Synthesis And Structure-Property Modulation Of Narrowband Red Emitting Cryolite A₂BB'F₆:Mn⁴⁺ For WLED Application

White light-emitting diodes?WLEDs?,fabricated by blue-emitting InGaN chip and yellow phosphor Y₃Al₅O₁₂:Ce³⁺(YAG:Ce³⁺),are widely used in general illumination,owing to their superiorities of high efficiency,energy saving,and durability.However,this package suffers from high correlated color temperature?CCT>4500 K?and low color rendering index?CRI<80?due to the deficiency of red emission from YAG:Ce³⁺with limitation in indoor lighting.Fortunately,this drawback can be resolved by adding a suitable red phosphor into the aforementioned design.Currently,commercial nitrides of CaAlSiN₃:Eu²⁺and Sr₂Si₅N₈:Eu²⁺have been found to be the most successful red phosphors.However,these materials own broadband emission with low color purity and part of wavelength out of human eyes sensitivity,as well as extremely broad excitation along with serious re-absorption problem between red and YAG:Ce³⁺yellow phosphor.Additionally,the harsh synthetic conditions,such as high temperature,high pressure,should also be considered.In contrast,Mn⁴⁺activated fluorides owning broad excitation at⁴60 nm,line emission at⁶30 nm,good thermal stability,and facile preparation,have gained increasing attention and become alternative red light candidate for WLED applications.However,these fluoride red phosphors usually endure serious moist erosion,which has inevitably limited their practical applications.Tuning the crystal structure of fluoride host can obviously improve the moisture resistance,whereas the thermal stability and luminescence efficient were also simultaneously changed.It is a fundamental challenge to achieve Mn⁴⁺activated phosphors with both good stability and high efficient.Therefore,to promote their significant development,it is crucial to probe the relationship between the Mn⁴⁺emission stability of fluorides and their host structural stability.Herein,this dissertation aims to probe impacts of structure rigidity of matrix on Mn⁴⁺emission stability in double perovskite fluorides A₂BB?F₆,thus to design highly efficient and stable narrowband red phosphor for high-quality warm white LED applications.This thesis is divided into seven chapters.In chapter 1,the research progresses of WLED and red phosphors for WLED applications are briefly reviewed.Especially,the photoluminescence?PL?characteristics,the latest advances,and remaining challenges of Mn⁴⁺activated fluorides red phosphors are summarized,thus,the research subject is proposed.The synthesis and characterizations of the samples are presented in chapter 2.For chapters 3-6,the impacts of structure rigidity of matrix on Mn⁴⁺emission stability,as well as the high-quality warm white LED applications are systematically studied in cryolites.The final chapter summarizes the whole work and proposes a prospect.The specific achievements are presented as follows:?1?A new phosphor Na₃GaF₆:Mn⁴⁺has been exemplified to probe the PL property of Mn⁴⁺ion nonequivalently doped in A₂BB?F₆.Upon blue light excitation,Na₃GaF₆:Mn⁴⁺emits sharp line red light with intense zero phonon line?ZPL?.The effects of raw materials,doping concentration and environmental temperature on PL properties and crystal structure have been investigated and discussed in detail.Such newly synthesized red phosphor shows high PL efficiency and excellent anti-thermal quenching behavior.Moreover,refined contrasts of crystal strucute and spectrum between K₂SiF₆:Mn⁴⁺,K₂TiF₆:Mn⁴⁺and Na₃GaF₆:Mn⁴⁺,it is found that the structure defects induced by charge compensation could reduce the local structure symmetry to enhance ZPL emission with possible increasing color purity.?2?Based on the rich structural prototypes of cryolites,a novel hydrous fluoride K₂GaF₅?H₂O?with special[GaF₅?H₂O?]group has been discovered.Interestingly,further rapid adding K₂MnF₆ into the co-precipitation reaction system as K₂GaF₅?H₂O?,results in anhydrous cryolite phosphor K₃GaF₆:Mn⁴⁺.Detailed characterizations,such as phase identification,morphology,elemental composition analysis,and IR spectra,record the time-dependent evolution process.The investigation on possible formation mechanism unravels that hydrous K₂GaF₅?H₂O?can act as intermediate to transform into anhydrous K₃GaF₆:Mn⁴⁺induced by K₂MnF₆.It presents good moisture tolerance?50%of initial luminous efficacy at 48h?,which might due to the absence of coordinated water molecules make host matrix less hydrophilic.Further evaluations on thermal-dependent luminescence behaviors and device performances illustrate that K₃GaF₆:Mn⁴⁺possesses high stability and shows promising for warm WLEDs applications.Herein,the inductive transition from hydrous K₂GaF₅?H₂O?to moisture-proof K₃GaF₆:Mn⁴⁺is helpful for the investigation on moisture resistance of Mn⁴⁺activated fluorides.Moreover,these results probably propose a feasible strategy to exploit stable Mn⁴⁺-activated fluoride phosphors for warm WLEDs applications.?3?Three isotypic double perovskites A₂BAlF₆:Mn⁴⁺?A=Rb,Cs;B=K,Rb?were exemplified to probe the impacts of structure rigidity of matrix on Mn⁴⁺emission stability.XRD Rietveld refinements and Raman spectra indicate that neighboring smaller A-site or larger B-site cation in A₂BAlF₆:Mn⁴⁺lowers structural stability,but suppresses[AlF₆]octahedron to increase crystal field strength and bond strength of Al-F bond.A combination of static and dynamic emission degradations demonstrates that the water resistance of Mn⁴⁺emission mainly relies on structural stability of host matrix;meanwhile,the thermal stability highly depends on rigidity of its local accommodation.The variations of temperature-dependent WLED performances further evidence their emission stability sequence.?4?Based on the aforementioned structure-property relationship,the highly efficient and thermally stable?>100%@423 K?Rb₃AlF₆:Mn⁴⁺phosphor has been designed and synthesized.The differences of Mn⁴⁺emission stability and quantum efficiency?QE?in A₂BAlF₆:Mn⁴⁺?A=Rb,Cs;B=K,Rb?have been analyzed from structure.It is found that the distorted[AlF₆]octahedron in Rb₃AlF₆ shorten Al-F bond length and enhance the thermal stability of Rb₃AlF₆:Mn⁴⁺;furthermore,the asymmetric coordination also facilitate its QE.By using Rb₃AlF₆:Mn⁴⁺as red component,a superior WLED with luminous efficiency of 157lm/W,CCT³255 K,Ra⁹0.1,and R9⁶8.3 has been fabricated.The slight fluctuations of drive current and working temperature dependent device performances indicate that Rb₃AlF₆:Mn⁴⁺red phosphor shows promising application on high quality warm WLEDs with low light degradation,durability and high CRIs.