Co,Zn共掺铌酸锂电子结构和吸收光谱的第一性原理研究

First-principle calculation of electronic structures and absorption spectra of lithium niobate crystals doped with Co and Zn ions

利用基于密度泛函的第一性原理的计算方法,研究了Co单掺及Co和Zn共掺LiNbO_3晶体的电子结构和吸收光谱.研究显示,各掺杂体系铌酸锂晶体的带隙均较纯铌酸锂晶体变窄. Co:LiNbO_3晶体禁带宽度为3.32 eV; Co:Zn:LiNbO_3晶体, Zn的浓度低于阈值或达到阈值时,禁带宽度分别为2.87或2.75 eV. Co:LiNbO_3晶体在可见-近红外光波段2.40, 1.58, 1.10 eV处形成吸收峰,这些峰归结于Co 3d分裂轨道的跃迁;加入抗光折变离子Zn^(2+),在1.58, 1.10 eV处的吸收峰增强,可以认为Zn^(2+)与Co^(2+)之间存在电荷转移,使e_g轨道电子减少,但并不影响t_(2g)轨道电子.结果表明,晶体中的Co离子在不同共掺离子下可充当深能级中心(2.40 eV),或可充当浅能级中心(1.58 eV),两种情况下,掺入近阈值的Zn离子均有助于实现优化存储.

In this paper, the electronic structures and absorption spectra of Co doped and Co, Zn co-doped LiNbO3 crystals are studied by the first-principle using the density functional theory, to explore the characteristics of charge transfer in Co, Zn co-doped LiNbO3 crystals, and to build the relationship between these characteristics and the holographic storage quality. The basic model is built as a supercell structure of 2 × 1 × 1 of near-stoichiometric pure LiNbO3 crystal with 60 atoms, including 12Li atoms, 12 Nb atoms and 36 O atoms. Four models are established as the near-stoichiometric pure LiNbO3 crystal（LiNbO3）, the cobalt doped LiNbO3 crystal（Co：LiNbO3）, the zinc and cobalt co-doped LiNbO3 crystal [Co：Zn（L）：LiNbO3] with doping ions at Li sites, and the other zinc and cobalt co-doped LiNbO3 crystal [Co：Zn（E）：LiNbO3）] with zinc ions at Li sites and Nb sites. The last two models would represent the concentration of Zn ions below the threshold（6 mol%） and near the threshold, respectively. The charge compensation forms are taken as Co＋Li-V-Li, Co＋Li-Zn＋Li-2 V-Liand CoLi-＋-ZnNb^3--2 ZnLi-＋respectively in doped models. The results show that the conduction band and valence band of pure LiNbO3 crystal are mainly composed of O 2p orbit and Nb 4d orbit respectively, and energy gap is 3.48 e V. The band gap of the doped LiNbO3 crystal is narrower than that of pure LiNbO3 crystal, due to the Co 3 d and Zn 3d orbit energy levels superposed with that of O 2 p orbit energy levels, and thus forming the upside of covalent bond. The band gap of Co：LiNbO3 crystal is 3.32 e V, and that of Co：Zn：LiNbO3 crystals are 2.87 e V and 2.75 e V respectively for Co：Zn（L）：LiNbO3 and Co：Zn（E）：LiNbO3 model. The Co 3d orbit is split into eg orbit and t2 gorbit with different energies. The absorption peak at 2.40 e V appears in the band gap of Co：LiNbO3 crystal, which is attributed to the transfer of the Co 3 d splitting orbital t2 gelectrons to conduction band. The absorption peaks of 1.58 e V and 1.10 e V could be taken as the result of eg electron transfers of both Co2＋and Co3＋in crystal, especially the latter ion.These two absorption peaks are obviously enhanced in Co：Zn（E）：LiNbO3 crystal compared with in other samples in this paper. Based on that, it could be proposed that a charge transfer between Zn^2＋ and Co^2＋ as Co^2＋ Zn^2＋→Co^3＋ Zn-＋ exist in the crystal, which results in the decrease of eg orbital electron number, but hardly affect the t2 gorbital electron.The Co ion in crystal could act as the deep-level center（2.40 e V） or the shallow-level center（1.58 e V） with the different accompanying doped photorefractive ions in the two-light holographic storage applications. In both cases, the choice of Zn ion concentration near threshold could be helpful for the photo damage resistance and recording light absorption in storage applications.