Co-precipitation synthesis, photoluminescence properties and theoretical calculations of MgWO₄:Eu³⁺ phosphors

Abstract

A novel red light emitting phosphorescent material MgWO₄:Eu³⁺ is successfully synthesized by the co-precipitation method. The structure, luminescent properties and fluorescence lifetime of the prepared phosphors are characterized using X-ray diffraction (XRD), photoluminescence and luminescence decay spectra. The results indicate that an effective electric dipole transition ⁵D₀–⁷F₂ of Eu³⁺ red emission with maxima at 615 nm can be achieved in the MgWO₄ host while being excited by near-UV light (393 nm) or blue light (464 nm). The CIE chromaticity coordinates of MgWO₄:Eu³⁺ phosphors exhibit emission in the red-light region. Moreover, to explain the fluorescence spectra of MgWO₄:Eu³⁺ phosphors, a complete energy matrix is successfully built by an effective operator Hamiltonian including free ion and crystal field interactions. The fluorescence spectra for the Eu³⁺ ion at the monoclinic (C_2v) Mg²⁺ site of the MgWO₄ crystal are calculated from this matrix. The fitting values are in agreement with the observed results.

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1. Introduction

Luminescent materials containing rare earth ions are able to absorb energy in the ultraviolet or blue regions and emit visible light. Recently, these materials have drawn increasing interest due to their potential applications in display devices, fluorescent labels, white light emitting diodes, etc.^1–3 Rare earth ions have been playing an important role in phosphors due to the abundant emission color based on their 5d–4f or 4f–4f transitions, such as the trivalent europium ion, which is a red emitting activator due to its ⁵D₀–⁷F_J (J = 0–6) transitions. The Eu³⁺ ion is expected to be one of the most promising species that provides optical devices in the red color region and many investigations have been carried out in various hosts. For example, Eu³⁺ activated tungstate or molybdate phosphors have excellent luminescent efficiency, color purity and stability.^4–6 They can be well excited by ultraviolet (393 nm) or visible light (464 nm) which is nicely in agreement with the widely applied near-UV or blue LED chips. Therefore, they have attracted much attention in recent years. Many investigations, such as those on Ba_1−xMoO₄:xEu³⁺,⁷ CaMoO₄:Eu,⁸ (Ba_1−xEu_x)WO₄,⁹ SiO₂@CaMoO₄:Eu³⁺,¹⁰ (Ca_1−x−yLu_y)MoO₄:xEu³⁺,¹¹ and (Ca_1−x−yLn_y)MoO₄:xEu³⁺ (Ln = Y, Gd),¹² etc., have been developed to compensate LED for the red deficiency of the output light. Comparison of two methods of preparation of the traditional solid-state and co-precipitation method, the solid-state method for preparing phosphors has a few disadvantages, such as high calcination temperature and large grain size of the samples (generally more than 1100 °C and 2 μm).^8,13 Compared with the conventional solid-state method, the co-precipitation method is a low-cost, controllable particle size and low energy consumption synthesis technique for preparing phosphors.^14,15 Thus, in order to improve the crystallite size and its distribution, Eu³⁺ doped MgWO₄ phosphors are fabricated by using co-precipitation method instead of the common solid-state reaction. Their structure, luminescent properties and fluorescence decay time are studied in detail. At the same time, taking into account that a fundamental theoretical understanding of the fluorescent mechanism for Eu³⁺ ion at the monoclinic (C_2v) Mg²⁺ site of MgWO₄ crystal has not been set up, the explanation for energy levels of MgWO₄:Eu³⁺ by crystal-field theory is necessary. Thus, in the present study, the fluorescent spectra for Eu³⁺ ion at the monoclinic (C_2v) Mg²⁺ site of MgWO₄ crystal are calculated by the complete diagonalization (of energy matrix) method. The calculated data are compared with the experimental results.

2. Experimental section

Firstly, Eu₂O₃ (99.99%), MgCl₂ (A.R.), Na₂WO₄ (A.R.), HCl (A.R.) aqueous solutions were prepared, respectively. Then according to the appropriate stoichiometric ratio, MgCl₂ and/or Eu₂O₃, HCl solutions were added sequentially into Na₂WO₄ solution under stirring to form a clear and homogeneous mixture. Thirdly, this mixture solution was neutralized by the precipitating aid agent into deposit precursor at 80 °C, wherein NH₃·H₂O and NH₄HCO₃ were taken as precipitating aid agent respectively. At last, the precursor was pre-fired at 400 °C for 2 h, and fired at 800 °C for 2 h to improve the crystallization and luminescent intensity of MgWO₄:Eu³⁺.

