Studies On The Photoluminescence Properties And Moisture Resistance Of Mn⁴⁺Doped Hexafluoride

In recent years,with unique and efficient narrow band red light emission and broad band blue light excitation characteristics,Mn⁴⁺doped fluoride red phosphor has received more and more attention.Compared to Eu²⁺doped nitride,the Mn⁴⁺doped fluoride presenting emission within human sensitive curve is ideal red phosphor in application of high performance WLED（white light-emitting diode）with high luminous efficacy and color rendering index.However,due to the ease of dissociation of the fluoride matrix in water and the instability of MnF₆^2-after dissociation,the Mn⁴⁺doped fluoride red phosphor suffers from serious fluorescence degradation during use.Moreover,for Mn⁴⁺doped fluoride red phosphor,the phtoluminescencce properties are closely related to structure and affect the performance of packaged WLED devices.Therefore,to promote the Mn⁴⁺doped fluoride red phosphor into practical application,it is critical to develop novel Mn⁴⁺doped fluoride red phosphor,study the structure-performance relationship in Mn⁴⁺doped fluoride red phosphor and improve the moisture resistance of Mn⁴⁺doped fluoride red phosphor.Our work is dedicated to the above research content,and the results obtained are as follows:（1）A Mn⁴⁺doped red nano phosphor K₂NaGaF₆:Mn⁴⁺with internal quantum yield of 61%is found.Its crystal structure is determined to be Fm3m by Rietveld refinement.The energy level of Mn⁴⁺in K₂NaGaF₆ is calculated by exchange charge method,where the broad excitations at 360 nm,460 nm are ascribed to Mn⁴⁺⁴A₂→⁴T₁,⁴T₂ transition;the narrow band emission at around 630 nm is assigned to Mn⁴⁺²E→⁴A₂.Although the Mn⁴⁺substitutes at high symmetry Ga³⁺site,the valence and radii difference between them leads to partial lattice distortion,and thus the relative strong ZPL（zero phonon line）intensity in emission spectrum.The decay curves of K₂NaGaF₆:Mn⁴⁺are fitted by Inokuti-Hirayama model,illustrating that dipole-dipole interaction is the main machanism for concentration quenching.By employing K₂NaGaF₆:Mn⁴⁺as red component,a WLED device with improved color rendering index（89.4）and decreased correlated color temperature（3779 K）is fabricated.（2）A high performance Mn⁴⁺doped heterodialkaline fluorogermanate CsNaGeF₆:Mn⁴⁺with internal quantum yield of 95.6%is found,and the structure-performance relationship of Mn⁴⁺doped fluoride is revealed by making comparison in Cs₂GeF₆:Mn⁴⁺,Na₂GeF₆:Mn⁴⁺and CsNaGeF₆:Mn⁴⁺.The crystal structure of CsNaGeF₆ is determined to be Pbcm by analysis of the single crystal X-ray diffraction data.In CsNaGeF₆,the Mn⁴⁺subsitituts at the only Ge⁴⁺site with C_s symmetric.Since the energy level of Mn⁴⁺undergoes crystal field splitting and spin-orbit coupling under low symmetry,two ZPLs,whose relative intensity changes with temperature,and relatively wide emission peak appear in the emission spectrum.It is found that in three Mn⁴⁺doped materials,the ZPL intensity is determined by the degree of distortion rather than the symmetry of the site;from Na₂GeF₆,CsNaGeF₆ to Cs₂GeF₆,the covalence is increased and the emission of Mn⁴⁺²E→⁴A₂ transition shifts to longer wavelength.The CsNaGeF₆:Mn⁴⁺has a high Mn⁴⁺critical concentration（9%）,higher internal quantum efficiency than Na₂GeF₆:Mn⁴⁺（67.3%）,and higher spectral lumen efficiency(242 lm W^-1)than Cs₂GeF₆:Mn⁴⁺(216 lm W^-1).By employing CsNaGeF₆:Mn⁴⁺as red phosphor,a WLED with color rendering index of 92.5,correlated color temperature of 3783 K and luminous efficacy of 179 lm W^-1 is obtanied.（3）A Mn⁴⁺doped fluoride red phosphor Rb₂SnF₆:Mn⁴⁺with internal quantum yield of 78%is found;the protective homogeneous passivation layer is formed by surface Mn⁴⁺reduction to improve its poor moisture resistance.The crystal structure of Rb₂SnF₆ is determined to be P3m1 by analysis of single crystal X-ray diffraction data.Rb₂SnF₆:Mn⁴⁺has a higher luminescence intensity than synthesized commercial red phosphor K₂SiF₆:Mn⁴⁺and is also more susceptible to moisture-induced luminescence degradation.By comparing the luminescence properties and moisture resistance of Rb₂SnF₆:Mn⁴⁺treated with different reductive H₂C₂O₄,H₂O₂ solutions and different concentrations of H₂C₂O₄ solution,it is found that the treatment solution with appropriate reducing ability is the key to obtain Rb₂SnF₆:Mn⁴⁺with sametime high brightness and water resistance.The Rb₂SnF₆:Mn⁴⁺treated with low concentration of H₂C₂O₄ can maintain more than 95%of the initial luminescence intensity after being immersed in room temperature and boiling water for several hours.By employing the low concentration of H₂C₂O₄ treated Rb₂SnF₆:Mn⁴⁺as red phosphor,the packaged white light WLED can achieve color rendering index of 90,correlated color temperature of 3890 K,and luminous efficiency of 106.24 lm W^-1.（4）The photoluminescence efficiency and moisture resistance of synthesized commercial red phosphor K₂SiF₆:Mn⁴⁺are improved by simple antisolvent（ethanol）induced epitaxial growth.Specifically,K₂SiF₆:Mn⁴⁺is partially soluble in the synthetic solution HF,and the addition of ethanol as a precipitant at the end of the synthesis leads to additional epitaxial growth.The epitaxial growth layer K₂SiF₆:Mn⁴⁺under a certain amount of ethanol addition exhibits stronger photoluminescence due to the high Mn⁴⁺critical concentration,and thus enhances the luminescence intensity of the final product.In addition,with this epitaxial growth,a non-doped homogenous K₂SiF₆ shell layer is designed and grown on the K₂SiF₆:Mn⁴⁺to improve the moisture resistance.By employing the resultant K₂SiF₆:Mn⁴⁺@K₂SiF₆ as red phosphor,a WLED with correlated color temperature of 2879 K,color rendering index of 96.5 and luminous efficiency of 119.74 lm W^-1 is fabricated.Further stability studies reveals that this WLED device can maintain 93.5%of initial luminous efficiency after being aged in a high temperature and high humidity environment（85°C,85%）for 10 days.