Study On Synthesis And The Structure-property Relationship Of Ba₂SiO₄:Eu²⁺Based Phosphors

Ba2SiO4:Eu2+ based phosphors have significant application in wLEDs.However,due to their poor thermal stability,the performance of the wLED devicesare limited.Based on the Eu2+-doped orthosilicate Ba2SiO4,we have designed and synthesized Eu2+-doped(Ba,Sr)-2SiO4,(Ba,Ca)2SiO4 and(KxBa2-s)(Si1-xPx)O4 phosphors.We have conducted a systematic study on the influence of some structural factors,such as cation ordering,atom site splitting and polyhedral chain-like structure on the luminescence and thermal quenching properties of the phosphors.The main contents are as follows:(1)We have designed and synthesized successfully a series of(Ba1-xSrx)2SiO4:Eu2+ phosphors.In the(Ba1-xSrx)2SiO4:Eu2+ solid solutions,when Sr+ions are doped into the host lattice,the Sr2+ ions will occupy the smaller cation site preferentially,which means the distribution of Sr2+/Ba2+ ions in the crystal structure is ordered but not random.We introduced the cation ordering ? and order parameter s to elaborate this preferential occupation.We find that the variation of cation ordering against the composition is much similar to that of the thermal quenching temperature T1/2,and the intermediate composition BaSrSiO4:Eu2+ possesses both the maximum cation ordering and T1/2 values,drawing the conclusion that the enhanced thermal stability of the(Ba1-xSrx)2SiO4:Eu2+ phosphor in originate from the improved cation ordering.(2)We have designed and synthesized successfully a series of T-phase(Ba1-xCax)2SiO4:Eu2+ phosphors,which possess better thermal quenching stability than the end members.There are five cation sites,two Si sites and four O sites in the crystal structure,some of the crystallographic sites are split.By comparing the local coordination enviornments of the cation sites in the atom-site-split-model and atom-site-unsplit-model.respectively,we find due to the significant difference in ionic radius between Ba2+ and Ca2+ ions,there exsit two sets of coordination environments with different size to accommodate Ba2+ and Ca2+ ions to reach a more reasonable distribution and optimaized bonding between the central cations and their ligands in the T-pahse(Ba1-xCax)2SiO4 crystal structure.As a result,the structural rigidity has been improved,which directly leads to the enhancement of thermal stability of T-phase.The phase boundary of T-phase has been determined to be x = 0.28-0.4.While light emitting can be realized in single T-phase by co-doping Eu2+-Mn2+ ions into the host lattice.(3)Based on the same structure,we have designed and synthesized a series of Eu2+-doped KxBa1.97-x(Si1-xPx)O4:0.03Eu2+ solid solutions.We have introducedin-chain characteristic length d22 and outside-chain characteristic length d12 to investigate the variation of the chain-like structure against the composition on the luminescence properties of Eu2+ ions.This influence of chainlike polyhedra on luminescence can be understood through the inductive effect.As x values increase,the values of d22/d12 are gradually increased,which means "the chain-like structure effect" is gradually weakened.This results in the blue-shift of the excitation bands.In the meantime,the lowest 5d energy levels of Eu2+ are rised as increasing x values and present similar variation trend with the variation trend of(d22/d12 against x values.By constructing the HRBE diagram of the KxBa1.97-x(Si1-xPx)O4:0.03Eu2+solid solutions,it is found that the energy difference between the lowest 5d level of Eu2+ and the bottom of conduction band increase with x values increasing,which reduces the possibility of the thermal-assisted ionization of the 5d electron to the conduction band.This can explain the enhancement of the thermal stability of the KxBa1.97-x(Si1-xPx)O4:0.03Eu2+ solid solutions.