Compositional Design,Crystal Structure Tuning And Photoluminescence Of Solid Solution Phosphors For WLED

White Light Emitting Diode?WLED?has been widely regarded as the 4th generation of lighting source.Currently,most of the commercial WLEDs utilize the spectral conversion approach by combining a LED chip with down-converting phosphor materials,in order to control cost and obtain high luminous efficacy.The photoluminescent properties of phosphors are,therefore,critical for the performance and lifetime of WLEDs.Phosphor materials consist of a host compound and an optical activator;Eu²⁺ and Ce³⁺ are the most widely used activators,the emissions of which depend much on the crystal field splitting.In this thesis,the solid solution design method is employed to tune the local coordination environment of Eu²⁺/Ce³⁺,intending to optimize the luminescencent properties as well as to develop new phosphors.The main contents and results are:?1?The cation substitution-dependent phase transition was used as a strategy to discover new phosphors and to efficiently tune the emission of Eu²⁺ in the M₃?PO₄?₂:Eu?M=Ca/Sr/Ba?quasi-binary sets.Several new phosphors including the greenish-white SrCa₂?PO₄?₂:Eu,the yellow Sr₂Ca?PO₄?₂:Eu and the cyan Ba₂Ca?PO₄?₂:Eu were designed.Different behaviors of emission evolution in response to structural variation was verified among the three M₃?PO₄?₂:Eu joins.Sr₃?PO₄?₂ and Ba₃?PO₄?₂ form a continuous isostructural solid solution set in which Eu²⁺ exhibits a similar narrow-band blue emission centered at 416 nm,whereas Sr²⁺ substituting Ca²⁺ in Ca₃?PO₄?₂ induces phase transition and the peak emission gets red-shifted to 527 nm as approaching the phase transition point.In the Ca_3-xBa_x?PO₄?₂:Eu set,the validity of crystallochemical design of phosphor between a phase transition boundary was further verified.This cation substitution strategy may assist in developing new phosphors with controllably tuned optical properties based on phase transition.Besides,bimodal emission bands were observed in the Ca_3-xSr_x?PO₄?₂:Eu?0?x?2?range,the origin of which was identified on aspect of local coordination environment of Eu²⁺.?2?Starting from Lu₃Al₅O₁₂:Ce,the Al-Al in respective octahedral/tetrahedral coordination was simultaneously substituted by a Mg-Si pair forming the Lu₃(Al_2-xMgx)(Al_3-xSi_x)O₁₂:Ce?x = 0.5-2.0?series.With substitution of Al/Al by Mg/Si,the Al-tetrahedron site got shrinkaged linearly while the Al-octahedron site got expanded linearly.The CeO₈ polyhedrons were then distorted and the emission got red-shifted from green?542 nm?to yellow?571 nm?together with the spectral broadening.The evolution of,the unit cell,the local structural geometry as well as the Ce³⁺ luminescence in these garnet creations,in response to the gradual Mg-Si substitution for Al-Al,were studied by combined techniques of structural refinement and luminescence measurement.The new Lu₃Mg_0.5Al₄Si_0.5O₁₂ and Lu₃Al₃MgSiO₁₂ may be potential yittim-free yellow candidates for YAG:Ce without intellectual patent confilication.When using Y³⁺ to replace Lu³⁺,the emission can be further red-shifted,which may act as a way as effective as the commercial Gd/Y-Ga/Al substituting way in YAG:Ce to achieve an orange emission.?3?Starting from Y₃Al₅O₁₂:Ce,the MO₈/SiO₄?M=Ba/Sr/Ca/Mg?polyhedrons were employed to substitute the YO₈/AlO₄ polyhedrons,forming solid solution series of Y_3-xM_xAl_5-xSi_xO₁₂:Ce?x = 0-1?.The new garnets absorb blue light,and with M changing from Ba to Mg,the emission peaks red-shift from 532 to 550 nm.With bigger M ions,the power product contains some in-situ formed glass phase,the ratio of which depends on the size of M ion and the designed substitution ratio of M/Si for Al/Al.When M=Ba,the Y₂BaAl₄SiO₁₂:Ce is in a microcrystal-glass powder form,which can be regarded as a new form for phosphor.The introduction of MO₈/SiO₄ polyhedrons helps decrease the rare earth element usage and lower the synthesis temperature,which may permit a lower production cost.