【作者】 何娅丽；

【导师】 薛冬峰；

【作者基本信息】 大连理工大学 ， 无机化学， 2007， 硕士

【摘要】 本文从化学键的观点出发,综合考虑离子半径和化合价对离子占位的影响,提出了用键能方法来研究掺杂离子在铌酸锂（LiNbO₃）晶格中的占位以及不同掺杂离子对LiNbO₃晶体光学性能的影响。在LiNbO₃晶格中,掺杂离子M通常进入Li位或Nb位形成M-O键,根据键能越相近,化学键的性质越相似,掺杂引起晶格变形越小的观点,分别比较M-O键与正常格位Li-O键和Nb-O键的键能变化大小,判断出掺杂离子优先进入Li位还是Nb位。研究结果表明通过理论计算得到的掺杂离子占位情况与实验结果一致。通过分析掺杂离子占据不同格位的键能变化情况,发现光折变离子无论占Li位还是占Nb位键能都减小,而抗光折变离子占Nb位时键能增加。根据这一规律提出了用键能标准来区分光折变离子和抗光折变离子,并预测了一些新的抗光折变离子用于指导铌酸锂光学器件的研究与开发。运用键能方法定量计算了抗光折变离子在LiNbO₃晶体中的理论掺杂阈值。以目前研究得最为广泛且阈值浓度最为统一的同成分掺Mg铌酸锂晶体的阈值5.0 mol%为基准,分别计算了未掺杂LiNbO₃与掺Mg∶LiNbO₃晶体的键能,比较后发现掺Mg∶LiNbO₃晶体阈值处的键能比未掺杂LiNbO₃晶体的键能小2.14 kcal。根据不同抗光折变掺杂LiNbO₃晶体在阈值处所呈现出的共性,在相同的生长条件下,所有的抗光折变掺杂离子在阈值处都会使晶体的键能减小2.14 kcal。根据这一推论分别计算了抗光折变离子在同成分和近化学计量比LiNbO₃晶体中的理论掺杂阈值,定量计算的结果与实验报道结果很接近。从晶体化学结构式出发,提出了抗光折变离子在铌酸锂晶体中的掺杂取代机理,并在此基础上研究了晶体键能与缺陷结构之间的内在关系,发现晶体键能与本征缺陷Li空位的变化趋势相反,晶体键能随着掺杂量的增加先增加后减小而后又增加,而Li空位随着掺杂量的增加先减小后增加而后又减小。论文最后还研究了掺杂LiNbO₃晶体的缺陷能级。从晶体结构和化学键的观点出发,提出了计算掺杂LiNbO₃晶体缺陷能级的电荷迁移方法,计算结果表明理想的光折变中心在LiNbO₃晶格中应当具有适当的能级。通过计算所得到的掺杂离子在LiNbO₃晶格中的相对能级大小对开发应用前景广阔的双掺杂非挥发全息存储器件具有很重要的理论指导意义。更多还原

【Abstract】 From the viewpoint of chemical bond, a bonding energy model is proposed to study the occupancy and properties of dopants in the lithium niobate （LiNbO₃） single crystals based on the impact of the ionic radii and valence state.Dopants usually incorporate into Li or Nb sites in the LiNbO₃ crystallographic frame. Generally speaking, the properties of chemical bonds are more similar if they have closer bonding energies. Thus dopants make the crystal lattice change little. According to this opinion, the dopant occupancy in the LiNbO₃ matrix can be determined by comparing the deviation of its bonding energy in different lattice location at both Li and Nb sites. The theoretical occupancy of dopants agrees well with the experimental results.By analyzing the variation of bonding energy of dopants in different lattice sites （i.e., Li or Nb sites）, we find that photorefractive （PR） ions decrease the crystal bonding energy whether they occupy Li or Nb sites, whereas those optical damage resistant （ODR） dopants have the ability to increase the crystal bonding energy when they move to occupy Nb sites. It gives us a bonding energy criterion to distinguish ODR and PR ions and predict some ODR ions to explore new optical devices for the practical applications.The threshold concentration of ODR ions in the LiNbO₃ single crystals is quantitatively obtained using bonding energy method. Take the reliable threshold value 5.0 mol% of Mg doped congruent LiNbO₃ as reference, the crystal bonding energies of pure and Mg doped LiNbO₃ are calculated and the bonding energy of Mg doped LiNbO₃ at the threshold decreases 2.14 kcal compared to that of undoped congruent LiNbO₃. The threshold concentration of ODR ions in both congruent and near-stoichiometric LiNbO₃ crystals can be calculated in the light of the same reduced bonding energy （2.14 kcal） at the same growth condition, and it is assumed due to the same property at the threshold of various ODR dopants. The calculated results agree well with the literature reports.The doping mechanism of ODR ions in the LiNbO₃ crystal is proposed according to the structure formula. The internal relation between crystal bonding energy and defect structure is investigated. We find that with the increased doping concentration, the crystal bonding energy first increase and then decrease and then increase, while the change trend of intrinsic defect Li vacancy is opposite, it first decrease and then increase and then decrease.In addition, based on the crystal structure and chemical bonding, a charge transfer model is proposed for the calculation of energy gap of defects in LiNbO₃ single crystals. It is shown that the PR centers should have appropriate energy level in the LiNbO₃ crystals, which provides us a theoretical guide to obtain promising nonvolatile holographic recording devices through incorporating impurities intentionally.更多还原