Cation-Induced Variation of Micromorphology and Luminescence

Article pubs.acs.org/IC

Cation-Induced Variation of Micromorphology and Luminescence Properties of Tungstate Phosphors by a Hydrothermal Method Lihua He,† Xiao Zou,† Tao Wang,† Qiaoji Zheng,† Jie Liao,† Chenggang Xu,† Yongfu Liu,*,‡ and Dunmin Lin*,† †

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

‡

S Supporting Information *

ABSTRACT: Eu3+-doped MWO4 (M = Zn, Cd, Ca, Sr, or Ba) nanorods and rodlike, spherical, dumbbell-like, and doubletapter-like grains have been obtained via a hydrothermal method. The distinct differences in cationic radius lead to a special morphology, which is attributed to the symmetry of the crystal structure and the differences in the growth rates of various crystals, and it further leads to the variation of luminescence. It was found that the charge transfer band of MWO4:0.04Eu3+ exhibits a blue shift with an increasing cationic radius, and the shift is ascribed to less covalency being caused by an increase in the cationic radius. The emission intensity obviously increases with cationic radius, increasing for the samples with a monoclinic phase; however, it is the opposite for the samples with a tetragonal phase, and CaWO4:0.04Eu3+ exhibits an optimal emission intensity. In addition, the possible reasons for the decay lifetime are also discussed in detail. Our results indicate that cations can effectively control the crystal structure, micromorphology, and luminescence in tungstate phosphors, and thus, our approach is effective for obtaining materials with the desired morphology and crystal structure.

1. INTRODUCTION Among inorganic phosphors, divalent tungstates have attracted more attention in recent years because of their excellent spectra, stable chemical properties, wide intrinsic spectra, and high average refractive indices.1 Meanwhile, as a kind of typical self-activating material, tungstates exhibit high-efficiency blue emission under ultraviolet light excitation due to the charge transfer between activated 2p orbitals of O2− and the empty orbitals of the central W6+ ions in the host materials.2 As a result, tungstates with excellent physicochemical character have been extensively investigated and show potential applications in medical devices,3 microwaves,4 humidity sensors,5 catalysts,6−9 scintillates,10 phototropism,11 and white light-emitting diodes (wLEDs).12 Rare earth ions are known to play indispensable roles in inorganic photoluminescent materials because of their 4f−4f or 4f−5d transitions. In particular, Eu3+ has been frequently used as an activator because of its intense and narrow emission.13 The 4f−4f transition of Eu3+ is sensitive to its host; thus, the Eu3+ luminescence can be tuned by doping or changing the host crystal structure. Take tungstate as an example. A number of investigations have been performed to modify the luminescence properties of MWO4:Eu3+. For instance, an excellent luminescence of CaWO4:Eu3+ was obtained for its morphology changes caused by altering the Eu3+ doping concentrations.14 The luminescence of SrWO4:Eu3+ also was © XXXX American Chemical Society

enhanced greatly after the annealing processes during the synthesis by a hydrothermal method.15 Many factors in a hydrothermal method play important roles in determining morphologies and crystal structures that will influence the Eu3+ luminescence in tungstates.16−21 For example, a controllable microstructure can be achieved in CaWO4:Eu3+ by changing the concentration of sodium citrate or by adjusting the pH during the synthesis process, resulting in an improvement in luminescence.20,21 Much effort has been devoted to the investigation of the dependence of luminescence on cation substitution in various matrices.22−26 Actually, various cations M for MWO4:Eu3+ also have a strong effect on the morphology and crystal structure of obtained samples. However, there has been no investigation of the morphology or crystal structure induced by the M cations. Taking M = Zn, Cd, Ca, Sr, or Ba, in this work, we report for the first time the relationship between the morphology and crystal structure with cations in MWO4:0.04Eu3+. By a hydrothermal method, cations (M) with different radii were introduced into the MWO4:0.04Eu3+ host to adjust the morphologies of obtained samples, and Eu3+ luminescence has been enhanced. In this work, the relationship between morphology and luminescence properties of the samples has Received: September 30, 2016

A

DOI: 10.1021/acs.inorgchem.6b02352 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry been studied systematically, and it provides a method for adjusting and acquiring the anticipated micromorphology and crystal structure.

