First-priciples Study And Phenomenological Model Analysis Of The Spectral Parameters Of Rare Earth Ions

Rare earth ion-doped luminescent materials have comprehensive applications in lighting, display, scintillator detectors, solid-state lasers, quantum storage, solar cells, and biological fluorescent labels, due to the unique energy-level structures and spectroscopic properties of rare earth ions. The phenomenological approaches, which are based on the effective Hamiltonians for the4fN and4fN-15d configurations and the effective dipole moment operators for the4fN-4fN transitions of rare earth ions, have been used to investigate the energy levels and transitions of rare earth ions in crystals. With the continual improvement of the computer configurations and the theoretical calculation methods, the first-principles calculation methods have received growing attention and have been widely used to study the rare earth-doped luminescent materials. In this dissertation, we investigate the local coordination structures, the4f and5d energy-level structures, and the spectral parameters of rare earth ions (Especially Ce3+ion) in the luminescent materials via the combination of the first-principles calculations and the phenomenological approaches.This dissertation consists of six chapters. In the first chapter, we give a brief introduction to the electronic structures of rare earth ions, the specific applications of rare earth-doped luminescent materials, and the main work of this dissertation.In the second chapter, we first introduce the parametrization of the4fN and4fN-15d configurations of rare earth ions in crystals, and explain the physical meanings of the interaction operators and the corresponding parameters in the effective Hamiltonians. Then, the method of fitting the experimental energy-level data and two simple calculation models for the crystal-field parameters (Point charge electrostatic model and superposition model) are presented. At last, we briefly introduce the Judd-Ofelt theory on the intensities of the4f-4f transitions of rare earth ions.In the third chapter, we briefly introduce the Hartree-Fock and Post-Hartree-Fock methods based on the wavefunctions, and the density functional theory (DFT) based on the density of electrons. In this dissertation, the rare earth ion-doped luminescent materials are studied via the Ab-initio model potential (AIMP) embedded cluster method. On the basis of the crystal structures obtained from the DFT geometry optimization calculations, we construct the embedded cluster to simulate the rare earth-doped systems, and subsequently perform the CASSCF/CASPT2/RASSI-SO calculations to obtain the energy levels, the wavefunctions, and the dipole moment of4fN-4fN transitions of rare earth ions. On the basis of the first-principles calculated results, we can construct the effective Hamiltonian and dipole moment operator, and then extract the crystal-field parameters and the intensity parameters. Finally, the first-principles calculation software packages (VASP and MOLCAS) adopted in this dissertation are also introduced.In the fourth chapter, the local coordination structures, the4f1and5d1energy levels, and the crystal-field parameters of Ce3+ion doped in a series of fluoride (CaF2, YF3, LaF3, KMgF3, LiYF4, K2YF5and KY3F10) and oxide compounds (X1-Y2SiO5and X2-Y2SiO5) are systematically researched via the calculation method presented in the third chapter. The calculated energy levels and crystal-field parameters are in good agreement with the experimental and the fitted results. On the basis of the calculated crystal-field parameters of Ce3+ion, we obtain the spectral parameters of the other lanthanide ions in the same host via the least squares fitting of the experimental data. Also, the onsets of the4fâ†'5d transitions of lanthanide series are obtained via the empirical model of Dorenbos, once the onsets of the4fâ†'5d transitions of Ce3+ion in the same host are calculated.In the fifth chapter, on the basis of the superposition-model analysis of the first-principles calculated crystal-field parameters of Ce3+in the fluoride compounds, the general expressions for superposition-model (SM) intrinsic parameters are derived. The SM intrinsic parameters are used to depict the physical interactions of the doping ion with its surrounding ligands. Ce3+-doped Cs2NaYF6(Oh case), CaF2(Both Oh and C4v cases), and KMgF3(Oh case) are then chosen as the model systems to derive the general expressions. Furthermore, we apply the derived expressions for intrinsic parameters to Ce3+-doped LiYF4and KY3F10systems to calculate the crystal-field parameters of Ce3+ion. The calculated crystal-field parameters are compared with the first-principles calculated as well as the fitted results to examine the accuracy of the general expressions for the intrinsic parameters.In the sixth chapter, we extend the previous calculation method of crystal-field parameters so that the electric dipole intensity parameters for4fN-4fN transitions of rare earth ions can be calculated. On the basis of the embedded cluster model for the rare earth-doped system, the CASSCF/RASSI-SO calculations are performed to obtain the wavefunctions and the electric dipole moment for4fN-4fN transitions of rare earth ions. Analogous to the method of constructing the effective Hamiltonian, we construct the effective dipole moment operator for4fN-4fN transitions. Then the intensity parameters are extracted according to the parameterization of the electric dipole intensity. Finally, for Ce3+-doped LiYF4, KY3F10, YAG and CaF2(C4v case) systems, the electric dipole intensity parameters for the4fN-4fN transitions of Ce3+ion are extracted and discussed.