The compounds containing lutetium and tungsten atoms have large effective atomic number (Zeff) and high stopping ability for high energy radiation because both lutetium and tungsten atoms are heavy with large atomic number. In order to obtain a new red-emitting phosphor with high efficiency for X-ray detection, the trivalent europium ion (Eu3+) activated alkaline double tungstate phosphor NaLu(WO4)2:Eu3+ was prepared by high temperature solid state reaction. The crystalline structures of synthesized phosphor were determined by powder X-ray diffraction (XRD) and elucidated using Topas Academic software, and the morphologies were characterized by thermal field emission scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). The Rietveld structural refinement results suggest that the cell parameters become larger with the increasing of Eu3+ doping concentration. The emission spectra of NaLu(WO4)2:Eu3+ under UV and X-ray radiation were measured, respectively. It was observed that this micro-particle phosphor shows intensive red emission under X-ray radiation, which implies that NaLu(WO4)2:Eu3+ phosphor has potential application for X-ray detection.
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X-ray phosphors which can absorb X-ray and convert the absorbed energy efficiently into ultraviolet or visible emission, are used in the application of light intensity measurement under continuous X-ray radiation. A high-performance X-ray phosphor should be a high-density material or contain heavy elements because the absorption coefficient increases strongly with the atomic number . The first X-ray phosphor found by Pupin was calcium tungstate CaWO4, which was quite good in transferring X-ray radiation to fluorescent light . CaWO4 with a density of 6.06 g·cm−3 has served all needs for X-ray intensifying screens for a century. Only with the advent of rare-earth phosphors, CaWO4 has lost its dominant position. And with the availability of individual element of rare-earth of high purity, many new X-ray phosphors have been studied and developed, such as Gd2O2S:Tb [3,4], LaOBr:Tb , BaFCl:Eu , YTaO4:Tb , and GdTaO4:Tb .
Alkaline rare-earth double tungstates with the formula of MRE(WO4)2 (M = alkaline ions, RE = trivalent rare-earth ions) which are structural derivatives of CaWO4, have been used as host materials for phosphors and laser crystals due to their good thermal and chemical stability. Examples are NaLa(WO4)2 [9,10], RbNd(WO4)2 , KGd(WO4)2 , KYb(WO4)2 , etc. These double tungstates doped with trivalent rare-earth ions have shown good optical properties.
Sodium lutetium double tungstate NaLu(WO4)2 containing lutetium and tungsten atoms has large effective atomic number (Zeff = 66.10) compared with other common X-ray phosphors like Gd2O2S (Zeff = 59.37) and LaOBr (Zeff = 49.33) and high stopping ability for high energy radiation because both lutetium and tungsten are heavy with large atomic number. So, NaLu(WO4)2 is an efficient host for X-ray luminescent material. To our knowledge, the radio-luminescence properties of Eu3+ doped NaLu(WO4)2 have not been studied yet, though the red luminescence of Eu3+ ion has been extensively used in the lighting and displaying fields. In order to develop a new red-emitting phosphor for X-ray detection application, a series of samples NaLu(WO4)2:Eu3+ with different Eu3+ doping concentration were prepared by the solid state reaction, and the photoluminescence and radio-luminescence properties under UV-visible light and X-ray excitation were investigated in present work.
Samples with nominal composition NaLu(1-x)Eux(WO4)2 (x = 0.05, 0.10, 0.15, 0.20, 0.50, 1.0) were prepared by solid state reaction at high temperature. The raw material WO3 (A.R. grade), NaHCO3 (A.R. grade), Lu2O3 and Eu2O3 (99.99% purity) were mixed according to the following stoichiometric ratio and ground in agate mortar.
The structures of the samples NaLu(WO4)2:Eu3+ were recorded at room temperature (RT) by X-ray powder diffraction (XRD) using Cu Kα radiation on Burker D8 (Bruker Co., Germany), operating at 40 kV and 40 mA. A step size of 0.02 ° was used with a scanning speed of 10 °/min. And the scanning speed was changed to 0.2 °/min when the data was conducted by the Rietveld structural refinement. The morphology and elemental composition of the as-synthesized samples were measured by a thermal field emission environmental scanning electron microscope (FEI Quanta 400) equipped with an energy-dispersive X-ray spectrometer.