2. EXPERIMENTAL SECTION 2.1. Synthesis of MWO4:0.04Eu3+ (M = Zn, Cd, Ca, Sr, or Ba). A facile hydrothermal method was used to synthesize the MWO4:0.04Eu3+ powders. All chemicals were used directly without any treatment. Distilled water was used throughout. In a typical synthesis, Eu2O3 (99.99%) was dissolved in 65% HNO3 to form a Eu(NO3)3 solution. Zn(NO3)2·6H2O (99%) and Na2WO4·2H2O (99.5%) in a 1:1 molar ratio were dissolved in distilled water; the resulting tungstate-containing solution was added dropwise to the solution containing rare earth ions while being vigorously stirred at room temperature (except for BaWO4, at 60 °C), and finally, the pH was adjusted when a small amount of ammonium hydroxide was added to the solution. The resultant bulk volume was fixed at 37 mL. Then, the resulting suspension was transferred into a 100 mL Teflon flask held in a stainless steel autoclave, and the autoclave was sealed and heated at 180 °C for 22 h in an oven. After the autoclave had naturally cooled to room temperature, the white precipitates were collected by centrifugation, washed several times with distilled water and ethanol, and finally dried at 80 °C in air for 20 h. Other MWO4:0.04Eu3+ materials (M = Cd, Ca, Sr, or Ba) were prepared using Cd(NO3)2· 4H2O (99.99%), Ca(NO3)2 (99%), Sr(NO3)2 (99.97%), and Ba(NO3)2 (99.5%) via a similar process. 2.2. Characterization. The crystal structures of the powders were examined using X-ray diffraction (XRD) with Cu Kα radiation (Smart Lab), and the powder diffraction data were calculated by Rietveld refinement using the General Structure Analysis System (GSAS).27,28 The microstructures were observed with a scanning electron microscope (SEM, FEI-Quanta 250). The spectral information was recorded with a fluorescence spectrophotometer (F-7000) with a xenon lamp. All the measurements were taken at room temperature.

Figure 1. XRD patterns of Eu3+-doped MWO4 (M = Zn, Cd, Ca, Sr, or Ba).

parameters a, b, c, and V increase as the cationic radius increases for the same space group. The refinement results further confirm that the host lattice expansion occurs when a larger cation is introduced into the tungstate host. Figure 3 shows the unit cell structure of all samples and coordination environments of M2+ and W6+. For ZnWO4 and CdWO4, they display a monoclinic crystal structure (β = 91.46°). W6+ is surrounded by four oxygen atoms and forms the [WO4] tetrahedral cluster, and Zn2+ and Cd2+ are coordniated by six oxygen atoms and form the [ZnO6]/ [CdO6] octahedral cluster.29,30 As M = Ca, Sr, or Ba, they all exhibit a tetragonal crystal structure. For CaWO4, SrWO4, and BaWO4, Ca2+, Sr2+, and Ba2+, respectively, are coordinated by eight oxygen atoms, forming the [CaO8]/[SrO8]/[BaO8] cluster, and W6+ is coordinated by four oxygen atoms, forming the [WO4] cluster.31−33 Obviously, MWO4 possesses a distinct crystal structure with a different M2+ cation. 3.2. Morphology. The powder morphologies of the MWO4:0.04Eu3+ (M = Zn, Cd, Ca, Sr, or Ba) samples are shown in Figure 4. It is apparent that the samples present distinctly different morphologies during crystal growth. ZnWO4:0.04Eu3+ exhibits nanorods with an average diameter of 32 nm and a length of 66 nm (Figure 4a). Both ZnWO4:0.04Eu3+ and CdWO4:0.04Eu3+ crystallize into a monoclinic structure. However, CdWO4:0.04Eu3+ displays irregular rodlike particles with an average length of 0.78 μm and a width of 0.24 μm (Figure 4b), which is much larger than that of ZnWO4:0.04Eu3+. CaWO4:0.04Eu3+ shows a torispherical morphology with an average size of ∼3 μm (Figure 4c). SrWO4:0.04Eu3+ consists of dumbbell-like particles with bottom diameters of 2 μm and lengths of 3 μm, and some pores and defects also can be observed from the particle surface (Figure 4d). BaWO4:0.04Eu3+ crystallizes well, and some

