| Rare-earth ion-doped luminescent materials have extensive applications in many fields such as white LEDs, laser, long afterglow and scintillators due to the abundant electronic energy levels and optical transitions of rare-earth ions. Meanwhile, the optical transitions between the hyperfine energy levels of rare-earth ions have long coherent time and inhomogeneous linewidth, which makes the rare-earth ion-doped inorganic crystals show tremendous application prospects in quantum information. In this dissertation, we investigate the structural, spectral and paramagnetic properties of rare-earth ion-doped inorganic systems in the combination of first-principles calculations and crystal-field parametric model analyses. This dissertation comprises six chapters.In chapter one, we briefly introduce the spectral properties of rare-earth ions, some specific application fields of rare-earth ion-doped materials and the main work of this dissertation.In chapter two, we first introduce the crystal-field parametric models of 4fN configuration and 4fN-15d configuration of rare-earth ions in crystals, based on which the approaches to fit the experimental energy levels of rare-earth ions are introduced. Then we give a brief introduction to the theoretical foundations of the first-principles calculations, the wavefunction-based quantum chemical methods and the electron density-based density functional theory. Next, we introduce the ab-initio model potential embedded cluster approaches to study rare-earth ion-doped inorganic systems. By means of the CASSCF/CASPT2/RASSI calculations, we obtain the information such as the energy levels and wavefunctions of the systems. By combining the parametric Hamiltonians, the crystal-field parameters and the energy-level g factors of rare-earth ions can be extracted. Finally, we briefly introduce the program packages adopted in this dissertation.In chapter three, we fit the experimental energy-level data sets of rare-earth ions doped in LiYF4 based on the crystal-field parametric model of 4fN configuration, and then obtain the energy-level parameters of a series of rare-earth ions. The main energy-level parameters still have regular variation trends when rare-earth ions are located at the sites with low symmetry. It is of great significance because we can derive the crystal-field parameters of other rare-earth ions from this conclusion and the crystal-field parameters of Ce3+which can be obtained on the basis of the first-principles calculations. Besides, the energy-level g factors of rare-earth ions are calculated and then compared with the experimental ESR results, which can be used to check the reliability of the obtained parameters. The results show that it will increase the accuracy of parameters if the g factors are taken into account in the energy-level fittings.In chapter four, we investigate the structural and spectral properties of Ce3+-doped strontium silicates (Sr3SiO5 and Sr2SiO4) in the combination of first-principles calculations and crystal-field parametric model analyses. First, we perform the geometry optimization of Ce3+-doped strontium silicates crystals to investigate the local structures and stability of Ce3+sites based on the DFT total-energy calculations using supercell models. Second, based on the optimized crystal structures, the Ce3+-centered embedded cluster models are constructed and the CASSCF/CASPT2/RASSI calculations are performed to obtain the 4f and 5d energy levels and wavefunctions of Ce3+. The calculated 4fâ†'5d transition energies are consistent with the experimental optical spectra. Third, on the basis of the calculated energy levels and wavefunctions, an effective Hamiltonian is constructed to extract the crystal-field parameters of Ce3+via the combination of the parametric Hamiltonians of 4f and 5d configurations of Ce3+. Finally, based on the parametric Hamiltonians, we calculate the anisotropic g tensors and corresponding principal values of the lowest 4f and 5d energy levels. Moreover, the sign of the product of three principal values of a g tensor is determined via the analyses of wavefunctions.In chapter five, we investigate the site occupation, the local coordination structures, the 4f and 5d energy levels, the crystal-field parameters and the g factors of five Ce3+centers in KMgF3 crystal based on the combination of first-principles calculations and crystal-field parametric model analyses. Then we explain the charge compensation mechanism of Ce3+centers, which is consistent with the experimental optical spectra and ESR spectra. First, based on the DFT total-energy calculations with supercell models, we perform the geometry optimization of Ce3+-doped KMgF3 crystal and analyze the stability of different Ce3+sites. Second, on the basis of CASSCF/CASPT2/RASSI calculations with embedded cluster models, we obtain the 4f and 5d energy levels and corresponding wavefunctions of five Ce3+centers, from which the crystal-field parameters and energy-level g factors of Ce3+are extracted. The calculated 4fâ†'5d transition energies are in good agreement with the experimental optical spectra. Finally, by means of the comprehensive analyses of the experimental data and the theoretical calculation results, we identify the Ce3+centers in KMgF3 and associate four Ce3+centers occupying K+sites with the ESR spectra. Especially, the Ce3+center assigned to occupying a Mg2+site in literature is re-assigned to occupying a K+site.At last, we summarize the research work of this dissertation, and give meaningful analyses and discussion to the present research methods and somewhere to be improved.The research work of this dissertation demonstrates that the combination of first-principles calculations and crystal-field parametric model analyses is an effective method to study the structural, spectral and paramagnetic properties of rare-earth ion-doped inorganic systems. |
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