The Synthesis And Luminescent Properties Of Rare-Earth Polyborates

This thesis concentrates on the synthesis and luminescent property of trivalent rare earth polyborates.Different hydrated and anhydrous rare earth borates have been prepared by soft chemical methods,such as hydrothermal technique,sol-gel method,borate acid flux growth and thermal decomposition method.Besides,we studied the luminescent properties of different luminescent centers(Eu³⁺ or Tb³⁺)to seek out superior new phosphors.In addition,we demonstrated the critical role of Bi³⁺ substitutions in the stabilization of metastable ?-REB₃O₆?RE = Sm-Er,Y?at ambient pressure,where the Bi³⁺-to-RE³⁺ substitutions would efficiently reduce the synthetic temperature of the high temperature-high pressure phase.In this study,we researched for the luminescent properties of high-pressure phase systematically for the first time.Many other high-pressure phase can be obtained at ambient condition by inducing Bi³⁺,which would enrich the kinds of borates materials greatly.The main experimental results are as follows:1.We successfully lower the synthetic temperature by introducing Bi³⁺ into RE³⁺ sites in an appropriate concentration.Partial solid solutions of ?-RE1-xBixB₃O₆?RE = Sm-Er,and Y?were successfully synthesized under ambient pressure at the temperature range of 735-820 °C.The solid solution ranges of Bi³⁺ in different host compounds were confirmed by the annealing XRD patterns and unit cell volumes obtained by Le Bail fitting for all samples.For ?-GdB₃O₆:Bi³⁺,Tb³⁺ compounds,the emission color can be varied from blue to green continuously with the increasing Tb³⁺ concentration.For ?-GdB₃O₆:Bi³⁺,Eu³⁺,the emission color varies from blue to red with the increasing Eu³⁺ concentration.Single-phased white light emitting phosphor was prepared in ?-GdB₃O₆ host by controlling the doping concentrations of Bi³⁺,Tb³⁺ and Eu³⁺ based on energy transfer.The activator energy for Bi³⁺,Tb³⁺ and Eu³⁺ calculated by Arrhenius equation are 0.45?0.19 and 0.23 eV,respectively,which can certify the good thermal stability of the white light emitting phosphor.2.La_1-xEu_x[B₈O₁₁?OH?₅]?0? x ?0.135?and La_1-xEu_x[B₅O₈?OH?₂].1.5H₂O?0? x ?0.40?compounds were obtained by hydrothermal technique and borate acid flux technique,respectively.This two hydrated phosphors can be excited by near-UV LED chips efficiently,which can be used in WLEDs.By annealing the hydrated polyborates,we obtained a new anhydrous compounds,named ?-La_1-xEu_xB₅O₉.In addition,we found this anhydrous phosphor exhibit pure red emission under UV excitation.3.By using sol-gel method,another anhydrous polyborates,?-GdB₅O₉,was obtained.We learned the luminescent property of Ce³⁺ and Tb³⁺ in this host matrix.The f-f transition in excitation spectra of Tb³⁺ is more superior than f-d transition,so this green phosphor have potential applications for near-UV pumped LEDs.As a sensitizer,Ce³⁺ can transfer its energy to Tb³⁺,and as a result,the luminescent intensity of Tb³⁺ was enhanced greatly compared to ?-GdB₅O₉: Tb³⁺samples.4.A new structural isomer of ?-GdB₅O₉ and ?-LaB₅O₉,?-La_1-xEu_xB₅O₉,was synthesized by sol-gel method and its structure was determined by powder X-ray diffraction.Different content of Eu³⁺ and different temperature result in different resultant.If the concentration of Eu³⁺ is proper,a phase transition from ????? can be observed by annealing at different temperature,and this process is irreversible.