| White LEDs have been extensively applied in daily lighting,liquid crystal display,automobile lighting and so on,due to their high efficiency,energy saving,and mercury-free.Rare-earth luminescent material is one of the key components in white LEDs.However,for most rare-earth luminescence materials with promising applications,the crystal structure and chemical composition present to be complex(such as multiple-site occupation,charge compensation,occupation disorder,etc.),and their correlation with luminescent properties is difficult to clarified.Conventional experimental techniques are unable to give a clear understanding of the structure-property relationship and,meanwhile,empirical theoretical models for structural analysis also have limitations in this respect.Understanding of luminescence mechanism for regulation of optical properties remains to be further studied.This dissertation is mainly concerned with some typical Ce3+/Eu2+-doped complex luminescence materials.The density functional theory(DFT)and multiconfigurational ab initio calculations are combined to investigate the structure-property relationship of luminescence materials,starting from the chemical composition and atomic structure of the host crystal.The results can provide theoretical clues for further optimizing luminescence properties.The first chapter firstly describes basic principles of luminescence and knowledge of rare-earth elements.Secondly,the related rare-earth luminescent materials are introduced.Finally,the research background,main contents,as well as computational design of this work are given.The second chapter describes theoretical calculation methods.Firstly,the basic concepts of first-principles calculations are provided.Secondly,the theoretical foundations of this method are stated,namely,the wavefunction-based quantum chemistry methods and the charge density-based DFT.Finally,two program packages adopted in this dissertation are presented,i.e.,the plane wave basis set-based VASP package and the ab initio model potentials basis set-based MOLCAS package.The third chapter is concerned with thermodynamic and spectral properties on intrinsic point defects and dopants Ce3+in SrLiAl3N4.Lanthanide-doped SrLiAl3N4 are among the most promising phosphor materials for solid-state lighting due to their superior luminescence properties.Native point defects occur naturally during their synthesis at high temperature and may act as charge-compensating centers for aliovalent lanthanide substitutions or as trapping centers of electrons from the conduction band,but their nature and influence on luminescence properties are not well understood.In this chapter,we first perform a systematic study on intrinsic point defects in SrLiAl3N4 by combining hybrid DFT and multi-configurational ab initio calculations.DFT-based defect formation energies predict the most favorable native point defect,and the associated defect-induced energy levels in the band gap are derived and discussed in relation with experimental findings.Besides,the interaction of the acceptor-type defects with dopants Ce3+is examined,based on which the relative site preference of Ce3+at the two Sr sites is clarified.Finally,by combing the calculated Boltzmann weighted occurrence probabilities and 4f→5d transition energies of Ce3+,the luminescence origins of experimentally identified major and minor Ce3+activators are clearly clarified.Meanwhile,the specific structural parameters responsible for the relative spectral shift of two CeSr centers are revealed on the basis of the wave functions from the multi-configurational ab initio calculations.The present results can also serve as a theoretical basis for the engineering of nitridolithoaluminate-based phosphors with targeted luminescence properties.The fourth chapter investigates site occupancy and spectral properties of Ce-doped Ba2Y5B5O17 and Ba3Y2B6O15 crystals with multiple cation sites and occupational disorder.Ce3+-activated Ba2Y5B5O17 and Ba3Y2B6O15 compounds displayed efficient and thermally stable blue luminescence under UV or near-UV excitation.However,the site occupations of Ce3+are not yet clarified due to existence of multiple cation sites with occupational disorder.Firstly,by conducting combinatorial-chemistry(enumeration)based-DFT calculations,the most stable undoped crystal structure is obtained.Then,the stable local structures of Ce3+are derived by using hybrid DFT calculations,and the reasons are analyzed.At last,the 4f→5d transition energies of the dopant Ce3+are calculated by using a wave-function-based embedded cluster method,and thus the origins of blue emissions in Ce3+-doped phosphors are claimed.The host-referred Ce3+4f ground-state and the lowest 5d excited-state levels within the host band gaps are obtained and discussed in association with local structures and luminescence thermal stabilities.This investigation is expected to enhance our understanding of lanthanide luminescence in cation-disordered phosphors.The fifth chapter is focused on site occupation and spectral assignment in Eu2+activated β-Ca3(PO4)2-type phosphors.Eu2+-activated β-Ca3(PO4)2-type phosphors have attracted significant interest for the presence of multiple cation sites.However,the site occupation and associated spectral assignment of dopant Eu2+,and hence the mechanism behind the site-regulated emission tuning,still remain elusive.On the basis of the host composition and structure of host crystal,taking into account temperature effects using Boltzmann distribution,the relative occupation probabilities of Eu2+at different sites of Ca3(PO4)2,Ca10M(PO4)7(M=Li,Na,K)and Ca3(PO4)2:Mg are obtained.And the emission spectra of the phosphors are interpreted with respect to the substituted sites,which is also verified by a comparison between the calculated 4f→5d transition energies by configuration interaction and experimental excitation spectra.In addition,two factors(site size and charge compensation)which govern the occupation of Eu2+ions are discussed from the shifts of the nearest-neighbor atoms and distortion information by Eu2+doping,and the mechanism behind the composition dependence of the emission color is consistently elucidated.Our results provide a new perspective on the site preference of Eu2+in β-Ca3(PO4)2-type compounds and may also serve as a theoretical guideline on the structure-property relationship for the design of other Eu2+-activated phosphors. |
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