The photoluminescence (excitation and emission) spectra and the decay curves were determined on a FLS 920 spectrometer (Edinburgh Instruments) with red-sensitive PMT. For steady-state spectra, a 450 W xenon lamp was used as the excitation source. For luminescence decay spectra, a 60 W μF flash lamp was used. Low temperature (LT) for spectra measuring was realized by liquid helium. The slit width was 0.2 mm. The photoluminescence emission spectra were corrected for the spectral sensitivity of the recording system and the photoluminescence excitation spectra were not normalized to equal the number of incident photons. The X-ray excited fluorescent emission spectra were carried out in F50-100 bedside X-ray Machine (Shanghai Huaxian Medical Nuclear Instrument Co. Ltd) in the voltage 45 kV and filament current 30 mA. Time of exposure was 0.2 S. The emission signals were recorded using a fiber spectrometer (Ocean Optics QEB0388) with a charge coupled device (CCD) camera at RT.
3. Results and discussion
3.1 Structure and morphology of NaLu(WO4)2:Eu3+
Figure 1 shows the XRD patterns of the synthesized samples NaLu(1-x)Eux(WO4)2 (x = 0.05, 0.10, 0.15, 0.20, 0.50, 1.0). All patterns are compared with the standard card JCPDS 79-1118 of the isostructural compound NaLa(WO4)2. The characteristic diffraction peaks of samples NaLu(WO4)2:Eu3+ are similar with that of NaLa(WO4)2 except that the peaks of NaLu(WO4)2:Eu3+ have small shift to high angle because the radius of both Lu3+ ion (C.N. = 8, R = 97.7 pm) and Eu3+ ion (C.N. = 8, R = 106.6 pm) are less than that of La3+ ion (C.N. = 8, R = 116 pm) , leading to NaLu(1-x)Eux(WO4)2 having relative smaller cell parameters than NaLa(WO4)2. And with the increasing of Eu3+ doping concentration, the deviation between the characteristic diffraction peaks of samples NaLu(1-x)Eux(WO4)2 and that of NaLa(WO4)2 becomes smaller because the ionic radius of Eu3+ is closer to that of La3+, which indicates that the samples NaLu(1-x)Eux(WO4)2 are in single crystalline phase with Eu3+ ions occupying the sites of Lu3+ ions in the crystal structure.
To indicate the evolvement of the cell parameter accurately, Rietveld structural refinements from powder XRD profiles were further carried out. Figure 2 exhibits the refinement diffraction pattern of NaLu0.5Eu0.5(WO4)2 as an example. All fitted diffraction peaks match with XRD data to high extent. According to the refined results, it can be obtained that the structure cell parameters of NaLu0.5Eu0.5(WO4)2 are as following: a = b = 5.212 Å, c = 11.293 Å, V = 306.776 Å3 and ρ = 7.119 g·cm−3. The refined results of other samples with different Eu3+ doping concentration are listed in Table 1 . It is confirmed that the unit cell parameters are related to the Eu3+ doping concentration in the host NaLu(WO4)2. With the increasing of Eu3+ content, the unit cell of NaLu(1-x)Eux(WO4)2 becomes larger, which is an agreement with the XRD analysis.
NaLu(WO4)2 having a scheelite (CaWO4) structure, is a member of the MRE(WO4)2 family with C (I41/a) space group. Na+ and Lu3+ ions occupy Ca2+ sites in the original scheelite CaWO4 crystal structure with a ratio of 1:1, and form distorted NaO8 and LuO8 dodecahedra sharing oxygen atoms. The LuO8 polyhedra form a single zigzag chain in the  direction sharing O-O edges. The local site symmetry of the Lu3+ ions in the host NaLu(WO4)2 is C2 asymmetry.
