Synthesis And Properties Of Rare Earth Doped Lanthanum Metaphosphate And Pyrophosphates M₂P₂O₇（M = Sr, Mg, Ba）phosphors

Rare-earth phosphates have been considered as excellent hosts for luminescence materials for their special physical and chemical properties they possess. Andthe relatively low luminescence efficiency and high cost of rare-earth orthophosphate have limited their applications. However, rare earth polyphosphates have potential applications as phoshors in the areas of lighting and display owing to their relatively low cost and high quenching concentration. Therefore, it is important to develop the synthesis technologies and explore the relation between the structures and properties for the applications of rare earth polyphosphates luminescent materials.Rare earth doped La P3O9 and M2P2O7:Eu3+(M=Sr,Mg,Ba) phosphors luminescent materials were synthesized by a wet chemistry method. The crystal structures, thermal behaviors and optical properties of the as-synthesized samples were investigated by X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FT-IR), Thermal gravimetric analysis(TG-DTA) and Fluorescence spectroscopy(FL). Some new results were obtained and listed as follows.A polyacrylamide gel mehtod was developed to synthesize rare earth doped La P3O9 and M2P2O7:Eu3+(M=Sr, Mg, Ba) phosphors. The influence of the calcination temperature, calcination time, p H of precursor solution, and the value of n(P)/n(La+RE) in the reaction system on the crystal structures and optical properties of the as-prepared samples were investigated.. The results reveal that the calcination temperature and the value of n(P)/n(La+RE) in the reaction system play the key roles in the phases of products, and the preparation process conditions also have singnificant influence on the optical properties of the samples. When the calcination temperature is 850 oC, calcination time is 6 h, p H value is 4, and the value of n(P) /n(La+RE) is 3, single-phase orthorhombic structure La P3O9:Eu3+ crystals can be obtained. The La P3O9:Eu3+ samples emitted orange-red light characteristic of Eu3+ under ultraviolet light excitation. Series of Eu3+, Tb3+, Ce3+ doped/ co-doped La P3O9 luminescent materials with different doping contents were synthesized via a polyacrylamide gel route. The phase structures and fluorescence properties of doped/co-doped La P3O9 phosphors were also discussed. The XRD results show that the doping of samll contents of rare earth ions had no significant influence on the crystal structure of the samples. The excitation spectra of La P3O9:Eu3+ samples are composed of the O2–-Eu3+ charge transfer(CT) band and the characteristic excitation bands of Eu3+. The relative intensity of the O2–-Eu3+ charge transfer(CT) band is far greater than that of the Eu3+ characteristic excitation bands. Under ultraviolet light excitation, the samples exhibited a characteristic orange-red light of Eu3+. The Eu3+ critical concentration is 40% in the La P3O9:Eu3+. The excitation spectra of La P3O9:Tb3+ products are composed of the O2–-Tb3+ charge transfer(CT) band and the characteristic excitation bands of Tb3+. The O2–-Tb3+ charge transfer(CT) band possess a higher excitation strength than that of the Tb3+ characteristic excitation bands. Under ultraviolet light excitation, the La P3O9:Tb3+ samples emitted a dominating green light attributing to the characteristic Tb3+ transition. Interestingly, the quenching concentration of the 5D4�'7Fj(j=6,5,4,3) transitions and the 5D3�'7Fj(j=6,5,4) transitions is 6% and 4%, respectively. The emission spectra display that the emission intensity of the Eu3+ increased with increasing the co-doping contents of Tb3+ for the La P3O9:Eu3+,Tb3+ samples, indicating that the energy transfer from Tb3+ to Eu3+ occurred. A similar enhancement of the energy transfer from Ce3+ to Tb3+ induced green emission was observed in the as-synthesized La P3O9:Eu3+,Tb3+ crystals.A polyacrylamide gel process combined with a hydrothermal route, and coprecipitation method have been applied to synthesize La P3O9:Eu3+ luminescent materials. The effect of the hydrothermal temperature, calcination temperature, and p H of solution on the crystal structures and optical properties have been discussed.. The results reveal that it is hard to obtained single phase La P3O9:Eu3+ crystals via the polyacrylamide gel process combined with a hydrothermal route. When the hydrothermal temperature is 140 °C, calcination temperature is 850°C, the p H is 13, the almost single phase of La P3O9:Eu3+ crystals can be prepared, although there still exist slight La PO4:Eu3+ crystlas. The La P3O9:Eu3+ samples have been also synthesized at 850 oC by coprecipitation method with Na3P3O9 as the starting materials. The FL spectra show that the as-synthesized La P3O9:Eu3+ samples obtained by coprecipitation method exhibited a good luminescence properties. The structures and the properties of samples prepared by three different methods, polyacrylamide gel technique, polyacrylamide gel process combined with a hydrothermal route, coprecipitation method, has been comparatively investigated. The products obtained by polyacrylamide gel method exhibited pure phase La P3O9:Eu3+ cyrstals and improved luminescence with smaller diameter compared with the samples prepared by other techniques.Pure phase Sr2P2O7:Eu3+ phosphors were synthesized by a polyacrylamide gel technique. The influence of synthesis conditions on the structures and properties of samples have been also investigated. The results reveal that, in the experimental range, the calcination temperature, calcination time, p H, and Eu3+ doping contents play the key roles in the luminescent properties of samples, while have less influence on the phases of products. According to the combined phase structures and luminescent properties of samples, the optimum prepared conditions were as follows: calcination temperature is 900 °C, calcination time is 6 hours, and p H value is 6. Single-phase Sr2P2O7:Eu3+ crystals with excellent luminescent properties were obtained under the conditions as above. Under ultraviolet light excitation, the Sr2P2O7:Eu3+ crystals emitted an orange-red light. The emission spectra show that and the relative intensity of the 5D0-7F2 emission peak is stronger than that of the 5D0-7F1 band. The concentration quenching was observed if the Eu3+ doping content above 7%. Moreover, pure phase M2P2O7:Eu3+(M= Mg, Ba) phosphors were synthesized by a polyacrylamide gel technique at 1000 oC. Under ultraviolet light excitation, Ba2P2O7:Eu3+ and Mg2P2O7:Eu3+ phosphors also displayed an orange-red light, while the relative intensities of the 5D0-7F1 emission band of Ba2P2O7:Eu3+ and Mg2P2O7:Eu3+ crystals are all higher than that of the 5D0-7F2 emission peaks. The emission spectra of phosphors indicate that the Eu3+ critical concentration is 5% in the Ba2P2O7:Eu3+ and Mg2P2O7:Eu3+ matrix. Moreover, Mg2P2O7:5%Eu3+ crystals exhibit stronger emission compared with the Ba2P2O7:5%Eu3+ samples.