First-principles Study Of A New Superhard Carbon Phase:C₂₀-T Carbon

Carbon is one of the most important elements in the nature,and is also one of the earliest elements of human contact.Carbon has a rich variety of structural allotropes due to its ability to form sp,sp²,and sp³ hybridized chemical bonds.These carbon allotropes play an important role in life and production of people and scientific research.Superhard materials are of particular importance in industrial applications owing to their superior performance,such as high hardness,high strength,and chemical inertness.So far,among all of the superhard carbon materials which have been found by people,the diamond is the hardest.The diamond has strong sp³ hybridized bonds,and can fulfill most of industrial demands,for example,cutting,polishing,and drilling tools,and surface-protecting coatings,and so on.Over recent decades,extensive experimental and theoretical efforts have been devoted to discovering new superhard carbon allotropes.Here,a new stable superhard cubic carbon allotrope with a full sp³ bonding network is predicted using first-principles calculations.This new carbon allotrope has a cubic T symmetry and contains twenty atoms in its primitive unit cell,so we termed it as C₂₀-T carbon.We identified the dynamic stability,mechanic stability and thermal stability of this new carbon phase by some first-principles calculations.Interestingly,we found that C₂₀-T carbon has a high Vickers hardness value of 72.76 Gpa,though C₂₀-T carbon is a porous carbon phase which has a large cavity with a diameter of approximately 3 ? in its structure.We also simulated x-ray diffraction?XRD?patterns of C₂₀-T carbon to provide more structural characteristic of C₂₀-T carbon,which may be helpful to its identification in future experiments.The main contents and results are as follows:In the dissertation,we researched the structural and electronic properties of a new stable three dimensional superhard carbon phase C₂₀-T carbon.We calculated the total energy,phonon mode,elastic constants of C₂₀-T carbon and have also performed first-principles molecular dynamics simulations with the canonical?NVT?ensemble at temperature of 1500 K.The structural stability of C₂₀-T carbon is verified by the above analyses.The Vickers hardness value of C₂₀-T carbon is even higher than BC12 carbon and the famous superhard material c-BN by calculating and comparing the Vickers hardness of C₂₀-T carbon and some other carbon allotropes.We have also calculated the ideal strengths?ideal tensile and shear strength?,which is defined as the maximum of stress in the stress–strain curve of the weakest tensile stretch or shear slip direction.The calculated ideal tensile and shear strength of C₂₀-T carbon are 71.1 and 54.5 GPa,respectively,which are comparable to those of c-BN.For the purpose of providing more information and characterizing the material for possible experimental observation,we simulated and compared the x-ray diffraction?XRD?spectrum of C₂₀-T carbon and some other carbon allotropes.We have calculated the electronic band structures and the projected density of states of C₂₀-T carbon using a hybrid density functional method based on the Heyd–Scuseria–Ernzerhof scheme?HSE06?.The C₂₀-T carbon exhibits an insulating character with a wide indirect band gap of 5.44 eV.This new carbon phase has great potential for application in mechanical devices,hydrogen storage,and related fields.