In order to illustrate the particle morphology and size of as-synthesized samples NaLu(1-x)Eux(WO4)2, SEM image making the sample NaLu0.5Eu0.5(WO4)2 as an example is shown in Fig. 3 . It can be seen that all the prepared samples NaLu(1-x)Eux(WO4)2 are homogeneous polycrystalline powder and have good crystallinity with size around 3 μm. For further analyzing the detailed composition of the prepared samples, EDS spectra were measured. The inset in Fig. 3 confirms the presence of Na, Lu, Eu and W elements in the product. And the Na:Lu:Eu:W atomic ratio is determined to be 1:0.49:0.45:2.16, which agrees well with the theoretical atomic ratio (1:0.5:0.5:2) of the NaLu0.5Eu0.5(WO4)2.
3.2 Photoluminescence properties of NaLu(WO4)2:Eu3+
Figure 4 shows the emission spectrum of sample NaLu0.5Eu0.5(WO4)2 under 395 nm excitation at 10 K. The high strength sharp peaks at 615 nm are originated from the 5D0 → 7F2 transition of Eu3+ as the hypersensitive electric dipole transition and the relative weak emission peaks at 591 nm are assigned to 5D0 → 7F1 transition as the magnetic dipole transition. In general, the Eu3+ emission peaks are highly sensitive to the coordinating environment. If Eu3+ occupies the lattice site of a non-centrosymmetric environment in the host, the electric-dipole transition would be dominant. From this spectrum, it can be observed that the intensity of the electric dipole transition at about 615 nm is much higher than that of the magnetic dipole transition at about 591 nm, which means that Eu3+ doped in the host NaLu(WO4)2 situates at Lu3+ site of C2 asymmetry without inversion symmetry. Those peaks at about 660 nm and 705 nm are ascribed to 5D0 → 7F3 and 5D0 → 7F4 transitions within Eu3+ ions. As indicated by the tenfold magnified spectrum in the range from 500 nm to 600 nm, a single weak peak at 581 nm from 5D0 → 7F0 transition of Eu3+ suggests that there exists only one kind of site for Eu3+ in the surrounding coordination . And a series of weak emission peaks at 513 nm, 537 nm, 555 nm and 586 nm are attributed to 5D1 → 7FJ (J = 0, 1, 2, 3) transitions of Eu3+.
Figure 5 presents the Eu3+ content dependence of emission spectra for samples NaLu(1-x)Eux(WO4)2 (x = 0.05, 0.10, 0.15, 0.20, 0.50, 1.0) in the region between 450 nm and 750 nm excited by ultraviolet radiation at 395 nm corresponding to the 7F0 → 5L6 transition within Eu3+ ions at RT. The spectra essentially consist of sharp emissions with wavelengths ranging from 580 to 720 nm, which are associated with the 5D0 → 7FJ (J = 1, 2, 3, 4) multiplet transitions from the excited state to the ground state. The inset is the Eu3+ doping concentration dependence of the relative emission intensity due to 5D0 → 7F2 transition at 615 nm. It can be noticed that the red emission at 615 nm gradually increases with the increasing incorporation of Eu3+ and there is no concentration quenching even all Lu3+ ions in NaLu(WO4)2 are substituted by Eu3+ ions to form the isostructural compound NaEu(WO4)2, which indicates that the energy absorbed directly by Eu3+ ions at 395 nm can be effectively converted to red emission at 615 nm. The concentration quenching is absent because the emission causes big Stokes shift, indicating that the relaxed emission states and the adjacent particle cannot resonant. In addition from the crystal structure, the spatial arrangement with isolated EuO8 polyhedra by means of Eu-O-W-O-Eu can block resonance energy transfer among Eu3+ ions .
