorm Hirshfeld surface at every pressure. Variation in intermolecular interactions can be estimated by analyzing the dnorm Hirshfeld surface at every pressure. A shorter π···π stacking distance leads to the increasement in C···C contacts under the increasing pressure, the amount of H-H contacts reduces, in favor of a larger percentage of C···H contacts. The metallization of pentacene may be ascribed to the increasement ofπ-electron delocalization induced by high pressure.Rubrene with orthorhombic structure was compressed up to 10 GPa without any structural transition by means of synchrotron X-ray diffraction measurements. The band-edge of optical absorption of rubrene shifted red under pressure and the bangap ranged from 1.94 eV at ambient pressure to 1.34 eV at 10 GPa. Similar to the case of pentacene, HSE06 was applied to explore the electronic structure of rubrene under pressure. At 30 GPa the number of calculated bands increased from 288 to 384. Band structure and DOS shows that rubrene became a metallic phase at 80 GPa. During compression up to 80 GPa, side benzene groups gradually tended to be paralleled and backbone tetracene undergone bend with the deviation from its planarity. Experimental and theoretical results reach an agreement with the same range of pressure. Under the increasing pressure, the rearrangement of the phenyl groups reduces π···π stacking distance, which leads to the increasement in C···C contacts, the amount of H···H contacts and in favor of a larger percentage of C···H contacts and C···C contacts. Increasing π-electron delocalization induced by high pressure plays an important role to the metallization of rubrene.