First Principles Studies On The 4f-5d Transitions Of Ce³⁺-doped Materials

The interactions between electrons in partially-filled 4f shell of rare-earth ions lead to a large numbers of energy levels that are available for optical transitions. The abundant luminescent energy levels combined with the excellent properties of host materials make rare-earth-ion-doped optical materials having important applications in various fields. The optical properties have attracted extensive studies for decades and are continuing to be research focus nowadays. Recent years have witnessed tremendous developments in computer hardware and parallel algorithms, which made the first-principles calculations become indispensable tools in scientific research. This thesis reports on studies of optical properties of Ce³⁺-doped materials by density functional theory using modern high-performance computers.The thesis is organized as follows:The first chapter mainly introduces some important applications of rare-earth elements and basic knowledge about luminescence of lanthanide ions.The second chapter describes the general framework of the density functional theory and the developments of the theory achieved in recent years. Moreover, the calculation tool ABINIT package is introduced The third chapter is the main body of this thesis. The 4f–5d transition of Ce³⁺ in a LuAlO₃ host crystal has been of interest as a promising scintillation detector. First principles treatments of this transition are complicated by the excited nature of the 5d state and the strong correlation of the 4f electron. We have investigated properties of the compound associated with the transition by using the periodic density functional theory. A hybrid functional has been used for the 4f states and a constrained approach been employed for the excited 5d state. It is found that the average distance between Ce³⁺ and the eight nearest-neighbor O atoms decrease by 0.05 ? on going from 4f to 5d state. The calculated Stokes shift is in good agreement with experiment. Based on the optimized structure around Ce³⁺, the energy level scheme of the 5d states has been evaluated using the angular overlap model, with reasonable agreement with experiment.The fourth chapter presents a DFT study on Sr_3–xA_xAlO₄F(A=Ca,Ba) which is a new host material for rare-earth luminescence. The structure and electronic properties of Sr₃AlO₄F were studied at first, and then the site preference of doping Ca²⁺ or Ba²⁺ ions on the Sr sites were investigated. In this compound the fluoride and oxide ions are orderly located in different layers, and there are two different lattice sites for strontium ions since the noticeably different coordination environment. Our total energy calculations evidenced that the Ba²⁺ cations show a strong site preference for the 10-coordinate Sr sites while the Ca²⁺ cations prefer the 8-coordinate ones, which agrees with the experiment results. The site preference was tentatively interpreted based on the difference of the local structures.