The Predictionr Of Two-dimensional Carbon Dirac Material And Its Mechanical Phase Tranision

Since there are four valence electrons in the outermost layer of carbon atoms,carbon atoms can form various chemical bonds containing sp,sp²,and sp³-hybridized modes.Therefore,carbon atoms can form allotropes of different sizes and different properties.With the successful synthesis of graphene in experiments,people have opened the door to the study of two-dimensional carbon structure.Two-dimensional carbon nanomaterials have great applications in various fields due to their rich and unique physical properties.For next-generation carbon-based nanoelectronics,it is highly desirable to search for easily obtained 2D carbon allotropes with various appealing properties.Based on first-principles calculations,a new 2D carbon Dirac semimetal is identified,which is composed of a carbon skeleton of para-xylene and acetylenic linkages.The calculations of stability reveal not only that palgraphyne is dynamically,thermally（above 1000 K）,and mechanically stable,but also that it is energetically more preferable to the recently synthesizedβ-graphdiyne andγ-graphdiyne.Due to the particular atomic-framework,the calculations of Young’s modulus show that palgraphyne is mechanically anisotropic with a sizable ratio between the maximum and minimum value up to 3.29.Due to the anisotropy of structure and mechanics,its electronic transport properties also exhibit anisotropy and its distorted Dirac cone has different sizes in different k directions.Interestingly,the highest Fermi velocity of palgraphyne reaches 8.89×10⁵m/s,which is very close to that of graphene（9.0×10⁵m/s）.According to the hybridization mode of carbon atoms,we call the two-dimensional carbon alltropes with acetylene bonds as graphyne.Based on the investigation of the two-dimensional carbon allotropes,especially graphyne,this paper selects three typical graphyne structures,γ-graphyne,B-graphyne and 6,6,12-graphyne,to systematically explore its intrinsic physical properties and focused on the mechanical phase transition between them.By introducing the concept of two-dimensional enthalpy（H=E₀+?A）,we quantitatively explored the mechanical phase transition from the experimentally synthesizedγ-graphyne to two other structures.The calculations show that the biaxial pressure required to achieve the phase transition fromγ-graphyne to B-graphyne is-1.11N·m^-1,while the biaxial pressure required to achieve the phase change fromγ-graphyne to 6,6,12-graphyne is-6.12 N·m^-1.