The XRD measure of phosphor samples was taken on a X-ray diffractometer (Shimadzu, XRD-7000S/L) with Cu Kα radiation (λ = 0.15406 nm) operating at 40 kV and 30 mA. The morphology and energy dispersive X-ray microanalysis (EDX) spectrum of the product were characterized using a scanning electron microscopy (SEM, MIRA3, Tescan). A fluorescence spectrometer (Hitachi, F-4600 type) was used to record the photoluminescence spectra of the samples. The decay time was characterized by using FLS-980 fluorescence spectrometer (Edinburgh Instruments). All measures were recorded at room temperature.

3. Results and discussions

3.1 Crystal structure

The results of XRD patterns for all the samples present similar profiles which are shown in Fig. 1(a). XRD examination of Mg_1−xWO₄:xEu³⁺ samples with x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05 reveals a monoclinic sanmartinite-type of structure. It is found that the peaks from XRD are the phase of MgWO₄ for low doping (below about x = 0.05). In Fig. 1(b) we present the EDX spectrum of a little rectangular region, which shows that the elements of the region are Mg, W, Eu and O. It is thus safe to conclude that the phosphor observed here comes from the phase Mg_0.96Eu_0.04WO₄. The nanocrystallite size can be estimated from the Scherrer formula,¹⁶ D = kλ/β cos θ, where k (0.89) is a constant and D is the average crystallite size, λ is the X-ray wavelength (0.15406 nm), β is the full width at half maximum (FWHM) of an observed peak and θ is the diffraction angle. By using the Scherrer formula to the FWHM of the (1̄11), (111), (001) and (110) diffraction peaks, the average crystallite sizes of MgWO₄:Eu³⁺ can be calculated as 49.1 nm.

Fig. 1 (a) XRD patterns of Mg_1−xWO₄:xEu³⁺ (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05). All peaks can be indexed by a monoclinic sanmartinite-type of structure, indicating that the phase here is Mg_1−xEu_xWO₄. (b) The EDX spectrum of Mg_0.96Eu_0.04WO₄. The inset in (b) shows a SEM picture. The little rectangle marks the position where we took the EDX spectrum.

3.2 SEM image of MgWO₄:Eu³⁺

The typical SEM image is shown in Fig. 2. From Fig. 2, it reveals that the surface morphology of the sample (4 mol% of Eu³⁺ doping case) is irregular. It is made of numerous tiny particles. These particles sizes of MgWO₄:Eu³⁺ are mainly in the range of 0.2–1 μm. It is worth noting that the particle size of the result of SEM is generally different from the crystallite size from the observation in XRD. One particle may be one or more crystallite grains of the aggregates. Thus, the sizes are different in SEM and XRD results.¹⁷

Fig. 2 SEM image of Mg_0.96Eu_0.04WO₄ phosphor.

3.3 Luminescence of the samples

The excitation (EX) spectra of MgWO₄:Eu³⁺ phosphor are shown in Fig. 3(a). It can be seen that the EX spectrum monitored under 615 nm compose of a wide band centered at 320 nm and several intense sharp lines. The broad band from 290–350 nm is attributed to the charge transfer (CT) transition of O²⁻ → W⁶⁺ and O²⁻ → Eu³⁺.²⁰ Several intense sharp lines in the range of 350–500 nm originate from the intra-configurational 4f → 4f transitions of Eu³⁺ ions in the host. The peaks centered at 393 nm and 464 nm are corresponding to the ⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂ transitions respectively. The absorption of ⁷F₀ → ⁵L₆ at 393 nm and ⁷F₀ → ⁵D₂ at 464 nm are strong, which is coupled well with the characteristic emission from near-UV and blue GaN-based LED chips and their intensities are much stronger than that of CT band. Other peaks locate at 361, 375, 381, 413, 455, 470 and 490 nm, corresponding to ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵L₇, ⁷F₀ → ⁵L₇, ⁷F₀ → ⁵D₃, ⁷F₀ → ⁵D₂, ⁷F₀ → ⁵D₂ and ⁷F₀ → ⁵D₁ transitions of Eu³⁺.

Fig. 3 Photoluminescence spectra of Mg_1−xWO₄:xEu³⁺ (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05): (a) the emission at λ_em = 615 nm; (b) the excitation at λ_ex = 393 nm, and (c) λ_ex = 464 nm.

The EM spectra of MgWO₄:Eu³⁺ phosphor under the excitation of 393 and 464 nm respectively are also shown in Fig. 3(b) and (c). All emission (EM) spectra consist of sharp lines with wavelengths ranging from 500 to 750 nm, which are associated with the ⁵D₀ → ⁷F_J (J = 1, 2, 3, and 4) transitions from the excited levels of Eu³⁺ to the ground state.^18,19 It can be observed from Fig. 3 that the intensities of samples increase with the increasing of the dopant concentrations and reach a maximum at 4 mol% Eu, which may be due to the improvement of the lattice sites of the activated centres of the phosphors. Then the EM intensities decrease significantly as the concentrations continuously increase from 4 mol% Eu, which is attributed to the concentration quenching. It indicates that the concentration of activators has an important influence on the lattice of phosphors.

