掺杂稀土离子激发组态电声子耦合的第一性原理研究

The theoretical study of electron-phonon couplings of excited configurations of doped rare-earth(RE) ions is the key to developing the theory of the RE-doped lattice dynamics, understanding the ultraviolet and vacuum-ultraviolet spectral mechanism of RE ions and designing the RE-based luminescent materials applied in the high energy spectral region. Since last century, the method of spectral generating functions and parameterized crystal-field (CF) theory have been proposed and applied to the study of the electron-phonon coupling problems, but these empirical models are rather rough and so weak in prediction as to greatly restrict their application range. Recently, applying the first-principle calculation to the RE-doped system is very popular due to the strong ability of the first-principle study in prediction. Thus, based on the first-principle technique, the present project focusing on the electron-phonon couplings of the excited configurations of doped RE ions in solids will be carried out by studying the following three aspects: 1) by employing the periodic density-functional solid calculation for defect system, the equilibrium geometry structures of the ground and excited configurations of the doped RE ions are optimized and the related vibration modes are analyzed with considering the Jahn-Teller effect of 5d electron; 2) a series of the embedded clusters containing RE ions obtained by relaxing the equilibrium structures along the directions of various vibration modes are calculated by performing the three MOLCAS calculations(i.e., the state-average complete-active-space (CAS) self-consistent-field study, the CAS second-order many-body perturbation theory calculations with a scalar relativistic many electron Hamiltonian and the restricted-active-space state-interaction spin-orbit calculation with the former wave-functions and the latter energies to account for the spin-orbit effect), and then these calculation results are used to extract the numerical expression of the electron-phonon interaction in terms of the effective CF Hamiltonian theory; 3) the simulation of the phonon sidebands in the high energy spectra and the calculation of non-radiative relaxation rates between highly-excited states are implemented based on the calculated electron-phonon coupling expression.

掺杂稀土离子激发组态电声子耦合的理论研究是发展稀土晶格动力学理论、理解稀土离子紫外和真空紫外光谱机理以及设计高能稀土发光材料的核心和关键。人们先后使用了光谱生成函数理论和参数化晶场理论来研究这一问题，但是这些唯象模型的粗糙性和弱预言性极大地限制了他们的应用。本项目将从第一性原理的角度研究稀土离子激发组态的电声子耦合问题，拟：1)通过周期性密度泛函理论的固体缺陷计算，优化稀土离子基组态和激发组态的平衡构型和分析相应的振动模式，并讨论5d电子的Jahn-Teller效应；2) 在沿振动模式方向弛豫各平衡构型的基础之上，对一系列弛豫获得的稀土扭曲镶嵌团簇进行基于波函数的全组态空间自洽场加二级多体微扰及自旋-轨道耦合相对论计算，再借助于有效晶场哈密顿理论从这些计算结果中解压缩出电声子耦合作用的数值表达；3）应用这样的表达到稀土离子高能光谱声子边带的模拟以及高能激发态之间的无辐射弛豫速率的计算中。

掺杂稀土离子激发组态电声子耦合的理论研究是发展稀土晶格动力学理论、理解稀土离子紫外和真空紫外光谱机理以及设计高能稀土发光材料的核心和关键。人们先后使用了光谱生成函数理论和参数化晶场理论来研究这一问题，但是这些唯象模型的粗糙性和弱预言性极大地限制了他们的应用。本项目从第一性原理的角度出发研究稀土离子激发组态4f^(n-1)5d^1的电声子耦合效应问题，主要完成了四方面的工作：1）荧光材料电子结构的纯密度泛函理论（DFT）理解（包含带宽，镧系离子4f和5d在带内的位置等）；2）用于计算5d电声子耦合效应的DFT和晶场理论杂化的计算方案的构建；3) 运用计算方案到一些实际例子上，特别是在Ca2(Al1-xMgx)(Al1-xSi1+x)O7: Eu2+和MB2Si2O8 (M=Sr, Ba): Ce3+这两个系统上获得了极大的成功；4）使用DFT技术探索了部分太阳能电池基质材料，以期在这些材料的声子能量研究上匹配稀土荧光粉的应用。围绕这些既定的工作目标，本项目共计发表SCI收录论文25篇，其中以通讯作者发表在Journal of the American Chemical Society（顶级期刊）1篇，较彻底地解决了掺Eu2+或者Ce3+材料中所涉及到的电声子耦合效应问题，所提出的计算方案已经在Phosphor Safari 2015等国际学术大会上作为邀请报告宣讲，并获得国际同行的肯定。