3. RESULTS AND DISCUSSION 3.1. Crystal Structure. The XRD patterns of all asprepared MWO4:0.04Eu3+ (M = Zn, Cd, Ca, Sr, or Ba) powders are shown in Figure 1. All diffraction peaks are strongly consistent with the corresponding phase structure. For M = Zn and Cd, it can be seen that the diffraction peaks match well with the wolframite-type monoclinic phase of ZnWO4 (JCPDS Card No. 15-0774) and CdWO4 (JCPDS Card No. 14-0676), respectively. Their space group is P2/c. Meanwhile, diffraction peaks of CdWO4:0.04Eu3+ shift toward the lower 2θ value compared to that of ZnWO4:0.04Eu3+, indicating that the crystal lattice expansion occurs because of the increase in the cationic radius [RZn2+ (0.74 Å) < RCd2+ (0.95 Å), sixcoordinated].21 For M = Ca, Sr, and Ba, all of the diffraction peaks perfectly match with the scheelite-type tetragonal phase of CaWO4 (JCPDS Card No. 77-2233), SrWO4 (JCPDS Card No. 85-0587), and BaWO4 (JCPDS Card No. 72-0746), respectively. Their space group is I41/a. It is also interesting to find that the diffraction peaks shift to lower 2θ values with an increase in the cationic radius (RCa2+ = 1.0 Å; RSr2+ = 1.18 Å; RBa2+ = 1.35 Å; eight-coordinated).21 All the XRD patterns exhibit a single crystal phase, and no impurity phase is detected, implying that the Eu3+ ions are doped into the host lattice successfully. Figure 2 depicts the Rietveld refinements for all samples. The refinement results and the corresponding cell parameters are listed in Table 1. The reliability parameters for all samples are good enough except for those of BaWO4:0.04Eu3+, implying that the diffraction data match well with the corresponding initial model. In Table 1, it can be clearly seen that cell B


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Inorganic Chemistry

Figure 2. Rietveld refinements of XRD profiles for (a) ZnWO4:0.04Eu3+, (b) CdWO4:0.04Eu3+, (c) CaWO4:0.04Eu3+, (d) SrWO4:0.04Eu3+, and (e) BaWO4:0.04Eu3+.

Table 1. Refined Lattice Parameters of MWO4:0.04Eu3+ (M = Zn, Cd, Ca, Sr, or Ba) sample composition 3+

ZnWO4:Eu CdWO4:Eu3+ CaWO4:Eu3+ SrWO4:Eu3+ BaWO4:Eu3+

space group

a (Å)

b (Å)

c (Å)

V (Å3)

Rwp

Rp

x2

P2/c (No. 13) P2/c (No. 13) I41/a (No. 88) I41/a (No. 88) I41/a (No. 88)

4.6980 5.0282 5.2504 5.4180 5.6115

5.7491 5.8684 5.2504 5.4180 5.6115

4.9497 5.0920 11.3897 11.9474 12.7316

133.683 150.193 313.976 350.716 400.904

2.52 6.06 2.26 2.99 10.90

2.08 4.69 1.76 2.36 8.29

2.20 2.75 2.19 1.12 18.36

double-tapter-like particles with an average diameter of 5 μm and a length of 30 μm are observed in Figure 4e. On the basis of the analysis presented above, it can be inferred that the crystal structure and composition have a significant influence on the growth of the alkaline earth tungstate microarchitecture. The different crystallographically structure gives rise to a distinctly different morphology. As shown in panels a and b of Figure 3, both ZnWO4 and CdWO4 particles possess similar anisotropic monoclinic cell structure,

leading to an anisotropic growth of particles, and thus, the rodlike particles are obtained. However, the tetragonal CaWO4:0.04Eu3+ seeds have an isotropic cell structure as shown in Figure 3c, inducing isotropic growth, so the morphology behaves like a sphere to minimize the surface energy of crystal facets. Compared to those of CaWO4:0.04Eu3+, the particles of SrWO4:0.04Eu3+ and BaWO4:0.04Eu3+ with the relative anisotropic crystal structure grow along crystallographically reactive directions, and thus, the C


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Inorganic Chemistry

Figure 3. Crystal structures of (a) ZnWO4, (b) CdWO4, (c) CaWO4, (d) SrWO4, and (e) BaWO4.

