| It is always a hot field in materials science to search for superhard materials with hardness exceeding diamond.Currently,the known superhard materials mainly includes two categories: the first class is B-C-N-O light element compounds,the second class is heavy transition metals and light-element compounds.The carbn monomer with a three dimensional strong covalent network has always been the research focus of superhard materials.Generally speaking,the smaller the atomic distance,the stronger the chemical bond is,and the harder the external force destroys materials structure,thus its hardness may be higher.Pressure will change the macroscopic and microscopic structure of matter and make it possess superior physical properties;especially under ultra-high pressure,the material structure will be more stable and difficult to be destroyed.In a large number of previous studies,it is generally believed that the ultrahigh pressure carbon bc8 structure(bc8-C)has the hardness comparable to that of the diamond at atmospheric pressure.The hardness test of the indentation,however,is often a dynamic process,and the previous study generally uses static parameters,which may cause the result to deviate from the actual situation.Therefore,a reasonable dynamic ideal shear strength is adopted to evaluate the hardness of bc8-C.In this paper,the mechanical and dynamic stability of bc8-C and diamond are studied based on the first principle calculation method.Then the ideal shear strength of bc8-C at normal and high pressure is calculated systematically compared to that of diamond.It is found that bc8-C and diamond have no virtual frequency in the whole Brillouin region under normal and high pressure,thus the dynamic stability is confirmed.Based on the Pugh theorem,we confirm that the calculated elastic stiffness coefficient satisfies the criterion of mechanical stability.In addition,the modulus of elasticity of bc8-C at normal pressure can almost be comparable to that of diamond,and the modulus of elasticity of bc8-C will be gradually higher than that of diamond with the change of pressure.The ideal shear strength of the diamond is well Coincided to the previous work at atmospheric pressure ideal shear calculation.Subsequently,it is found that the ideal shear strength of bc8-C at different crystal directions along different slip planes is significantly lower than that of diamond.In order to explore the difference of ideal shear strength between them,we compared the enthalpy,volume and bond length,respectively.It is found that the energy barrier of the transition process of bc8-C to graphitization is far lower than that of diamond,which reveals that the ideal shear strength of the normal pressure bc8-C is lower than that of the diamond.The ideal shear strength of high pressure shows that the ideal shear strength of bc8-C is much higher than that of diamond(external pressure >200GPa).In order to explain the difference between ambient and high pressure ideal shear strength of bc8-C and diamond,we examine the enthalpy,volume and bond lengths for two materials at different pressures.It is noted that the fracture conditions of the two substances in the critical state has changed significantly,resulting in significant changes in energy and volume with the change of pressure during the shearing process.Furthermore,in order to describe the bond strength,bond density and fracture mode of the chemical bonds more vividly,we have also plotted the crystal structure deformation and the ELF diagram.From this,we can find that the shear deformation mode is restrained due to the pressure confinement effect under high pressure,which significantly strengthens the ideal shear strength of the material.Therefore,we propose a new mechanism of bc8-C ideal shear strength higher than diamond under high pressure. |
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