Computer Simulation Of The Defect Properties Of Several Advanced Ceramic And Electronic Materials

Defects widely exist in materials and usually have significant effects on the host properties.When taking advantage of intrinsic properties of materials for investigating or manufacturing,defects should be avoided as much as possible.On the other hand,decorating defects in materials to modify the host basic properties is usually utilized to achieve the functionalization of materials.Therefore,studying the defects properties of materials becomes crucial.In this dissertation,based on the density functional theory,we have performed a series of studies on the defect properties of some advanced ceramic and two-dimensional electronic materials.The main conclusions are as follows:1.A systematic study has been carried out to investigate the C doping effects in?-Al₂O₃.The work helps develop an atomic-level understanding of the intrinsic character of C defects in the?-Al₂O₃:C crystals.Our results reveal that,under the reducing?Al-rich?synthesis conditions,only defect ofC_O^-2 has the low enough formation energy to occur in a relevant concentration in?-Al₂O₃ under the thermal equilibrium conditions.It implies that C tends to act as an acceptor directly involved in modifying the intrinsic electronic properties of?-Al₂O₃ by substituting on the oxygen site.This is consistent with the proposal that a large number of F⁺centers are formed as charge compensators to C_O^-2 ions in?-Al2O3.Based on the most stable charge state of-2 and the??-1|-2?thermodynamic transition level?TRL?of 3.82 eV with respect to the VBM,C defects are expected to act as double acceptors,inducing considerable deep electron traps in?-Al₂O₃,which has significant effects on the dosimetry properties of the?-Al₂O₃:C materials.2.Ab initio molecular dynamics?AIMD?method is employed to simulate the low energy recoil events in Ti₃AlC₂?TAC?and Ti₃Si C₂?TSC?,to compare their radiation responses and explore the origin of the discrepancy.The results reveal that,for each recoil event,the defect generation is dominated by similar mechanisms in these two materials.The threshold displacement energies?E_ds?of TAC and TSC are comparable to each other when recoil events occur almost within the Ti₃C₂ block.This results from the similar structural properties of the Ti₃C₂ in these two materials.When Al/Si atoms are involved to form antisite defects with Ti recoils,or the whole recoil events occur in the Al/Si plane,leading to the formation of Al/Si Frenkel pairs,TSC has slightly larger E_d values compared with TAC.This results from the moderate difference between the energy barriers during the recoil pathway.It is found that the largest discrepancy in the displacement energy between TAC and TSC corresponds to the Al/Si recoil along the[001]directions,which is due to the significant discrepancy in the energy barriers during the pathway.3.A systematic study has been carried out to investigate the effects of vacancy defects and chemical element doping on the electronic and magnetic properties of the GaSe monolayer.Our results show that magnetic states can be induced by decorating V-Ga,V-Ga2,V-GaSe₃ and Ga₂Se₆,or by selectively doping specific transition-metal?TM?atoms as well as A group atoms at the Ga or Se site.Besides,it is found that the n-type conductivity can be induced in the GaSe monolayer by doping Si and Cl at the Ga and Se sites,respectively.As a result,the electronic and magnetic properties of the GaSe monolayer can be manipulated by introducing some intrinsic and extrinsic defects.4.A systematic study has been carried out to investigate the vacancy effects in two dimensional?2D?d⁰ materials Sn S₂ and ZrS₂ with and without applying strain.The results show that V-Sn,V-Zr,and V-SnS₆ are intrinsic magnetic defects.The semiconducting and magnetic properties of V-Sn doped SnS₂ are preserved under the external strain,while the magnetic moments of V-Zr doped ZrS₂ monolayer decrease/increase under the external compressive/tensile strain.The electronic and magnetic properties of V-Sn S₆ doped SnS₂is robust against the tensile strain,while they can be manipulated under the compressive strain.As a result,by decorating special vacancy defects,magnetism can be introduced in the single-layered SnS₂ and ZrS₂,which can be manipulated by applying external strain.5.A systematic study has been carried out to investigate the transition-metal?TM?atom?Sc-Zn?doping effects in the SnS₂ and ZrS₂ monolayers.Our results demonstrate that nontrivial magnetisms can be induced by doping V/Cr/Mn/Fe/Co at the cation sites.The behaviors of the magnetic exchanges of TM atoms exhibit some differences between Sn S₂and ZrS₂,which are found strongly dependent on the dopants'chemistry and spatial positions.Specifically,stabilized by double exchange and/or p-d exchange interactions,V/Co/Fe and V/Co/Cr can induce both short-and long-range ferromagnetism?FM?in SnS₂ and ZrS₂,making these doped systems promising candidates for 2D dilute magnetic semiconductors.