Er^(3+)掺杂TiO_(2)的局域结构及电子性质的第一性原理研究

First-principles calculations of local structure and electronic properties of Er^(3+)-doped TiO_(2 )

三价稀土铒离子(Er^(3+))掺杂二氧化钛(TiO_(2))因其优异的光电性能,在众多稀土掺杂发光晶体材料中脱颖而出,具有非常广泛的应用前景.利用CALYPSO(Crystal structure AnaLYsis by Particle Swarm Optimization)结构搜索方法和第一性原理计算,成功地预测和探究了三价铒离子掺杂二氧化钛(Er^(3+):TiO_(2))晶体的基态结构.首次报道Er^(3+)掺杂TiO_(2)的最低能量结构具有P4m2空间对称性.当Er^(3+)离子掺杂基质晶体时,Er^(3+)离子占据Ti^(4+)离子的位置,并造成结构畸变,最终使得Er^(3+)离子的局域对称性由D2d降低为C2v.通过电子结构计算发现Er^(3+)掺杂TiO_(2)的能带隙值约为2.27 eV,这表明Er^(3+)离子的掺杂会导致体系的带隙值降低,但没有改变其半导体性质,从而在光伏电池和半导体激光器等领域具有更广泛的应用.这些发现不仅为进一步探索Er^(3+):TiO_(2)晶体的性质和应用提供了数据参考,也为探究其他稀土掺杂晶体材料提供了最新的方法.

Trivalent rare earth erbium ion(Er^(3+))doped titanium oxide(TiO_(2))can possess a very wide range of applications due to its excellent optoelectronic properties,thus standing out among many rare-earth-doped luminescent crystals.However,the issues regarding local structure and electronic properties have not been finalized.To address these problems,the CALYPSO(Crystal structure AnaLYsis by Particle Swarm Optimization)method combined with the first-principles calculations is employed,and many converged structures of Er^(3+)-doped TiO_(2) are successfully obtained.Further structural optimization is performed by using the VASP(Vienna ab initio simulation package)software package,and we report for the first time that the lowest energy structure of Er^(3+)-doped TiO_(2) has the P4m2 symmetry.It can be observed that the doped Er^(3+)ions enter into the host crystal and occupy the positions of Ti^(4+) ions,resulting in structural distortion,which eventually leads the local Er^(3+)coordination site symmetry to reduce from D_(2d) into C_(2v).We speculate that there are two reasons:1)the difference in charge between Er^(3+)ions and Ti4+ions leads to charge compensation;2)the difference between their electron radii is obvious:the radius is 0.0881 for Er^(3+)ion and 0.0881 for Ti4+ion.In addition,during the structural search,we also find many metastable structures that may exist at a special temperature or pressure,which play an important role in the studying of structural evolution.When the electronic band structure of the Er^(3+)-doped TiO_(2) system is calculated,we adopt the method of local density approximation(LDA)combined with the on-site Coulomb repulsion parameter U to accurately describe the strongly correlated system.For the specific value of U,we adopt 3.5 eV and 7.6 eV to describe the strong correlation of 3d electrons of Ti^(4+) ions and 4f electrons of Er^(3+)ions,respectively.According to the calculation of electronic properties,the band gap value of Er^(3+)doped TiO_(2) is about 2.27 eV,which is lower than that of the host crystal(E_(g)=2.40 eV).The results show that the reduction in the band gap is mainly caused by the f state of Er^(3+)ions.The doping of Er ion does reduce the band gap value,but it does not change the conductivity of the system,which have great application prospect in diode-pumped laser.These findings not only provide the data for further exploring the properties and applications of Er^(3+):TiO_(2) crystals,but also present an approach to studying other rare-earth-doped crystalline materials.