Yttrium Lithium Fluoride Crystals Of Intrinsic Defects And Doping Mechanism Research

The usefulness of fluoride crystalline materials as the active media in solid statelasers and scintillators is well established. The best example is LiYF₄（YLF）, which isthe most widely used fluoride laser host crystal with several emission bands,depending on its active center. Most theoretical studies on laser crystals concentratedprimarily on the spectroscopy of the energy levels of the doping element in the localenvironment of a given host material, developed in the framework of the ligand fieldtheory. The atomic multiple levels were typically calculated with crystal-fieldparameters fixed by experimental data, while the physical properties of the host itselfwere frequently ignored. The electronic structures and color centers in many lasercrystals have not been extensively studied, while such investigations can contribute tothe understanding of many salient features of laser operation and may even aid to thediscovery of new laser host materials. In particular, the study of color center creationis a useful approach, which can be applied to understand the relevance of degradationprocesses due to e.g., pumping radiation and/or external radiation and theirmicroscopic mechanisms.The thesis mainly contains five parts. The first part is the introduction. The secondpart introduces the basic calculation theory and related computer programs. The thirdpart is the computer simulation of intrinsic defects in YLF crystal. The fourth partdiscusses the electronic structures and physical properties of YLF crystal related tointerstitial oxygen atoms. The fifth part is the computer simulation of Yb³⁺doping YLFcrystal.The first part of the thesis introduces the research background of YLF crystal,status of the research, and the research works in the thesis.The second part, the fundamental knowledge of the computer simulation method,such as the Density Function Theory and pseudo-potentials, the usage of the ABINITcode is discussed.The third part of the thesis is the research of the existent mechanism of intrinsicdefects of the YLF crystal. The point defects possibly existed in the crystal are primarilydiscussed. It is indicated that there may be three types color centers existed in the crystal, they are F, M and H center respectively. The H center may be created accompanied bythe creation of F center. M center may be created as increasing the radiation dose. SinceH center is the definition of an interstitial fluorine atom, the possible interstitial sitesthat the fluorine atom may occupy are discussed according to the knowledge of thedefect chemistry. The electronic structures of YLF containing H center are calculated.The calculation results indicate that the interstitial fluorine atom should combine withthe normal lattice fluorine ions to form the molecular ion in two different ways andcause the optical absorption near260nm. Hence the260nm absorption band is attributedto the H center. The calculation on the electronic structures of YLF containing either Fcenter or M center indicate that the existence of the two color centers would introduce anew energy level within the forbidden gap of the crystal respectively. The transitionenergy of the electron on the new level introduced by F or M center is3.73eV and2.42eV corresponding to333nm and513nm respectively. The calculation coincides withthe experiments330nm and505nm well.In the fourth part, the effects of an interstitial oxygen atom in YLF on the physicalproperties are studied. The calculation reveals that the interstitial oxygen atom wouldsimilarly combine with the normal lattice fluorine ions to form molecular ion H centerand cause260nm absorption.Part five, the effects of the Yb doping in YLF are discussed. It is analyzed that Ybdoped into YLF may simultaneously exist in the crystal in the form of Yb³⁺and Yb²⁺.The calculation indicates that there would be VFcenter created in the Yb²⁺: YLF crystaland would cause340nm absorption. VFdenotes a diatomic molecular ion formed bynearest neighbor lattice fluorine ions share a hole compensating to the electricalnegative caused by Yb²⁺.Part six, conclusions.