The First Principles Study Of Fluorocarbons

Since the discovery of superconductivity in 1911,superconductivity has been a research focus in the field of physics and materials.A large number of theoretical and experimental studies have found that superconductors can be obtained by doping.Some people predict that graphene doped with holes can be a traditional superconductor and have a superconducting transition temperature higher than the boiling point of liquid nitrogen.In addition,high pressure is important for studying the phase transition of structures.Pressure can lead to new physical and chemical phenomena and new structures.Exploring new materials with different structures is a hot topic in theoretical calculations.This article studies the crystal structures and electronic properties of fluorocarbons based on first-principles methods.The main contents are as follows:We used fluorinated graphene as the research object,using first-principles software based on density functional theory?DFT?,combined with virtual crystal approximation to study the the superconductivity of 6%,8%,10%p-doping C₆F₆.Intrinsic C₆F₆ is an insulator,and the doping of the holes causes the Fermi surface to move downwards,which increases the density of the electronic states at the Fermi level.The increase of the doping concentration will cause the increase of the electroacoustic coupling parameter?.According to the calculation of the modified McMillan equation,the result that the superconducting critical temperature T_c increases with the increase of the hole doping concentration is obtained.When the p-doping concentration is 10%,the superconducting critical temperature reaches 16 K.Based on the first-principles method of density functional theory,we use crystal structure search software combined with VASP software to explore the phase transition of C₆F₆ and CF₄ under high pressure.C₆F₆ at low pressure is mainly a molecular crystal,which gradually changes from chain to layer with increasing pressure.We predict two high-pressure C₆F₆ polymerization phases,which are chain P2₁/m and layered P-3m1structures.Among them,the energy band structure of the P2₁/m structure is shown as a semi-metallic state,and the density of electronic states shows that the band gap disappears at 30 GPa,and the semiconductor changes to a metal state.At 400 GPa,P-3m1,Pbca and P2₁/c-1 structures are all semiconductors,with band gaps of 1.80 eV,0.35 eV,and 0.40 eV,respectively.We also predicted the crystal structure of CF₄ insulating molecules under high pressure,and gave the curve of the relationship between the enthalpy difference of the structure and the pressure.Among them,the structure where CF₄ is stable under normal pressure is P4₂/nmc,and the most stable between 100?170 GPa is the C2/c structure.At higher pressures after 170 GPa,P4₂/nmc is the most stable structure.The phonon spectrum of these new predicted high-pressure fluorocarbon phases has no imaginary frequency in the corresponding pressure range,indicating that these new predicted high-pressure phase structures are dynamic stable.