Microwave Synthesis And Properties Of Rare Earth Ions Doped Calcium Molybdate Luminescent Materials

CaMoO₄ with the tetragonal scheelite-type structure,have been widely applied in the fields of fluorescent lamp,flat panel display and solid-state lasers because of their high thermal and chemical stability,excellent luminescence,low synthesis temperature etc.Therefore,CaMoO₄ may be a perfect choice for luminescence host materials.In this paper,a new route—microwave radiation method were developed for the synthesis of CaMoO₄-based phosphors,and a series of rare earth doped CaMoO₄ phosphors were prepared successfully by this method.The phosphors were analyzed and characterized by means of X-ray powder diffraction,scanning electron microscopy,fluorescence spectrophotometry and diffused reflectance spectrum,respectively.The influence of co-doped ions on the photoluminescence properties and the possible energy transfer mechanism were discussed in detail.The main research contents and results are as follows:?1?A series of yellow-green emitting phosphors CaMoO₄:Dy³⁺ were synthesized rapidly via microwave radiation method.The results reveal that the as-synthesized samples are pure tetragonal CaMoO₄.The particles of sample are approximately cubic in shape,and cubic big particle is composed by a large number of spherical-like grains.The excitation spectra of samples are attributed to the Mo–O,Dy–O charge transfer bands and the f–f transitions of Dy³⁺.The emission spectra consist of a series of sharp peaks,and the main peak is located at 572 nm,which is ascribed to the 4F9/2?6H13/2 transition of Dy³⁺.Meantime,the effects of doping charge compensator?Li+,Na+,K+?,PO₄^3-,and charge compensator together with PO₄^3-on the phase structure and luminescent properties of the phosphors were discussed in detail.The results show that the introduction of appropriate charge compensator and PO₄^3-could effectively improve the crystallinity and the emission intensity of the samples.Especially,the introduction of K+ as charge compensation agent has the best effect.The fluorescence intensity shows the tendency of increase at first,and then decrease with a further increase of K+ concentration.When the doping amount of K+ is 0.10,the intensity of main emission peak reaches to the maximum,which improves about 116% than that of the undoped-K+ sample.The optimum doping content of PO₄^3-is 0.04.In this case,the sample achieves an improvement of 19% of the main emission intensity compared with the undoped-PO₄^3-sample.The main emitting intensity of co-doped Li+/PO₄^3-,Na+/PO₄^3-and K+/PO₄^3-samples reaches to the maximum when the doping content is 0.12,0.12 and 0.10,respectively,which is about 103%,127% and 95% higher than that of the undoped samples,respectively.?2?Color-tunable phosphors Ca0.98-xMo O4:Dy³⁺0.02,Eu3+x were fabricated by a rapid microwave radiation method.The as-synthesized samples are pure tetragonal CaMoO₄ structure,which consist of cubic particles and some irregular polyhedron particles.The big cubic particles are assembled by many closely packed quasi-spherical grains.Under 301 nm excitation,the blue-green emission of Mo O42-,the blue and yellow emission of Dy³⁺,and the red emission of Eu3+ can be detected.The emission color of phosphors can tuned from yellow-green through yellow and orange-reddish and eventually to red by properly adjusting the content of Eu3+.Besides,the possible energy transfer mechanism of CaMoO₄:Dy³⁺,Eu3+ is ascribed to the energy transfer from the 4F9/2 level of Dy³⁺ to 5D0 level of Eu3+ through the phonon-aided nonradiative relaxation.?3?A series of orange reddish-emitting phosphors Ca1-xMo O4:Sm3+x?0?x?0.06?were prepared using a simple and efficient microwave radiation method,and the influence of microwave reaction time and doped concentration of Sm3+ on the phase structure and luminescent properties of the samples were discussed.It is found that when the microwave reaction time is 60 min,the as-synthesized samples are all tetragonal scheelite structure with I41/a space group.The sample consists of many cubic particles and some irregular polyhedron particles.The cubic particle is assembled by many closely packed quasi-spherical grains,and the grain size is about 0.2 2.0 ?m.The excitation spectra of the samples include a broad band and a series of sharp peaks.The broad band between 200 to 350 nm is attributed to the charge transfer bands of O2-–Mo6+ and O2-–Sm3+;the absorption peaks at 364,377,405,442 and 469 nm are assigned to the characteristic transitions of Sm3+.In the emission spectra,the broad band in the range of 400 nm to 550 nm is corresponding to the intrinsic emission of Mo O42-;and the three peaks around 562,605 and 645 nm are attributed to the 4G5/2?6H5/2 magnetic-dipole transition,4G5/2?6H7/2 and 4G5/2?6H9/2 electric dipole transitions of Sm3+,respectively.The microwave reaction time has obvious effect on the luminescent properties of the samples.The main emission intensity of Sm3+ at 645 nm firstly increases with the reaction time,then decreases.The emission intensity reaches to the maximum when the reaction time is 60 min.Moreover,the fluorescence intensity is related to the concentration of Sm3+.The optimum luminescence can be observed when Sm3+ concentration is 0.02.After that,the luminescence intensity reduces obviously,ie,the concentration quenching occurs,which is due to the dipole-dipole interaction and the four types of cross-relaxation processes between Sm3+.