Co,Zn共掺铌酸锂电子结构和吸收光谱的第一性原理研究

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.

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

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 12 Li 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⁺-2V_Li^- and Co_Li⁺-Zn_Nb^3--2Zn_Li⁺ 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 eV. The band gap of the doped LiNbO₃ crystal is narrower than that of pure LiNbO₃ crystal, due to the Co 3d and Zn 3d orbit energy levels superposed with that of O 2p orbit energy levels, and thus forming the upside of covalent bond. The band gap of Co:LiNbO₃ crystal is 3.32 eV, and that of Co:Zn:LiNbO₃ crystals are 2.87 eV and 2.75 eV respectively for Co:Zn(L):LiNbO₃ and Co:Zn(E):LiNbO₃ model. The Co 3d orbit is split into e_g orbit and t_2g orbit with different energies. The absorption peak at 2.40 eV appears in the band gap of Co:LiNbO₃ crystal, which is attributed to the transfer of the Co 3d splitting orbital t_2g electrons to conduction band. The absorption peaks of 1.58 eV and 1.10 eV could be taken as the result of e_g 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 Zn²⁺ and Co²⁺ as Co²⁺+Zn²⁺Co³⁺+Zn⁺ exist in the crystal, which results in the decrease of e_g orbital electron number, but hardly affect the t_2g orbital electron. The Co ion in crystal could act as the deep-level center (2.40 eV) or the shallow-level center (1.58 eV) 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.