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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LiNbO3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：LiNbO3晶体禁带宽度为3.32 eV；Co：Zn：LiNbO3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：LiNbO3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Zn2+，在1.58，1.10 eV处的吸收峰增强，可以认为Zn2+与Co2+之间存在电荷转移，使eg轨道电子减少，但并不影响t2g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 211 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 CoLi+-VLi-, CoLi+-ZnLi+-2VLi- and CoLi+-ZnNb3--2ZnLi+ 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 eg orbit and t2g 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 t2g electrons to conduction band. The absorption peaks of 1.58 eV and 1.10 eV 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 Zn2+ and Co2+ as Co2++Zn2+Co3++Zn+ exist in the crystal, which results in the decrease of eg orbital electron number, but hardly affect the t2g 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.
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Keywords:
- density functional theory /
- doped lithium niobate crystal /
- energy level structure /
- absorption spectra
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[1] Hesselink L, Orlov S S, Liu A, Akella A, Lande D, Neurgaonkaret R R 1998 Science 282 1089
[2] Zhao B Q, Zhang Y, Qiu X Y, Wang X W 2016 Acta Phys. Sin. 65 014212 (in Chinese) [赵百强, 张耘, 邱晓燕, 王学维 2016 物理学报 65 014212]
[3] Lee H J, Shur J W, Shin T I, Yoon D H 2007 Opt. Mater. 30 85
[4] Xia H P, Wang J H, Zhang J L, Zhang Y P, Nie Q H 2005 Chin. J. Lasers 32 965 (in Chinese) [夏海平, 王金浩, 章践立, 张约品, 聂秋华 2005 中国激光 32 965]
[5] Choi Y N, Park I W, Kim S S, Park S S, Choh S H 1999 J. Phys.: Condens. Matter 11 4723
[6] Zheng W, Zhou Y X, Liu C X 2003 Acta Photon. Sin. 32 1492 (in Chinese) [郑威, 周玉祥, 刘彩霞 2003 光子学报 32 1492]
[7] Zeng X L, Wang J H, Xia H P, Zhang J L, Song H W, Zhang J H, Yao L Z 2004 Chin. J. Lumin. 25 435 (in Chinese) [曾宪林, 王金浩, 夏海平, 章践立, 宋宏伟, 张家骅, 姚连增 2004 发光学报 25 435]
[8] Kong Y F, Li B, Chen Y L, Huang Z H, Chen S L, Zhang L, Liu S G, Xu J J, Yan W B, Liu H D, Wang Y, Xie X, Zhang W L, Zhang G Y 2003 J. Infrared Millim Waves 22 40 (in Chinese) [孔勇发, 李兵, 陈云琳, 黄自恒, 陈绍林, 张玲, 刘士国, 许京军, 阎文博, 刘宏德, 王岩, 谢翔, 张万林, 张光寅 2003 红外与毫米波学报 22 40]
[9] Zhang Y, Xu Y H, Li M H, Zhao Y Q 2001 J. Cryst. Growth 233 537
[10] Abrahams S C, Reddy J M, Bernstein J L 1966 J. Phys. Chem. Solid 26 997
[11] Iyi N, Kitamura K, Izumi F, Yamamoto J K, Hayashi T, Asano H, Kimura S 1992 J. Solid State Chem. 101 340
[12] Tsai P C, Sun M L, Chia C T, Lu H F, Lin S H, Hu M L, Lee J F 2008 Appl. Phys. Lett. 92 161901
[13] Fujita H, Inoue M, Phillips W 1978 J. Phys. Soc. Jpn. 44 1909
[14] Segall M D, Lindan P J D, Probert M J, Pickard C J, Hasnip P J, Clark S J, Payne M C 2002 J. Phys.: Condens. Matter 14 2717
[15] Wang W, Wang R, Zhang W, Xing L, Xu Y, Wu X 2013 Phys. Chem. Chem. Phys. 15 14347
[16] Zhang X C, Zhao L J, Fan C M, Liang Z H, Han P D 2012 Acta Phys. Sin. 61 077101 (in Chinese) [张小超, 赵丽军, 樊彩梅, 梁镇海, 韩培德 2012 物理学报 61 077101]
[17] Zeng F, Sheng P, Tang G S, Pan F, Yan W S, Hu F C, Zou Y, Huang Y Y, Jiang Z, Guo D 2012 Mater. Chem. Phys. 136 783
[18] Thierfelder C, Sanna S, Schindlmayr A, Schmidt W G 2010 Phys. Status Solidi C 7 362
[19] Lei X W, Lin Z, Zhao H 2011 J. At. Mol. Phys. 28 944 (in Chinese) [雷晓蔚, 林竹, 赵辉 2011 原子与分子物理学报 28 944]
[20] Gao P, Liu Q J, Zhang X J 2010 Acta Phys. Sin. 59 493 (in Chinese) [高攀, 柳清菊, 张学军 2010 物理学报 59 493]
[21] Gray H B 1964 J. Chem. Educ. 41 1
[22] Xia H P, Wang J H, Zhang J L, Zhang Y P 2005 J. Chin. Ceram. Soc. 33 1326 (in Chinese) [夏海平, 王金浩, 章践立, 张约品 2005 硅酸盐学报 33 1326]
[23] Yan W B, Li Y X, Shi L H, Chen H J, Liu S G, Zhang L, Huang Z H, Chen S H, Kong Y F 2007 Opt. Express 15 17010
[24] Wood D L, Remeika J P 1967 J. Chem. Phys. 46 3595
[25] Arizmendi L, Cabrera J M, Agullolopez F 1984 J. Phys. C: Solid State Phys. 17 515
[26] Mok F H, Burr G W, Psaltis D 1996 Opt. Lett. 21 896
[27] Xu J J, Liu S M, Wu Y Q, Zhang G Y 1991 Acta Phys. Sin. 40 1443 (in Chinese) [许京军, 刘思敏, 武原庆, 张光寅 1991 物理学报 40 1443]
[28] Psaltis D, Berben D, Buse K, Luennemann M, Berben Dirk, Hartwig Ulrich, Buse Karsten 2003 J. Opt. Soc. Am. B 20 1491
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