Studies Of Solid Methane At High Pressure By First-principles Methods

Recently, along with the rapid development of high-pressure science technology and Spectrum measurement technique, we find many novel situation. Many Molecular crystals changed into Atomic crystals and accompanied by metallic crystal phase transition under high pressure. For decades, scientists have been going to great effort to design high-temperature superconducting material. Hydrogen, the lightest element, was predicted to become metallic under very strong compression and will probably become an extraordinary high-temperature superconductor. However, hydrogen remains insulating up to extremely high pressures, at least up to ~342GPa. It was recently predicted that group IV hydrides would also present a high superconducting critical temperature, while becoming metallic at lower pressures due to chemical precompression. At present, Silane, germane, Sn4 have been proved superconducting at different pressure, while the metallic of methane has not been resolved by now. In 2006, Sun at el. believe that there is insulator–semiconductor phase transition at 288GPa at room temperature, and there is no structural phase transition until 288GPa.Methane (CH4) is the simplest hydrocarbon compounds, a simple molecular crystal under high pressure. The phase diagram of methane in its condensed phase is both rich and intricate, but not completely well understood up to now. The complication is attributed to the varying orientational disorder of its molecules and the different lattice constants at the various phases. Different experiments on methane were performed during the last few decades to reveal its phase diagram. Of the several known phases of solid methane, only structures in the three low temperatures and low pressure phases have been completely solved. In phase I, which at ambient pressure is stable below a melting temperature of about 90K and above T=20.4K, all of the tetrahedral molecules are orientationally disordered. In phase II, below this temperature, the orientation-dependent octupole-octupole interaction leads to a partial orientational order. There are six orientationally ordered sublattices and two disordered sublattices which space group is Fm3c. It is sometimes referred to as"antiferrorotational."Using high-resolution neutron powder diffraction and a direct-space Monte Carlo simulated annealing approach, the fundamental structure in phase III has been resolved. It is orthorhombic with space group Cmca, in which unit cell there are 16 molecules. Recently, Hirai et al. suggested that the structures of four high-pressure phases, phaseB, phaseHP1, phaseHP2 and phaseHP3, have cubic symmetry from about 12 GPa to 86 GPa at room temperature. But the structures at low temperature and high pressure phases have not been resolved by now.In this paper, we use the classical simulated annealing and first-principles calculations to study the structure and metallization of methane at high pressure. We use the structure of phase III and other random structures as the initial structure of annealing. We got a series of structures by annealing. The geometry optimization shows that the monoclinic P21/b structure with four methane molecules in the unit ceell has the lowest energy between 4 GPa and 90 GPa,and the four carbons , in the same plane, constitute a parallelogram. The phonon dispersion curves and the total DOS of P21/b structure have been calculated at pressure between 4 and 90 GPa. It is interestingly noted that the frequencies of the acoustic phonon modes are below zero before 10 GPa, indicating that the structure is not stable. With increasing pressure the imaginary frequency modes shift to higher energy. Up to 10 GPa, there is no imaginary frequency modes. The result is similar at above 10 GPa up to 90 GPa. The absence of imaginary frequency modes indicates that the structure is stable. Additional phonon calculations establish the stability range to be between 10 GPa and at least 90 GPa. It is believed that other structure may be existed in the pressure range from 4-10 GPa. In this article, the electronic band structure and the density of electronic states (DOS) have been calculated. The result shows that this structure has not metalized until 90 GPa with an energy gap of about 6 eV.