First-principles Study Of Photorefrective Ions Doped Lithium Niobate Crystals With Antiphotorefrective Ion Or Intrinsic Defect

LN crystal is a kind of photorefractive material which has important applications in many fields,especially in optical holographic storage.It is identifieded as one of the preferred materials for optical storage.The optical storage technology has the advantages of high storage density,large storage capacity,parallel transmission,associated addressing and many other advantages in the next generation of storage technology to show a good prospect for the future application.The LN crystal and its doping series crystal are the preferred materials for this technology.The photorefractive ions dopped in LN crystal play the the most important role in the optical storage processes,and the antiphotorefractive ions would resist optical damage,they would be used together in the LN crystal.There are generally intrinsic defects within LN crystal,these structures are conducive to doping,and might change the storage quality.In this paper,one research the electronic structures and the optical properties of the codoped LN crystal and its comparing group by the first-principles based on the density functional theory,the results show that: 1.For the Mn:In:LN crystal and its comparing group researchs,the extrinsic defect levels within forbidden band of Mn:LN crystal are mainly contributed by Mn 3d orbital electrons,and the extrinsic defects also effect the top of the valence band.The band gap of Mn:LN about 3.18 eV is narrower than that of LN;those of In:Mn:LN and In(E):Mn:LN sample are 2.82 eV and 2.93 eV respectively.The electron density of state(DOS)of Manganese codoped LiNbO3 crystal shows that the orbits of Mn3 d,Nb4d and O2 p superpose each other,i.e.forming covalent bonds,which result in conduction and valence bands shifting to low energy.The Indium ion doesn't contribute the extrinsic energy level within forbidden band,it effects the band gap through changing O2-electron cloud shape.The band gap narrows down if the Indium ions occupy Lithium ion positions,and becomes broad if the Indium ions occupy Niobium ion positions.It is found that the Mn:LN,In:Mn:LN and In(E):Mn:LN samples display the absorption peaks at 3.25 eV,3.11 eV,2.97 eV,2.85 eV,2.13 eV and 1.66 eV.The last absorption peak is contributed by the electron transferring from the Mn2+ energy level to conduction band,and the doping of Indium ions leads to attenuating of this peak.The peak at 2.13 eV is relative with the Mn3+,it is enhanced by the doped Indium ions.The Indium ions in crystal would influence the absorption,which is related to Manganese ions,by transforming Manganese ion valence via the formula as ++++nMn+?+ InMI2332 n,that is,with the doping of the Indium ions,the concentration of photorefractive center Mn2+ decreases,which is responsible to the absorption peak at 1.66 eV.It must be mentioned that the Mn2+ possesses not only the shallow levels as previous,but also the deep ones,and those are responsible for the absorptions at 2.85 eV and other high energies.For the Indium and Manganese codoped LiNbO3 crystals,if the recording light is chosen at near 1.66eV(748nm),the relatively low concentration of Indium ion is proposed to be chosen to achieve the high recording sensitivity.2.For the Fe:LN crystal with the intrinsic defect research,the extrinsic defect levels(within forbidden band)of Fe:LN are contributed by Fe3 d orbital and O2 p orbital;the band gap of Fe:LN(about 2.29eV)is narrower than that of LN;the band gap of Fe(P):LN,Fe(HP):LN and Fe(HB):LN sample is 2.26 eV,2.37 eV and 2.24 eV respectively.On the other hand,the Fe:LN,Fe(P):LN and Fe(HP):LN sample displays absorption peak at 2.05 eV,2.07 eV and 2.34 eV respectively in the visible region,which contributed by electron transfer of iron ions;Fe(HB):LN crystal has two peaks in the visible region at 2.34 eV and 1.96 eV,originated from Fe ions and biopolarons.Generally bipolarons will dissociate into small polarons by illuminating,however,the small polaron samples show non absorption peak in visible region,so bipolaron level hasn't any advantage for the double optical storage with the two-center.Comparing with the absorption peak(about 2eV),one consider that this peak is relative with the position(Nb or Li)occupied by Fe ions;in near stoichiometrical Fe:LN crystal,due to the charge compensation requirement of bipolaron,meanwhile coinciding with the absorption experiment dates,it can be proposed that the FeNb exists even if the dose of iron is low(<1mol%)in Fe:nSLN crystal.