First-Principles Study On Site Occupation And Spectral Properties In Eu²⁺/Bi³⁺-Activated Inorganic Luminescent Materials

A new generation of inorganic luminescent materials has brought revolutionary improvement in various application areas due to their high efficiency,high stability,and precise regulation.Eu²⁺and Bi³⁺ions are the common dopants of inorganic luminescent materials.They exhibit various spectral characteristics in different site environments,which favors the regulation of their luminescence properties,but also bring some new challenges.For example,when using a solid-state reaction to prepare Eu²⁺/Bi³⁺-activated inorganic luminescent materials,their luminescence properties can change with crystal preparation conditions,due to the complexity of the material structure（such as intrinsic defects or multiple-site occupantion）,it is difficult to systematically explain the luminescence mechanism in specific luminescent materials by a conventional experimental perspective.It hinders the further performance optimization of related luminescent materials.With the development of computer technology,first-principles calculations represented by density functional theory（DFT）have played an important role in the research of luminescent materials,and have shown great potential and advantages in explaining the luminescence mechanism.In this paper,the properties of defects in several recently developed Eu²⁺/Bi³⁺-doped inorganic luminescent materials prepared under different conditions（reducing atmosphere or air atmosphere）were theoretically investigated using DFT,and the luminescence mechanism was elucidated.The intrinsic relationship between crystal preparation conditions,defect properties and luminescence properties was explored,and the results could provide a theoretical basis for further performance optimization of the phosphors.It mainly includes the following three parts:（1）Eu²⁺-activated A₂CaPO₄F（A=K,Rb）phosphors was prepared in the reductive atmosphere by a solid-state reaction,but the site occupation of dopant Eu²⁺,and the mechanism behind the site-regulated emission tuning,still remain elusive.Herein,in the third chapter,we carried out systematic PBE+U density functional theory calculations on defect formation energies and optical transitions of Eu²⁺situated at different crystallographic sites with various local charge compensations.It shows that,both the 660nm emission and 480 nm emission are due to Eu²⁺located on A（A=K,Rb）site,but with the different charge compensation.On this basis,we discuss the relationship between Eu²⁺local environments,crystal preparation temperature and the phosphor spectral characteristics.The results provide a theoretical basis for further performance optimization of the phosphors.（2）It was recently reported that Bi³⁺-doped LiREGeO₄（RE=Sc,Y,Lu）compounds displayed strong ultraviolet-A persistent luminescence（Pers L）at～360 nm with a duration of tens of hours at room temperature,which were prepared in the air condition.However,the mechanistic origin of the Pers L remains to be unveiled.Herein,in the fourth chapter,we carried out a systematic study on optical transitions,formation energies,and charge-transition levels of dopants and intrinsic point defects in these compounds using PBE0hybrid density functional theory calculations.The results show that the efficient charging by 254 nm is due to the D-band transition of Bi³⁺and hence the charge carriers pertinent to Pers L are electrons originating from the dopants which are involved in the trapping and detrapping processes.The main electron-trapping centers are antisite defects Ge_Li⁰,interstitial defects Li_i⁰,and dopants Bi²⁺.These findings are further confirmed by comparison with calculated results for isostructural Na Lu Ge O₄ and Li Lu Si O₄,based on which the roles of Li and Ge elements in forming intrinsic defects with appropriate trap depths for Pers L are clarified.The results provide a theoretical basis for the rational design of novel Pers L phosphors containing lithium and germanium in the host compound.（3）Bi-activated Ba₃Sc₄O₉ phosphors have attracted attention due to their luminescent properties covering both the visible and near-infrared regions,which were prepared in the air condition.And the near-infrared emission intensity has been enhanced by further reducing treatment.However,the luminescence mechanism is still elusive.In the fifth chapter,we carried out a systematic study on intrinsic point defect properties and optical transitions of Bi³⁺at different crystallographic sites using HSE06 hybrid density functional theory calculations.Site preferences and valence state tendencies of Bi,as well as the charge compensation mechanisms under different synthesis conditions,were obtained,respectively.And the results show that the emission at 548 nm was due to Bi³⁺at the Sc1/Sc3 sites;the emission at 510 nm was originated from Bi³⁺at the Ba1/Ba2 sites.Besides,the near-infrared emission at 1350 nm comes from the Bi⁺at the Ba1/Ba2 sites,which is produced by the electron reduction of the excited states of Bi³⁺on the adjacent Bi³⁺.On this basis,the influence of different crystal preparation conditions on the defect properties in the crystal was studied.Our results not only assist in the understanding of experimental observations but also provide a theoretical basis for optimizing the properties of similar luminescent materials.