| Calcium tungstate (CaWO4) is known as a single-crystalline scintillation detector of ionizing radiation and was first used in the years following Roentgen’s discovery of X-ray. Since its excellent luminescence, chemical and thermal stability property, CaWO4has been extensively used as solid-state laser, display, scintillating media, fiber-optical communication, etc. In recent years, studies on the optical property of Eu3+doped CaWO4phosphor are carried out, and some satisfactory results with practical value are also obtained. It finds that Eu3+doped CaWO4phosphor can be excited by393nm or464nm and emits the red light with line spectrum (the main peak locates616nm), which is in agreement with the UV or blue LED chips, having the potential to replace the sulfides-based red phosphors. Therefore, many efforts mainly focus on how to enhance the emission intensity, but the results show that the strong emission intensity of Eu3+doped CaWO4phosphor only can be obtained at high Eu3+doping concentration. For Eu3+doped CaMoO4phosphor, it has the same optical property as Eu3+doped CaWO4phosphor. Usually, the excitation spectra of Eu3+doped CaMoO4phosphor monitored at616nm show the excitation peak at393nm stronger than that at464nm. However, upon careful study we found that the relative intensity of excitation spectra peaks at393nm and464nm can be changed with different Bi3+/Eu3+ratio in CaMoO4matrix, we will discuss this phenomenon in present paper. What is more, in additional to the fluorescence characteristics, there actually exists a red afterglow in CaWO4and CaMoO4phosphors with low Eu3+doping concentration and the afterglow property is seldom noticed in previous paper. Thus, a series of CaMoO4:Eu3+, Bi3+and CaMO4:Eu3+(M=W, Mo) red phosphors were prepared by the high temperature solid-state reaction and the formulas2Ca2+â†'Eu3++M+(M=Na+) was adopted to compensate the unbalance charge of some samples. For the red afterglow phosphors with low Eu3+doping concentration, a series of AMO4:Eu3+, Re3+(A=Ca, Sr, Ba; M=W, Mo; Re=Sm3+) samples were also prepared by solid-state method. The structure and the luminescence property of the obtained phosphors are characterized by the XRD, photoluminescence, decay and thermoluminescence (TL). The main results obtained in this paper are as follows:1) It is found that the optimal Eu3+doping concentration in CaMoO4phosphor was0.3. For Ca0.70BiyMoO4:Eu(0.30-y)3+samples, the excitation intensity at393nm is stronger than that at464nm when the y value is less than0.04, but the excitation intensity at464nm become stronger than that at393nm when the y value is equal or greater than0.04. What is more, the results indicate that the excitation intensity at464nm increases with the increases of y value, but the excitation intensity at393nm decreases with the increases of y value. We also investigate the optical property of Ca0.70-2yBiyMoO4:Eu(0.30+y)3+samples and the results show that the excitation intensity at393nm is stronger than that at464nm when the y value is less than0.02, but the excitation intensity at464nm become stronger than that at393nm when the y value is equal or greater than0.02. Therefore, the relative excitation intensity between393nm and464nm can be changed by the appropriate ratio of Eu3+/Bi3+co-doping in CaMoO4phosphor. It is believed that the causes of the change of the excitation intensity between393nm and464nm is from the energy transfer between Bi3+and Eu3+and the unbalance charge when the Ca2+is replaced by Eu3+and Bi3+. The formulas2Ca2+â†'Eu3+/Bi3++Na+can improve the optical property of the samples.2) With the investigation of NaxCa1-2x(MoO4)1-y(WO4)y:Eux3+red phosphors, we find that all prepared samples have a scheelite structure in the space group I41/a (NO.88), when y=0, the main diffraction peaks shift to the low angle with the increase of the x value. The results of the photoluminescence shows that the intensity of the charge charge-transfer (C-T) band reaches the maximum value when x=0.25, but the emission and excitation intensity of Eu3+do not decrease with the increase of x value, that is to say, the emission and excitation intensity of Eu3+reaches the maximum value when x=0.50(namely, Na0.50MoO4:Eu0.503+sample). With the study of Na0.50(Mo04)1-y(WO4)y:Eu0.503+samples, it finds that the optical property can be improved when appropriate amount of MoO42-is replaced by WO42-and Na0.50(MoO4)0.40(WO4)0.60:Eu0.503+is experimentally to be considered as the optimal sample. Upon excitation of393nm and464nm, all obtained samples can emit a good red component which is originated from the5D0â†'7F2transition of Eu3+.3) When the CaW04phosphors with low Eu3+doping concentration are excited by a mercury lamp (254nm), red long afterglow phenomenon can be observed. The red afterglow spectrum shows that the emission peaks of all samples are originated from the5Do-7FJ (J=0,1,2,3,4) transitions of Eu3+, and the main peaks locate at592nm and616nm. The afterglow property can be improved after the introduction of the alkali metal charge compensators. We assume that there are two ways of Eu3+arrangement in CaW04crystal lattice:on Ca2+sites and on W6+sites, and we believe that the afterglow-phenomenon mainly originated from the complex defects produced by the replacement of W6+with Eu3+. The afterglow duration of the optimal sample could last nearly40minutes in dark with the naked eyes after being excited at254nm for3min.4) Red afterglow can be observed when CaMoO4:Eu3+samples are excited by a mercury lamp (254nm). With the analysis of the TL curves, it finds that the TL peak at around74℃is the origin of the afterglow phenomenon. Different doping concentration of Eu3+in CaMoO4results in the different afterglow property, and the optimal Eu3+concentration is experimentally determined to be0.5%.5) The optical and long afterglow property can be improved when appropriate amount of MoO3is added in CaWO4:Eu3+. This may result from the fact that appropriate amount of MoO3can improve the efficient energy transfer from host to Eu3+. The optimal MoO3concentration for afterglow property was experimentally ascertained to be0.02.6) With the investigation of CaWO4:Sm3+, Eu3+phosphors, it finds that the optical property and long afterglow property can be improved when appropriate amount of Sm3+is added in CaWO4Eu3+. This may result from the energy transfer between Sm3+and Eu3+and Sm3+ions can transfer a part of its absorbed energy to Eu3+ions. With the analysis of the afterglow spectrum, it finds that the afterglow originates from the5D0-7FJ (J=0,1,2,3,4) transitions of Eu3+, and the main peaks locate at592nm and616nm, respectively. In the obtained samples, there are five TL peaks around at74,114,153,167and254℃. The optimal Sm3+doping concentration for afterglow property is experimentally ascertained to be0.05%.7) The results of MWO4:Eu3+(M=Ca, Sr, Ba) samples show that the intensity of the charge transfer (CT) band and Eu3+excitation spectra decreases when the Ca2+ions are completely replaced by the Sr2+ions or Ba2+ions in host. What is more, the CT band of the host shifts to the short-wavelength direction (namely, blue shift). There are five TL peaks in CaWO4:Eu3+sample and two TL peaks in SrWO4:Eu3+and BaWO4:Eu3+samples, respectively. The luminescence, afterglow duration and TL intensity are different and the sequence is CaWO4:Eu3+> SrWO4:Eu3+> BaWO4:Eu3+. All MW04:Eu3+(M=Ca, Sr, Ba) samples exhibit afterglow phenomenon. It is probably due to the charge transfer (CT) from the2p states of O2-to the5d states of W6+accompanied by the energy transfer to luminescent centers. Holes in valence band can be trapped by hole traps and their liberation leads to the CT and then the afterglow of Eu3+is generated by energy transfer from CT to the Eu3+ions. |
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