Ab Initio Studies On Crystal Structures And Properties Of Li-CH₄ Compounds Under High Pressure

High-pressure can not only make the crystal structures of materials transform, but it can also cause a great change in their physical and chemical properties, such as amorphous phase turns into crystal, insulators transform into the metals, etc. Therefore, numbers of researches have been performed to explore the novel structures and peculiar properties of materials under high pressure. In particular, high pressure bringing about high temperature superconductors has attracted the extensive attention.Methane is the simplest hydrocarbons. In recent years, theoretical and experimental researches were carried out to investigate the physical behaviors of methane under high pressure. These Studies show that methane remains an insulator under high pressure, and when pressure reaches a certain it becomes unstable and dissociates into C₂H₆ and H₂. But the phase diagram and critical decomposition pressure of methane under high pressure are still controversial. In this paper, using first principles calculation we find that methane will dissociate into C₂H₆ and H₂ at 145 GPa, and below 145 GPa there are three stable insulating molecular structures, namely Pmn2₁, P2₁2₁2₁ and P2₁/c. Below 4.7 GPa orthorhombic Pmn2₁ is the most stable structure, methane undergoes a phase transition from P2₁2₁2₁ to P2₁/c at 33.7 GPa. The band gap of monoclinic P2₁/c is 5.29 eV at 140 GPa. In the pressure range of 1¹00 GPa, enthalpy differences among P2₁2₁2₁,P2₁/c,Pnma and Cmcm are small, it means that differences in methods to describe the exchange-correlation potential and choosing calculation precision may lead to the distinction of predicting phase transition. It explains why the conclusions about the stable structures of methane under high pressure between Guoying Gao and He Lin who respectively used the generalized gradient approximation and the local density approximation in processing exchange correlation potential are conflicting.Some studies show that doping metal is useful not only in stabilizing the structures but also inducing or improving Tc of materials. On the other hand, it was found that the metal can activate the very inert C-H and C-C bonds of hydrocarbon, which is helpful in improving the practical value of methane and other hydrocarbons. In the paper we further investigated the influences on structures and properties of methane through doping the lightest metal atoms Li under high pressure at 1:1, 1:2 and 2:1 chemical ratio. It is found that doping metal Li causes the activation of C-H bond in methane and formation of C₂H₆（in P-1_LiCH₄）, CH₃ radical and CH₂ chain hydrocarbon（respectively in stable phases Cmm2 and P2₁/m-P of Li₂CH₄）. Li₂CH₄ possesses the lowest formation enthalpy among three kinds of compounds in range of 0-140 GPa, but it becomes stable only when the pressure reaches 15 GPa. We found two structures with Cmm2 and P2₁/m-P space groups of Li₂CH₄ below 140 GPa. Orthorhombic Cmm2 is the most stable structure in the range of 15-34 GPa, when above 34 GPa the orthorhombic structure with space group of Cmm2 transforms into monoclinic P2₁/m-P structure. In this paper we confirmed that LiCH₄ compound with triclinic structure P-1 possessing the second lowest enthalpy is stable in the range of 0-15 GPa. The P-1, Cmm2 and P2₁/m-P structures are all insulating, it means that under high pressure doping metal Li to methane cannot achieve the goal of metallizing methane. Nonetheless, with the increasing of pressure, the band gap of P2₁/m-P decreases almost linearly which reaches to 0.545 eV at 140 GPa.