Coulombic attraction that results from the electronic density between the positive charge (M2+/Eu2+) and negative charge (WO42−), the first MWO4:0.04Eu3+ grain forms. However, the cation radius and Coulombic attraction differ in MWO4:0.04Eu3+; as a result, the grain size differs significantly, which further influences the number of grains. The cation possesses a relatively small ionic radius. The grain formed early in the process is smaller. The rate of migration increases. Thus, the nucleation rate, i.e., crystal number of grains, increases. Consequently, the particles with the smaller cationic radii exhibit a higher rate of migration because they have fewer blocks; thus, the grain size is smaller. 3.3. Luminescence Properties. Figure 5 shows the PLE and PL spectra of MWO4:0.04Eu3+ and relative emission intensity with M = Zn, Cd, Ca, Sr, or Ba. It can be seen that all the excitation spectra (blue profiles) monitored at 615 nm mainly contain a broad band (200−350 nm) and some sharp peaks (300−600 nm). Usually, the WO42− group shows a charge transfer band (CTB) ranging from 220 to 280 nm, which arises from the O 2p orbitals to the empty W 5d orbitals, such as a CTB band that peaks at 243 nm for the CaWO4 host.42 Meanwhile, the CTB of Eu3+-O2− is also located at 200−300 nm.14,18,43,44 Thus, the broad excitation bands should be derived from the CTB of both WO42− and Eu3+-O2−. On the other hand, the CTB shifts from 290 to 256 nm as the cation changes from Zn to Ba. The covalency between the central ion M and the ligands L is stronger, and the energy of electric transfer is lower.45 Meanwhile, the covalency of the M−L bond can be easily affected by the coordination numbers of the central ion (M), the M−L bond length, and the difference in electronegativity between ligands.18 The electronegativities for different M cations in MWO4 are listed in Table 2. Generally, less overlap between the ligands (L) and the central ion (M) caused by the

corresponding crystallites form. Consequently, the main factor that determines the dramatic change in morphology should be the cell structure.34,35 Furthermore, the evolution of the morphology depends strongly on the diverse cations as the crystal structure is fixed, and the difference in the growth rates of various crystal axes caused by the variation of cation will result in different crystal shapes. In general, facets perpendicular to the fast growth directions have smaller surface areas, and slow-growing faces therefore dominate the morphology. In this situation, the difference in cationic radius has a significant effect on the difference in the growth rates between varying crystallographic directions. The morphology of the crystallites strongly depends on the difference in the growth rates of various crystal facets.36,37 The size of grains increases significantly from 32 nm to 30 μm with an increase in cationic radius [Zn2+ (0.74 Å) < Cd2+ (0.95 Å) < Ca2+ (1.0 Å) < Sr2+ (1.18 Å) < Ba2+ (1.35 Å)] as shown in Figure 4f. These results reveal that the cationic radius should be responsible for the size of MWO4:0.04Eu3+ grains. Generally, the process of crystal nucleation determines the size of the particles by the equation L = (CV/8N)1/3, where L is the grain size, C is the concentration, V is the molecular volume, and N is the number of crystals.38 From this equation, when the concentration of samples is fixed, the number of grains has inevitable effects on the size of grains. In our experiment, the reaction process is the formation of small particles of MWO4:0.04Eu3+, which happens upon addition of a stoichiometric ratio of Eu(NO3)3, M(NO3)2·6H2O (99%), and Na2WO4·2H2O (99.5%). In this solution, the dissociation rate of inorganic salts is tremendously enhanced by the solvation energy, and M2+/Eu2+ and WO42− are rapidly dissolved by the surrounding H2O.39,40 M2+/Eu2+ and WO42− attach to the surfaces of H2O molecules because of their negative charge and partial positive charge, respectively.41 Because of the strong D


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Inorganic Chemistry

Figure 4. SEM images of (a) ZnWO4:0.04Eu3+, (b) CdWO4:0.04Eu3+, (c) CaWO4:0.04Eu3+, (d) SrWO4:0.04Eu3+, and (e) BaWO4:0.04Eu3+. (f) Comparison of the average grain sizes for crystalline powders.

