中文摘要：

基于密度泛函理论的第一性原理，研究了LiNbO3晶体以及不同Mg浓度的Fe：Mg：LiNbO3晶体的电子结构和吸收光谱．研究结果显示：掺铁铌酸锂晶体的杂质能级由Fe的3d轨道和O的2p轨道贡献，禁带宽度为2．845eV；对于Mg，Fe共掺样品，Mg的浓度小于或等于阈值时，禁带宽度分别为2．901和2．805eV；掺铁铌酸锂晶体的吸收谱在2．3和2．66V处分别存在一个吸收峰，其强度因Mg的浓度不同而发生变化．研究结果还表明，不同浓度的Mg对晶体内Fe^2＋和Fe^3＋的浓度以及占位产生了不同的影响．还提出了光电子的形成不应单独考虑铁的轨道电子态，而应同时考虑与铁成键的氧的轨道电子态的观点．

英文摘要：

In this paper, the electronic structures and absorption spectra of LiNbO3 （LN） and Fe：Mg：LiNbO3 crystals are studied by the first-principles under the generalized gradient approximation. The supercell structures of the LN crystal are established with 60 atoms, including four models： pure LN crystal, Fe：LiNbO3 crystal （Fe：LN）, Fe：Mg：LiNbO3 crystal with Mg of 2 mo1%-3 mol% （Fe：Mg（L）：LN）, and Fe：Mg：LiNbO3 crystal with Mg of 5.0 mol% （Fe：Mg（E）：LN）. The electronic structures show that the extrinsic defect levels （within forbidden band） of Fe：LN are contributed by Fe 3d orbital and O 2p orbital, and the band gap of Fe：LN （about 2.85 eV） is narrower than that of LN. For Fe：Mg：LN crystals, the band gap changes to 2.90 eV and 2.81 eV respectively for the Mg ion concentration less than and equal to the threshold （~5.0 mol%）. The two absorption peaks at 2.3 eV and 2.6 eV are attributed to the Fe ions in crystal. Moreover, the intensities of these peaks vary with the concentration of Mg ion. It is revealed that the concentration of Mg ion influences the concentrations and the sites of Fe^2＋ and Fe^3＋ ions in crystal. From the absorption spectrum, the values of ratio Fe^2＋/Fe^3＋ in Fe：Mg（E）：LN and Fe：Mg（L）：LN can be obtained, and the ratio of first sample is smaller than that of the second one. With the one-center model, one can distinctly deduce that the photoconductivity of Fe：Mg（E）：LN is relatively weak compared with that of Fe：Mg（L）：LN, but this is inconsistent with many experimental results. One notices the contribution of O 2p orbital to extrinsic defect level in electronic structure. Therefore, it is reasonable to presume that the one-center model is not suitable enough for this condition. Based on the research work, we find that the formations of photoelectrons are related to orbital electron states of iron ions and oxygen atoms at extrinsic defect levels in Fe：LN and Fe：Mg：LN crystals.