Synthesis And Properties Of Rare Earth Doped LiRE?MoO₄?₂?RE=La,Gd? Phosphors

Conventional blue light chip combined with YAG:Ce phosphor to form white light emission,the color rendering index of the light source is low due to the lack of red light component.Therefore,exploring a high-efficiency red phosphor excited by ultraviolet or blue light has important practical significance for the development of white LEDs.Double alkaline rare earth molybdates are used in the host of phosphors due to their excellent physical and chemical stability.Its chemical formula is ARE?MoO₄?₂,wherein A is an alkali metal ion such as K,Na,Li,and RE is a rare earth ion such as Y,La,Gd or the like.For luminescent materials,MoO₄^2-has a strong energy absorption in the ultraviolet region,and the absorbed energy can enhance the luminescence intensity of the rare earth ions by energy transfer.It can be seen that the double alkaline rare earth molybdates is indeed suitable as a phosphor host material.In this paper,the LiLa?MoO₄?₂:Dy³⁺,Eu³⁺and LiLa?MoO₄?₂:Sm³⁺,Bi³⁺phosphors were prepared by high temperature solid-state method,and the LiGd?MoO₄?₂:Eu³⁺phosphors were synthesized by hydrothermal method.The energy transfer mechanism,crystal structure,morphology and luminescence properties were studied by XRD,SEM,TEM,fluorescence spectra and infrared spectra.The main research contents and conclusions are as follows:?1?Dy³⁺single-doped and Dy³⁺/Eu³⁺co-doped LiLa?MoO₄?₂-based phosphors were prepared by high temperature solid-state method.The optimum doping concentration of Dy³⁺for LiLa?MoO₄?₂:Dy³⁺phosphor is 5 mol%under excitation wavelength at 389 nm.The excitation spectra of Dy³⁺and Eu³⁺for LiLa?MoO₄?₂:Dy³⁺,Eu³⁺phosphors show that there is an energy transfer between Dy³⁺and Eu³⁺at excitation wavelength of 454 nm.According to Dexter's energy transfer theory,the energy transfer between Dy³⁺and Eu³⁺is determined as electric dipole-electric dipole interaction.By changing the doping concentration of Eu³⁺in LiLa?MoO₄?₂:0.05Dy³⁺,xEu³⁺phosphors,the color of the phosphors is tunable.?2?Sm³⁺single-doped and Sm³⁺/Bi³⁺co-doped LiLa?MoO₄?₂-based phosphors were prepared by high temperature solid-state method.When Sm³⁺is single-doped,the optimum doping concentration of Sm³⁺for LiLa?MoO₄?₂:Sm³⁺phosphor is 5 mol%.When Sm³⁺/Bi³⁺is co-doped,the luminescence intensity of Sm³⁺for LiLa?MoO₄?₂:Sm³⁺,Bi³⁺phosphor is nearly doubled compared with Sm³⁺single-doping at excitation wavelength of 405 nm.However,in this study,the action mode of Bi³⁺ions enhanced Sm³⁺ions emission is a non-sensitization mechanism,which is different from the traditional energy transfer mechanism between Bi³⁺ions and RE³⁺ions.The results indicate that the incorporation of Bi³⁺changes the intrinsic structure of the crystal,thereby forming crystal defects,affecting the crystal growth and accompanying the formation of subgrains.During the crystal growth,there is a phase transition of LiLaMo₂O₈:Sm,Bi to LiLa?MoO₄?₂:Sm,Bi,and there is also a competitive growth of subgrains,which will form a strong microstrain in the polycrystal.The microstrain will cause the distance R between the quenched Sm³⁺ions to be greater than the critical distance R_c,and the originally quenched Sm³⁺ions will be excited again,so the luminescence intensity of the Sm³⁺ions is enhanced.?3?LiGd?MoO₄?₂ host material and LiGd?MoO₄?₂:Eu³⁺phosphor were successfully synthesized by hydrothermal method.The hydrothermal synthesis conditions were obtained.Li₂MoO₄ was used as the key raw material and urea mineralizer was added.The reaction was carried out at a hydrothermal temperature of 240°C for 24 h,and the transition product formed by hydrothermal treatment was calcined at 625°C for 4 h,the LiGd?MoO₄?₂:Eu³⁺phosphor was obtained.The transition product undergoes a crystal phase transition during the formation of LiGd?MoO₄?₂,and the morphology changes from a smooth surface to a sheet with a rough surface similar to oatmeal.After the doped Eu³⁺ions undergo a crystal phase transformation process,the excitation spectra of LiGd?MoO₄?₂:Eu³⁺phosphors has changed greatly.Among them,the charge transfer band?CTB?has a large extent of broadening.Some characteristic peaks of Eu³⁺are covered by the CTB,but the ⁷F₀?⁵D₂ electronic transition peak of Eu³⁺at 465 nm is greatly enhanced.Based on this feature,LiGd?MoO₄?₂:Eu³⁺phosphors are more suitable for use in LEDs excited by blue chips.