emissions from the excited 5D0 level to ground states 7FJ (J = 0−4). The 5D0−7F2 emission at 615 nm from the electronic dipole transition is much stronger than the 5D0−7F1 emission at 593 nm that is from the magnetic dipole transition, indicating that Eu3+ ions occupy an asymmetry site in the MWO4 host. In addition, the ratios of the 5D0−7F2 emission to the 5D0−7F1 emission for different M cations are listed in Table 3. Compared to that of ZnWO4:0.04Eu3+, the ratio of the 5 D 0 − 7 F 2 emission to the 5 D 0 − 7 F 1 emission for CdWO4:0.04Eu3+ decreases, indicating that the symmetry increases with cationic radius for the wolframite-type monoclinic phase. However, it can be clearly inferred that symmetry decreases with an increase in cationic radius for the scheelite-type tetragonal phase. All samples exhibit an identical spectrum shape except for emission intensities. Figure 5f shows the dependence of integrated emission intensities on the cation. It can be seen that the emission intensity distinctly increases as the cationic radius increases from Zn to Cd for the monoclinic phase. However, it is contrary to the samples with the tetragonal phase, in which CaWO4:0.04Eu3+ exhibits optimal emission intensity. This phenomenon may be attributed to the morphology and defects of crystal particles. In general, the

longer M−L bond length as shown in Tables S1−S5 results in weaker M−L bond covalency. Furthermore, the larger difference in electronegativity between the ligands and the central ion indicates that the covalence of the M−L bond is weaker. For MWO4:0.04Eu3+ (M = Zn or Cd), when the coordination number of the central M is 6, the excitation spectra shift to a shorter wavelength with an increase in the cationic radius because of the greater difference in electronegativity and less overlap between the ligands and the central ion. The M sites in the MWO4 host tend to be replaced by Eu3+ due to their similar cationic radii. Therefore, the increase in the cationic radius results in weaker covalency, which makes the CTB shift to a shorter wavelength. Meanwhile, similar theories could explain the blue shift from Ca to Sr and Ba. The sharp lines beyond 300 nm could be ascribed to the 4f−4f electron transition of Eu3+, and the main peaks located around 393 and 464 nm are derived from the 7F0−5L6 and 7F0−5D2 transitions of Eu3+, respectively. For the PL spectra of MWO4:0.04Eu3+ in Figure 5 (pink profiles), all the emission spectra contain a broad band from the WO42− group and several sharp peaks from Eu3+. These peaks in the range of 550−710 nm are the characteristic Eu3+ E


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Inorganic Chemistry

Figure 5. PLE (blue profiles) and PL (pink profiles) spectra of MWO4:0.04Eu3+ with M = Zn (a), Cd (b), Ca (c), Sr (d), or Ba (e). (f) Dependence of emission intensity on cation.

more or less. As a consequence, it can be inferred that the luminescence properties of samples with various cations in this work may be attributed to the end effect of the size of particles and surface morphology, and the edges of the crystalline grains, a conclusion consistent with a previous report.49 3.4. Fluorescence Decays. Figure 6 shows the decay curves of MWO4:0.04Eu3+ (M = Zn, Cd, Ca, Sr, or Ba) monitored at 615 nm; it is obvious that all the curves fit the multiexponential decay model. In this situation, the average fluorescence lifetime can be estimated by the following formula:50−53

Table 2. Electronegativities of Different M Cations in MWO4 ZnWO4 CdWO4 CaWO4 catonic radius (Å) electronegativity of the M ion

0.74 1.65

0.95 1.69

1.00 1.00

SrWO4 BaWO4 1.18 0.95

1.35 0.89

Table 3. Ratios of 5D0−7F2 Emission to 5D0−7F1 Emission for Different M Cations ZnWO4 CdWO4 CaWO4 emission intensity at 593 nm emission intensity at 615 nm ratio

SrWO4 BaWO4

221

362

303

109

42

1392

2665

2811

686

114

0.16

0.14

0.11

0.16

0.37

τ=

∫0

∞

I (t ) d t

where I(t) represents the fluorescence intensity at time t with a normalized initial intensity. The lifetimes for M = Zn, Cd, Ca, Sr, and Ba phosphors are calculated to be 0.90, 0.73, 0.84, 0.70, and 0.85 ms, respectively. The SrWO4:0.04Eu3+ phosphor exhibits a lifetime shorter than those of others, which may be attributed to the excess surface defects. These defects offer the possibility of forming a

small particles with a large surface area introduce more defects into crystals. These defects provide nonradioactive pathways and result in the low emission intensity of the materials.46−48 Furthermore, the morphology, edges, apexes, and special junctions of crystal particles affect the luminescence properties F


