Theoretical Study On Structure And Electronic Properties Of Superhard Materials BC₄/BC₂N

Materials with Vickers hardness values above 40 GPa are defined as superhard materials.Due to the excellent properties,such as high melting point and corrosion resistance,superhard materials are widely used in many specific areas?e.g.,cutting tools,polishing tools,and wear resistance?.So far,the most commonly used superhard materials in industry are diamond and cubic boron nitride.However,the low ductility,high oxidizability and poor thermal stability of diamond limit its application.Although the thermal stability of cubic boron nitride is better than that of diamond,its hardness is only two thirds of that of diamond.Moreover,cubic boron nitride is easy to react with water at high temperature.Synthesizing superhard materials usually requires specific conditions of high temperature and high pressure,thus increasing the cost of research and development.Therefore,designing and synthesizing superhard materials with high hardness,high stability and low cost have been one of the important research topics in the field of condensed matter physics.With the development of computer hardware and software and the gradual advancement of basic theoretical research,theoretical calculation can accurately and quickly predict the synthesis conditions of materials,determine the structures of compounds,and explain experimental phenomena,etc.In this paper,we determined the ground state structures and electronic properties of BC₄ and BC₂N based on first-principles calculations and structure prediction method.The main works are as follows:?1?Our predicted BC₄ is a potential superhard material.Three stable BC₄ structures with space group of I4₁/amd,R-3m and Cm,respectively are discovered under high pressure using first-principles calculations and unbiased structure search.By calculating enthalpy difference curves of BC₄,including three new structures and two already reported structures?P4₂/mmc and Cmmm BC₄?,it shows that our predicted structures are more stable than the previously proposed ones.I4₁/amd BC₄ is the most stable structure in the studied pressure range.Besides,all of these three predicted structures are dynamically and mechanically stable.The three new structures exhibit different bonding patterns,i.e.I4₁/amd and R-3m structures adopt sp³ hybridizations,whereas Cm structure forms the mixture of sp² and sp³hybridizations.In addition,I4₁/amd and R-3m BC₄ are metallic,whereas Cm BC₄ is a semiconductor.The most stable I4₁/amd-structured BC₄ is an excellent superhard material from the standpoint of high bulk modulus?370 GPa?,high shear modulus?353 GPa?,large Young's modulus?804 GPa?,low Poisson's ratio?0.14?,and acceptable Vickers hardness value?55.7 GPa?.?2?We predicted that R3m BC₂N might be the ground-state structure synthesized experimentally.Experimental work reported that BC₂N presents high hardness and superior stability,which attracts extensive attention.However,its crystal structure has not been unambiguously determined thus far.Here,we identify a hitherto unknown hexagonal BC₂N with R3m symmetry,more stable than the earlier proposed phases through first-principles swarm structure calculations,which exhibits the interesting structure character of the honeycomb C sublattice and hexagonal B-N layer.Intriguingly,the simulated X-ray diffraction patterns,K-edge spectra,and hardness value of R3m BC2N are in good agreement with the experimental measurement.These results indicate that our predicted BC2N is the most likely experimental candidate structure,awaiting experimental confirmation.Moreover,the outstanding hardness?71 GPa?of BC2N can be attributed to the full sp³ bonding character and the higher number of C-C and B-N bonds compared to those of C-N and B-C bonds.The electronic properties show that R3m BC₂N is a wideband gap semiconductor with a band gap of 5.0 eV.