The excitation spectra of NaLu0.5Eu0.5(WO4)2 monitoring the emission at 615 nm from 10 K to 400 K in Fig. 6 contain a broad excitation band centered at 269 nm in the range from 220 to 320 nm that attributes to the charge transfer absorption from the 2p orbits of the oxygen ligands to the 5d orbits of the central tungsten atoms in the WO42- groups , indicating that the energy absorbed by WO42- can be transferred efficiently to Eu3+ for red emission. Noticing that from 10 K to 400 K the charge transfer band exhibits red shift from 265 nm to 273 nm, resulting from the longer W-O bond length when the temperature rising. However, the sharp lines from 300 nm to 550 nm corresponding to the f-f transitions of Eu3+ preserve the same position, where the two strongest absorption peaks at 395 nm and 465 nm are due to 7F0 → 5L6 and 7F0 → 5D2, respectively. The charge-transfer band of Eu3+-O2- was not clearly observed in the excitation spectra [18–20]. The peak at about 540 nm is ascribed to 7F0 → 5D1, and others are 299 nm (7F0 → 5F4), 304 nm (7F0 → 5F2), 319 nm (7F0 → 5H3), 328 nm (7F0 → 5H4), 362 nm, 366 nm (7F0 → 5D4), 377, 382, 385 nm (7F0 → 5L7) . The right-up corner inset in Fig. 6 shows the temperature dependence of the excitation intensities at 265 nm and 395 nm, respectively. It can be found that the intensities both at 265 and at 395 nm become weak gradually with temperature increasing sharing almost the same trend.
3.3 Radio-luminescence properties of NaLu(WO4)2:Eu3+
This series of synthesized samples NaLu(1-x)Eux(WO4)2 (x = 0.05, 0.10, 0.15, 0.20, 0.50, 1.0) were irradiated by X-rays and the X-ray excited emission spectra were detected using a fiber spectrometer with a CCD camera at RT. Figure 7 shows the X-ray excited emission spectrum of NaLu0.5Eu0.5(WO4)2 as an example. Under X-ray irradiation, this series of samples NaLu(1-x)Eux(WO4)2 (x = 0.05, 0.10, 0.15, 0.20, 0.50, 1.0) have intensive red emission at 615 nm due to 5D0 → 7F2 transition within Eu3+ ions and the intensity of this emission always increase as the growing of Eu3+ addition as shown in the inset of Fig. 7.
3.4 Luminescence decay properties of NaLu(WO4)2:Eu3+
Analyzing the luminescence decay curves provides the information on the luminescence processes. In order to evaluate the luminescence decay properties of this lutetium based red-emitting phosphor, the decay curves of NaLu(WO4)2:Eu3+ with different Eu3+ doping concentration have been measured at RT. Figure 8 is the decay curves for 5D0 → 7F2 (615 nm) emission of three samples NaLu(1-x)Eux(WO4)2 (x = 0.05, 0.50, 1.0) under excitation at 395 nm. All the decay curves are well fitted by the single-exponential equation as following, I = A + I0 exp(-t/τ), where I and I0 are the luminescent intensities at time t and t = 0. A is a constant, t is the time and τ is the lifetime value, respectively. Though the different content of Eu3+ in this series of samples, all the lifetime values keep the same value as 0.54 ms, which is relative fast. It is further verified that there is no concentration quenching phenomenon within the red-emission of Eu3+ doped in the host NaLu(WO4)2, because the decay curve of NaEu(WO4)2 still keep the same single-exponential process even all Lu3+ ions in NaLu(WO4)2 are substituted by Eu3+ ions.
Based on the fact that lutetium and tungsten atoms have high stopping ability for high energy radiation, a series of alkaline rare-earth double tungstate phosphor NaLu(WO4)2:Eu3+ with different Eu3+ doping concentration were prepared by the high temperature solid state reaction. The crystalline structure was elucidated from powder X-ray diffraction data using Topas Academic software. Data from XRD demonstrates the fact that Eu3+ substitutes the site of Lu3+ successfully, revealing the changing trend of cell parameter simultaneously as well. The emission spectra of NaLu(WO4)2:Eu3+ samples under UV and X-ray radiation were measured, respectively. Intense red emission originated from 5D0 → 7F2 and no concentration quenching can be observed, which implies that NaLu(WO4)2:Eu3+ phosphor has promising application for X-ray detection.
This work was financially supported by the National Basic Research Program of China (973 Program, No. 2007CB935502), National Natural Science Foundation of China (No. 20901085) and Ganzhou Qiandong Rare-earth Group Co., Ltd.
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