Photoluminescence Properties Eu²⁺,Mn²⁺ Co-doped Sr_2-xCa_xP₂O₇(x=0-2) And Sr₂Mg₃P₄O₁₅

In view of the advantages of high luminous efficiency, long lifetime andenvironmental safety for white LED against traditional fluorescent and incandescentlamps, it has gradually become the mainstream of research in the field of solid-statelighting in recent year. Employing blue and/or near UV GaN-based LED chips coatedwith phosphors is the most important method of fabricating white LED. The whiteLED employing near UV LED chips with tricolor phosphors has the advantages ofhigh color rendering index and color stability. Therefore, looking for tricolorphosphors that suitable for near-UV excitation is necessary.At present, phosphate luminescence materials have attracted much attention asphosphors for near UV excitation because of the advantages of high luminousefficiency, low synthesis conditions, low cost and high chemical stability. This thesisis aimed to searching for efficient tricolor phosphors for near-UV excitation, we studyon the photoluminescence properties of Eu²⁺, Mn²⁺codoped Sr_2-xCaxP2O7(x=0-2) andSr2Mg3P4O15for near UV excitation, the main results obtained are listed as follow:(1) The crystal phase and photoluminescence properties for Sr_2-xCaxP2O7: Eu²⁺(x=0-2) both vary with the content of Ca2+. Through the analysis of the XRD patternsand luminescence spectra for the samples, the following conclusions are obtained. Asincreasing x from0to2, the crystalline phase of the samples experiences pure Sr2P2O7phase(0≤x≤0.75), the mixing of deformed Sr2P2O7phase and Ca2P2O7phase(1≤x<2)and finally Ca2P2O7phase(x=2). As increasing x from0to1, the emittingband of Sr2P2O7: Eu²⁺shifts to the redside and the bandwidth is broadened; furtherincreasing x, the emitting band however keep unchanged. The emitting band ofCa2P2O7: Eu²⁺appears at x=1, then shifts to the redside and gets narrowed asincreasing x. The varying tendency of the position and bandwith for the emittingbands is explained by the crystal field strength and inhomogeneous broadening,respectively. We analyze the percentages of Sr2P2O7phase(normal and deformed)andCa2P2O7phase in the samples. The energy transfer process from the high energy Eu²⁺to the low energy one within the emitting band of Sr2P2O7: Eu²⁺is observed.(2) The crystalline phase of Sr_2-xCaxP2O7: Eu²⁺, Mn²⁺(x=0-2) for each x isidentical to that of Sr_2-xCaxP2O7: Eu²⁺. As increasing x from0to2, the emitting bandof Mn²⁺in Sr_2-xCaxP2O7first shifts to the redside (0≤x≤1.75) and then to the blueside(1.75≤x≤2). The shift of the emitting band position is explained by the change of thestrength and symmetry for the crystal field. The energy transfer process from Eu²⁺toMn²⁺is observed in the codoped samples, therefore, a strong blue emission and aorange one are both realized in a sample, demonstrating Sr_2-xCaxP2O7: Eu²⁺, Mn²⁺canbe used as blue-red dual color phosphor for white LED. Based on the applicability forthe white LED, we choose the Sr1.25Ca0.75P2O7: Eu²⁺, Mn²⁺(x=0.75) sample fordetailed investigation, because it is crystallizing in single α-Sr2P2O7type phase andexhibiting the widest spectra distribution in blue and red region as well as largestemission intensity ratio of orange to blue in this series of samples for fixed Eu²⁺andMn²⁺concentrations. We study the change of the emission spectra, fluorescencedecays and emission intensity ratios of orange to blue for fixed Eu²⁺concentration butvarying Mn²⁺concentrations, we also investigate its temperature characteristics.(3) We study the photoluminescence and energy transfer properties of Sr2Mg3P4O15:5%Eu²⁺,30%Mn²⁺. It is observed that Eu²⁺doped Sr2Mg3P4O15exhibits not onlythe well known blue emission band of Eu²⁺peaking at448nm but also a new one at399nm in violet. We attribute the violet and blue band to Eu²⁺occupying on the ten Ocoordinated Sr1site and six O coordinated Sr2site, respectively. Both this two Eu²⁺ band can transfer energy to Mn²⁺at610nm. We investigate the Mn²⁺luminescencethat deriving from the two Eu²⁺bands, respectively, the results obtained from theluminescence spectra are consistent with the one obtained from the fluorescencedecay curves. The Eu²⁺for the violet band can transfer energy to the red emittingMn²⁺more efficiently than Eu²⁺for the blue band, deducing attributed to the largerspectra overlap between the emission spectrum of violet band and excitation spectrumof Mn²⁺than that between blue band and Mn²⁺. The energy transfer efficiencies forviolet and blue band to Mn²⁺are70%and39%, respectively. Owing to the efficientenergy transfer from Eu²⁺to Mn²⁺, we get enhanced red luminescence in Sr2Mg3P4O15,demonstrating Sr2Mg3P4O15: Eu²⁺, Mn²⁺is a promising phosphor for enriching redcomponent of white LEDs.