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Inorganic Chemistry

WO42−. However, the energy level of Eu3+ has not changed in an obvious way. Therefore, the variation of CIE chromaticity coordinates from the border of the blue and red area (0.2632, 0.1796) to the blue region (0.2111, 0.1611) is ascribed to the difference in WO42− luminescence.

4. CONCLUSIONS In conclusion, cations with various cationic radii were introduced into MWO4:0.04Eu3+ (M = Zn, Cd, Ca, Sr, or Ba) to regulate the morphology and luminescence via a hydrothermal method. The crystal structure, morphology, luminescence properties, and decay lifetimes were studied in detail. For M = Zn or Cd, the samples exhibit a wolframite-type monoclinic phase, while for M = Ca, Sr, or Ba, the samples display a scheelite-type tetragonal phase. It is found that the crystal structure, composition, and growth rates of various crystal axes have a strong influence on the growth of the microarchitecture of the alkaline earth tungstate. Moreover, the cationic radius should be responsible for the size of MWO4:0.04Eu3+ grains. The CTB shifts to a shorter wavelength because of weaker covalency caused by an increase in cationic radius. The integrated emission intensity increases with cationic radius for MWO4:0.04Eu3+ (M = Zn or Cd) with a monoclinic phase, while the opposite is true for MWO4:0.04Eu3+ (M = Ca, Sr, or Ba) with a tetragonal phase, in which CaWO4:0.04Eu3+ exhibits the optimal emission intensity. The decay time has been examined in detail. This study gives us deeper insight into the underlying relationship among crystal structure, morphology, and luminescence properties. This study also is conducive to explaining the variation of luminescence caused by the crystal structure and inspires us to search for new tungstates.

Figure 6. Decay curves of MWO4:0.04Eu3+ (M = Zn, Cd, Ca, Sr, or Ba).

quenching center, increasing the level of nonradioactive relaxation of photons, and thus, the different morphology leads to a distinctly different decay lifetime. Generally, the Eu3+ ions near grain surfaces possess a short decay lifetime, whereas the Eu3+ ions at the core of the grains exhibit long decay lifetimes. On the basis of this theory, it can be easily inferred that the smaller grains with a large surface area make it more likely that Eu3+ ions exist on grain surfaces, resulting in the shorter lifetime. It can be seen that all samples are not consistent with most reports in which the materials with the strongest fluorescence exhibit the longest decay time. It is wellknown that the numbers of fluorescence centers, defects, energy transfer, and impurities have an inevitable influence on the decay of the kinetic behavior of the materials. However, the exact reason for the phenomenon is so complex that the process needs to be further studied systematically. 3.5. Color Coordinates. A better understanding of the trueness of color is very important for the applications of lighting and display devices. Figure 7 illustrates the CIE chromaticity coordinates of the as-prepared samples based on the PL spectra in Figure 5, and the luminescence consists of not only the emission of Eu3+ but also the characteristic emission of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02352. Selected bond distances of ZnWO4:0.04Eu3+ (Table S1), selected bond distances of CdWO4:0.04Eu3+ (Table S2), selected bond distances of CaWO4:0.04Eu3+ (Table S3), selected bond distances of SrWO4:0.04Eu3+ (Table S4), and selected bond distances of BaWO4:0.04Eu3+ (Table S5) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yongfu Liu: 0000-0002-9502-5897 Dunmin Lin: 0000-0002-2245-6717 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (NSFC 11404351), the Education Department of Sichuan Province (15ZA0037 and 15ZA0034), the Science and Technology Bureau of Sichuan Province (2016JY0225), and the large precision instrument

Figure 7. Representation of the CIE chromaticity coordinate diagram of MWO4:0.04Eu3+. G


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Inorganic Chemistry

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projects of Sichuan Normal University (DJ 2015-43, DJ 201642, and DJ 2016-52).

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