Investigations Of Crystal Structures And Properties Of Several Typical Diatomic Molecule Solids Under High Pressure

High-pressure properties of diatomic molecule solids,such as H₂,N₂,O₂,F-2,Cl₂,Br₂,and I₂,have attracted a lot of attention from physics and chemistry.Among these molecule solids systems,hydrogen is the simplest and most abundant element in the universe and metallic hydrogen was predicted to have possibility to be superconductor at room temperature.However,due to the strong bond interaction between internal hydrogen,neither metallic hydrogen nor molecular dissociation has been reported in experiments up to now.The crystal halogens are similar to solid hydrogen,so the typical three diatomic molecule solids H₂,F₂ and I₂ as our research objects are calculated by using first-principles calculations based on the density functional theory?DFT?in this paper.We conducted geometry optimization on the reported solid hydrogen structures?C2,P63/m,Pca21,C2/c,Cmca,Cmca-12,P63/mmc,P21/c,C2/c-1,Pbcn,Pa3?with four calculation strategies?ultrasoft pseudopotential and norm conserving pseudopotential with/without Van der Waals force?and acquired phase diagram and phase transition sequence of crystal hydrogen over high pressures and hope this can provide guides to future experiments and theories.Fluorine,the first of the halogens and the last of the first-row diatomic molecule solids elements,has many similar characters to solid hydrogen.C2/c is determined to be the better candidate of solid fluorine at low temperature and ambient pressure.Compressing solid fluorine up to 8 GPa,C2/c phase is found to undergo phase transition to a new structure with a space group of Cmca.Electronic energy band structures indicate the insulating feature of C2/c and Cmca with no bands across theFermi level.The IR?Infrared?and Raman of C2/c and Cmca at selected pressures are calculated to provide useful information to experiments in the future.Sufficient researches have been conducted to solid iodine.According to the reported theories and experiments,we summarized the phase transition sequence of solid iodine under high pressure and carried out geometry optimization onto the reported structures of Cmca and C2/m.The calculated elastic constants and phonon spectrum indicate that both Cmca and C2/m structures are mechanically and dynamically stable at 15 GPa.There are two different molecular covalent bond lengths in the crystal structure of C2/m,the bonds increase with pressure and iodine molecules have the trend to dissociate.The calculated electronic energy band indicates the C2/m structure is metallic at 15 GPa.