Cu,Fe掺杂LiNbO_3晶体电子结构和光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理对Cu,Fe单掺及共掺LiNbO_3晶体的电子结构和光学性质进行了计算.结果显示:Cu,Fe单掺杂LiNbO_3晶体禁带内均产生了杂质能级,主要由Cu3d,Fe3d轨道及O 2p轨道贡献;共掺LiNbO_3晶体禁带内出现了双能级结构,深能级由Cu3d和O2p轨道贡献,浅能级由Fe3d和O2p轨道贡献.Cu,Fe单掺和共掺LiNbO_3晶体带隙依次缩小,在可见光区的光吸收明显增强.共掺LiNbO_3在445和630nm左右分别表现出一个宽吸收峰,比单掺LiNbO_3晶体表现出更好的光吸收性质.研究表明,Fe占Nb位比Fe占Li位的双掺样品在双光存储应用中更有优势;同时,浓度比Fe2+]/Fe3+]值的适当降低有助于这种优势的形成.

First-principles study on the electronic structures and optical prop erties of Cu,Fe dop ed LiNbO3 crystals

The binding energies, electronic structures and optical properties of LiNbO₃ and Cu/Fe doped LiNbO₃ crystals are investigated by first principles based on the density functional theory in this paper. The supersell structures of crystals are established each with 60 atoms, including five models: pure LiNbO₃, LN1 (Cu²⁺ occupy Li⁺ site), LN2 (Fe³⁺ occupy Li⁺ site), LN3 (Cu²⁺ occupy Li⁺site and Fe³⁺ occupy Li⁺ site) and LN4 (Cu²⁺ occupy Li⁺ site and Fe³⁺ occupy Nb⁵⁺ site). The optimized results show that the total energies of all models can achieve certain stable values, which means that the models accord with the actual crystal structures. The impurity energy levels of Cu and Fe doped LiNbO₃ crystals appear within the band gaps, which are contributed by Cu 3d orbital, Fe 3d orbital and O 2p orbital; in co-doped LiNbO₃, Cu offers deep energy level and Fe offers shallow energy level within the band gaps. There are two wide absorption peaks appearing respectively at 445 nm and 630 nm in co-doped LiNbO₃ crystal, which correspond to the electron transitions from E_g orbital of Cu to Nb 4d orbital and T_2g orbital of Fe to Nb 4d orbital respectively; the absorption edge of Cu, Fe mono and co-doped LiNbO₃ crystals are red-shift successively, which coincides with the variation of band gape. The light absorption intensity of co-doped LiNbO₃ crystal is stronger than that of mono-doped LiNbO₃ crystal. The co-doped sample light absorption property is related to Fe site occupation. In this paper, it is suggested that the co-doped sample with Fe at Nb site is more competitive than that with Fe at Li site in optical volume holographic storage applications, and that reducing properly Fe²⁺]/Fe³⁺] value may be conducible to the formation of this advantage.