Studies On Physical Properties Of Novel Carbon Allotropes And Carbides

Carbon and its compounds have the wide applications in all fields of modern industry.Utilizing the crystal structure prediction technology,which is based on the particle swarm optimization method,and the first-principles calculations,some novel carbon allotropes and carbides were explored with the studies of physics properties.The researches on the mechanical,thermodynamical,electronic and optical properties of stable structures of different systems at pressures provide the theoretical foundations to experimental synthesis and explorations of other new materials.There are three novel carbon allotropes proposed,such as superhard tetragonal C₆₄,superhard metallic C₅ and light metallic mC₁₂.The superhard C₆₄ is predicted to have the Vickers hardness till to 60.2 GPa,and it is a quasi-direct band gap semiconductor with the band gap of 1.32 eV.The superhard metallic C₅ is consisted of the mixture of sp² and sp³carbons.The Vickers hardness is 58.5 GPa,and the structure shows the metallic nature.The novel hybrid sp-sp² monoclinic carbon allotrope mC₁₂ is a kind of promising light metallic material,owing to the small equilibrium density and large cell volume.The elastic constants and moduli reveal that mC₁₂ has a quite rigid mechanical property.For the carbides,the pressure-induced phase transition mechanism of Ca₂C reveals that the phase transition which transforms from metallic C2/m-Ca₂C to semiconducting Pnma-Ca₂C occurs at 7.8 GPa,and it is first-order with a volume drop of 26.7%.C2/m-Ca₂C₃ was found to be mechanically and dynamically stable only from 0 to 24 GPa,and can be regarded as a quasi-direct band gap semiconductor.The novel P4₂/mnm silicon carbides have two different stable structures:Si₈C₄ and Si₄C₈.They are approved to be the indirect semiconductors with the bandgap of 0.74 eV and 0.15 eV,respectively.The series of XC₆ and XC₁₂ phases,where the X atoms are from the elemental hydrogen to calcium,except noble gas atoms,have the mechanically and dynamically stable structures HC₆,NC₆,SC₆,BC₁₂,CC₁₂,PC₁₂,SC₁₂,ClC₁₂ and KC₁₂.The elastic properties,sound velocities,Debye temperatures and the electronic properties of these structures were also studied in detail.Moreover,the other novel semiconducting materials were studied.The cubic superhard phase of C₃N?c-C₃N?with estimated Vicker's hardness of 65 GPa is predicted to be the indirect semiconductor.The investigations of the mechanical anisotropy and the thermodynamic property of c-C₃N were made in detail.In the new stable phases of MoS,a phase transition order of Imm2?Pnma?Pm-3m was revealed with the transition pressures at 5.5 GPa and 45.3 GPa,respectively.Both the two phase transitions are first-order with the volume drop of 16.5%and 6.2%,respectively.The Imm2 and Pnma phases are indirect band gap semiconductors,whereas the Pm-3m phase shows metallic character.Finally,the optical properties of direct band gap silicon crystal D135-Si were systemically investigated.The imaginary part of the dielectric function shows that the effect of pressure on the optical absorption property of structure is the smallest,and the strain effects are gradually enhanced along c-,b-and a-direction,respectively.Based on the first-principles calculations,these novel structures exhibit intriguing properties,and they will be promising in applications if they could be successfully synthesized in experiments.With the increasing improvements of computing technologies and theories,the simulation and search of novel materials will acquire better achievements,and attract more researchers.