First-principles Study Of AlX?X=N,P,As? And Superhard B₂CO Compounds

Within this thesis,the potential crystal structures of AlX?X = N,P,As?and B₂ CO have been researched based on the crystal structure prediction CALYPSO package.Based on the first-principles calculations,their thermodynamic stabilities have been checked according to the energy comparison with known structures or the research on formation enthalpy;their mechanical stabilities have been analysed by independent elastic constants;and their dynamic stabilities have been verified by phonon dispersion curves.Then their mechanical and electrical properties have been studied,the influence of pressure on their properties has also been analysed.For the structure prediction of AlN,four orthorhombic metastable phases?Pmn21-,Pbam-,Pbca-,and Cmcm-AlN?with the energy between wz-AlN and rs-AlN at atmospheric pressure were proposed.They are all direct band-gap semiconductors,and possess high Vickers hardness between 13.2—15.2 GPa.In the pressure range of 0—20 GPa,their band gaps all increase with the pressure increase.For the structure prediction of AlP,three metastable phases?cI24-,hR18-,and oC12-AlP?with the energy between zb-AlP and CsCl-AlP at atmospheric pressure were proposed.They are all pressure driven structures.Based on the research on mechanical properties,the hardness of the three new AlP structures are higher than that of NiAs-AlP,especially,oC12-AlP possesses the highest hardness,even higher than wz-AlP and zb-AlP.At atmospheric pressure,hR18-AlP is a conductor;oC12-AlP is a semiconductor with indirect band-gap;cI24-AlP is a direct band-gap semiconductor.Their band gaps all decrease gradually until disappear with the pressure increase.For the structure prediction of AlAs,three metastable phases?oC12-,hP6-,and cI24-AlAs?with the energy between zb-AlAs,NiAs-AlAs and Cmcm-AlAs at atmospheric pressure were proposed.They are all pressure driven structures.The research on mechanical properties have revealed that oC12-AlAs and hP6-AlAs both possess the similar properties,and higher than cI24-AlAs.Based on the research on electrical properties,oC12-,and hP6-AlAs are both semiconductors with indirect band gaps of 0.468 eV and 1.356 eV,respectively;cI24-AlAs is a direct band gap semiconductor with gap value of 1.761 eV.Based on the structural prediction program CALYPSO,a new orthorhombic B₂ CO phase?oP8-B₂CO?with strong sp3 hybridization B-C/O covalent bonds was proposed.Based on different models for hardness prediction,oP8-B₂ CO is a superhard structure.The electrical properties' research indicated that oP8-B₂ CO is a semiconductor with indirect band gap,and the gap value is larger than these of superhard tetragonal phases tP4-B₂ CO and tI16-B₂ CO.The gap-pressure relationship research indicated that the gaps of all superhard B₂ CO increase with loading pressure within 0—100 GPa.