First-Principles Study On The New Three-Dimensional Carbon Structures

Exploring new carbon structures is one of the hot issues in condensed matter physics because of their fascinating properties.In this thesis,we identify by first-principles calculations several new three dimensional carbon structures,and systematically investigate their structural characteristics,stability,and electronic properties.To establish the connection with the experiments,we also simulated the x-ray diffraction?XRD?patterns.Firstly,we predict two diamond-like carbon structures,BC12 carbon and R16 carbon.The crystal structures of BC12 carbon and R16 carbon consist of six membered rings with sp³bonding type in Ia?3d and R?3c symmetry,as well as diamond and BC8 carbon.BC12 carbon can be viewed as a polymerized form of?3,0?carbon nanotube?CNT?,while R16 carbon can be regarded as a distorted crystalline form of BC8 carbon.The total energy and phonon calculations show their thermodynamic stability and dynamic stability.R16 carbon is more sta-ble than BC8 carbon below 167 GPa.The electronic band structure calculations present that BC12 carbon is a semiconductor with a band gap of 2.97 eV,and R16 carbon is an insulator with a band gap of 4.45 eV.They all exhibit direct band gap characters,which is similar to that of diamond and BC8 carbon.Simu-lated XRD patterns of BC12 carbon and R16 carbon provide an excellent match to the previously reported distinct diffraction peaks found in shock compression experiments and milled fullerene soot,respectively,and our calculated lattice pa-rameter of BC12 carbon is very close to experimental value.Therefore,they are likely candidate structures in the experiments.We secondly identify four carbon phases,t32,t32*,m32,and m32*carbon,originating from silicon phase due to the common valence electron configuration of carbon and silicon.These structures have large 32-atom conventional cells in all-sp³bonding networks with P?42₁c,P 4₃2₁2,P 2₁/c,and C2 symmetry,respectively.Meanwhile,these carbon phases are denser than diamond,and their bulk modulus are between 381 GPa and 390 GPa,which is close to the value for c-BN?396 GPa?.Their dynamically stable are verified by phonon mode analysis.Electronic band structure calculations show that they are insulators with band gaps in the range of 5.19 eV?5.41 eV.Among them,t32 carbon is a direct band gap insulator with the valence band top and conduction band bottom located at the?point.Simulated XRD patterns suggest that the proposed superdense carbon phases may be related to the n-diamond with a distinct diffraction peak around 50.8^?found in experiments.In addition,we study a cold-compressed graphite phase,W₃₂carbon,and find that it can be viewed as a distorted form of graphite through slip and buckling mechanism along the?100?direction.This structure in P mc2₁symmetry is made of exclusively sp³hybridized covalent bonds consisting of five-,six-,and seven-membered rings.It is also energetically more favorable than the proposed cold-compressed graphite phases.Compared with diamond,it shows indirect band gap behavior,and the band gap is estimated to be 5.91 eV.The calculated hardness is 95.1 GPa,which is slightly lower than that of diamond.Its simulated XRD pattern best matches the experimental data.These results indicate that W₃₂carbon may exist in cold-compressed graphite experiments.Finally,we study?1,3?armchair carbon foam termed as oC24 carbon,and?1,1.5?,?2,1.5?and?1,2?zigzag foams,termed as oC28,oC32,and oC36 carbon,respectively.They all have sp²-sp³bonding networks,and they are more stable than fcc-C₆₀,but comparable to?5,5?-CNT in energetics.Their volume is larg-er than graphite,showing the possible candidates for hydrogen storage.Elastic constants and phonon calculations show their mechanical stability and dynamic stability.The electronic properties of carbon foams is similar to that of carbon nanotube,depending on type and size:oC28 and oC32 carbon are metallic;oC36carbon is a semiconductor with band gap of 1.51 eV;oC24 carbon is a topological nodal line semimetal with two mirror-inversion symmetric nodal lines,and has a surface flat band nestled in a projected nodal line on the surface,suggesting great potential for superconducting materials,battery materials and catalytic materials.