Synthesis,performance Optimization And Application In W-LED Of Tantalate（niobate）luminescent Materials Activated By Mn⁴⁺ And Bi³⁺ Ions

Light-emitting diodes（LEDs）can effectively convert electricity into light.Because they have advantages such as high luminous efficiency and long lifetime,and is considered to the next generation of light sources.However,high correlated color temperature（CCT）and poor color-rendering index（CRI）are limited its application in indoor lighting,because of the lack of red-light components in the luminescence materials of traditional white light-emitting diodes.To solve the problem,a red-emitting phosphors with an excitation spectrum matching the chip is usually added to the white light system to improve the practicality of the device.Based on the above principles,a series of Mn⁴⁺activated luminescent materials were successfully prepared by traditional solid-state reaction.Their luminescence mechanisms,luminescence properties,and the effects of crystal structure and thermal stability on the luminescence properties were explored.By using Bi³⁺→Mn⁴⁺energy transfer,the emission color is changed.Their potential application value in white LEDs is discussed.This paper mainly includes three parts as follow:（1）The（M）LaZnTaO₆:xMn⁴⁺（M=Ba,Sr）phosphors were prepared.The crystal structure and phase purity of the synthesized materials were determined by X-ray powder diffraction（XRD）and Rietveld refinement.The morphology,element composition and distribution of the sample were obtained by SEM and TEM.The excitation（PLE）and emission（PL）spectra of the samples were studied separately.The as-prepared samples show good photoluminescence properties under near ultraviolet lamp excitation.The relationship between the emission intensity of the（M）LaZnTaO₆:xMn⁴⁺（M=Ba,Sr）and the temperature was explored through temperature-dependent PL spectra,etc.,and the thermal stability information of the luminescence materials was obtained by calculating the activation energy（Ea）.The relationship between the emission intensity of Mn⁴⁺ions and the doping concentration in the system was investigated.The results showed that the emission intensity increased first and then decreased with the increase of doping concentration.The study found that the luminescence quenching is caused by its energy transfer between Mn⁴⁺ions,and mechanism is dipole-dipole interaction.By comparing the analysis results,the luminescence performance of SrLaZnTaO₆:Mn⁴⁺is better than that of BaLaZnTaO₆:Mn⁴⁺,which is a potential red-emitting material for w-LEDs.We are discussed the application of materials in plant lighting.（2）The Ba₂GdNbO₆:Bi³⁺,Mn⁴⁺luminescence materials with double perovskite structure were prepared by high-temperature solid-state reaction.It was found that Bi³⁺/Mn⁴⁺replaced Gd³⁺/Nb⁵⁺lattice sites and entered the crystal,respectively.The study found that the optical band gap of the Ba₂GdNbO₆ after Bi³⁺/Mn⁴⁺substitution was change accordingly,and the paper analyzes this change.The activation energy（Ea）of Ba₂GdNbO₆:Bi³⁺,Mn⁴⁺was calculated by temperature-dependent PL spectra.Besides,the experimental results show that there is effective Bi³⁺→Mn⁴⁺energy transfer,and the Bi³⁺→Mn⁴⁺energy transfer mechanism and energy transfer efficiency are studied.The emission color of phosphors can be tuned by energy transfer as well as changing the doping concentration of Bi³⁺and Mn⁴⁺.The excitation spectrum shows that the Ba₂GdNbO₆:Bi³⁺,Mn⁴⁺luminescence material has a broad excitation band and the best excitation position is around 365 nm,which matches the near-ultraviolet chip.Using Ba₂GdNbO₆:Bi³⁺,Mn⁴⁺to encapsulate LED devices at different temperatures and study phosphor electroluminescence performance,the obtained devices have a high luminescence efficiency（55 lm/W）,confirming that Ba₂GdNbO₆:Bi³⁺,Mn⁴⁺is an excellent red emitting material for w-LEDs.（3）The CaLi₆La₂Nb₂O₁₂:Bi³⁺,Mn⁴⁺phosphor with high-efficiency red light emission were synthesized at 900 ^oC for 24 h,in which Bi³⁺/Mn⁴⁺ion replaces the crystal lattice of La³⁺/Nb⁵⁺ion.The clear lattice fringes of the CaLi₆La₂Nb₂O₁₂:Bi³⁺,Mn⁴⁺can be observed from TEM image,indicating that the material srystallizes well.The SEM image reveals the particle and elements distribution of the CaLi₆La₂Nb₂O₁₂:Bi³⁺,Mn⁴⁺.The absorption band and optical band gap of the phosphor were determined by diffuse reflectance spectra.The electron paramagnetic resonance（EPR）proves that manganese exists only in the state of Mn（IV）in CaLi₆La₂Nb₂O₁₂:Bi³⁺,Mn⁴⁺.The emission spectrum analysis of CaLi₆La₂Nb₂O₁₂:Bi³⁺,Mn⁴⁺shows that Bi³⁺replaces La³⁺lattice sites with emitting blue light,Mn⁴⁺replaces the Nb⁵⁺lattice sites with emitting red light.The energy transfer from Bi³⁺to Mn⁴⁺ions can be observed in the system.With the increase of Mn⁴⁺ions doping concentration,the emission color gradually changes from blue to red.An energy level diagram is used to explain the energy transfer between Bi³⁺/Mn⁴⁺ion.The activation energy of the CaLi₆La₂Nb₂O₁₂:Bi³⁺,Mn⁴⁺is estimated by using temperature-dependent PL spectra to judge the thermal stability.The chromaticity coordinates and EL spectra of the fabricated pc-WLEDs prove that CaLi₆La₂Nb₂O₁₂:Bi³⁺,Mn⁴⁺phosphor is red emission material with potential for application.