There are no observed differences for the EM band shape and position under the excitation of 393 or 464 nm except for luminescent intensity. Under the excitation of near-UV light (∼393 nm) and blue-light irradiation (∼464 nm), the EM spectra is composed of groups of sharp lines which belong to the characteristic emission of Eu³⁺ ion. The main emission is at 615 nm, originating from the electric dipole transitions (⁵D₀ → ⁷F₂) of Eu³⁺, which reveals that Eu³⁺ ions occupy the lattice sites without inversion symmetry. Other f–f transitions of Eu³⁺ ion, such as 579, 594, 657 and 706 nm, which are associated with the ⁵D₀ → ⁷F_J (J = 0, 1, 3, 4) transitions from the excited levels of Eu³⁺ to the ground state, are relatively weak. This indicates that it is advantageous to obtain a phosphor with good Commission Internationale de l'Eclairage (CIE) chromaticity coordinates. From the EM spectral data of Fig. 3, the average color coordinates (x ≈ 0.439, y ≈ 0.303) of MgWO₄:Eu³⁺ are obtained as you can see in Fig. 4 and they are in the red region. Therefore, the phosphors MgWO₄:Eu³⁺ can be effectively excited by near-UV and blue LED, and emit red light.

Fig. 4 Commission Internationale de l'Eclairage (CIE) average chromaticity diagram for MgWO₄:Eu³⁺ phosphors.

3.4 The decay curves

The typical fluorescence decay pattern of Mg_0.96WO₄:Eu_0.04³⁺ is shown in Fig. 5. The decay curve is nonexponential and well fitted into a two-exponential function^21,22

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (1)

in which τ₁ and τ₂ are time constants and A₁ and A₂ are coefficients. The average lifetime can be calculated by using the formula

Fig. 5 Fluorescence decay time of 615 nm emission of Mg_0.96WO₄:Eu_0.04³⁺ under excitation into the ⁵D₀ at 464 nm.

As can be seen, the average lifetime in Mg_0.96WO₄:Eu_0.04³⁺ is 718.16 μs, which is caused by the nonradiative relaxation channels and reduction of the ⁵D₀ lifetime in the Mg_0.96WO₄:Eu_0.04³⁺ phosphor.

3.5 Crystal-field calculations

The effective operator Hamiltonian for rare-earth ions in crystals with C_2v site symmetry can be written as^23–27

H = H_{free ion} + H_CF(C_2v) (3)

in which the free ion Hamiltonian can be expressed as^25–28

The physical meaning of these free ion parameters has been described by Crosswhite and Carnall et al.^26,27 The crystal field Hamiltonian with C_2v site symmetry can be written, in the Wybourne notation, as^28,29

H_CF(C_2v) = B₂₀C₂₀ + B₂₀(C_2,−2 + C_2,2) + B₄₀C₄₀ + B₄₂(C_4,−2 + C_4,2) + B₄₄(C_4,−4 + C_4,4) + B₆₀C₆₀ + B₆₂(C_6,−2 + C_6,2) + B₆₄(C_6,−4 + C_6,4) + B₆₆(C_6,−6 + C_6,6) (5)

in which B_kq are the crystal-field parameters, and C_kq are the Racah spheric tensor operators. A complete energy matrix is constructed from the effective operator Hamiltonian and then the energy-level calculations of Eu³⁺ ion in MgWO₄ crystal are carried out by means of this matrix. In the calculations, the free-ion parameters in the energy matrix are taken as follows,³⁰ E_av ≈ 63 786 cm⁻¹, the Coulomb repulsions F₂ ≈ 82 786 cm⁻¹, F₄ ≈ 59 401 cm⁻¹, F₆ ≈ 42 644 cm⁻¹, the spin–orbit coupling parameter ζ_4f ≈ 1332 cm⁻¹, the two-body and three-body interaction parameters α ≈ 19.8 cm⁻¹, β ≈ −617 cm⁻¹, γ ≈ 1460 m⁻¹, T₂ ≈ 370 cm⁻¹, T₃ ≈ 40 cm⁻¹, T₄ ≈ 40 cm⁻¹, T₆ ≈ −330 cm⁻¹, T₇ ≈ 380 cm⁻¹, T₈ ≈ 370 cm⁻¹, the Marvin integrals M₀ ≈ 2.38 cm⁻¹, M₂ ≈ 1.33 cm⁻¹, M₄ ≈ 0.90 cm⁻¹, the parameters related to the electrostatic correlated magnetic interaction P₂ ≈ 303 cm⁻¹, P₄ ≈ 227 cm⁻¹, P₆ ≈ 152 cm⁻¹. Thus, in the above complete energy matrix, only the crystal-field parameters are unknown. These parameters are often taken as adjustable data because they depend upon the central metal ion, the ligands and the nature of metal–ligand bonds. By fitting the calculated optical absorption spectra to the observed values of the Eu³⁺ center in MgWO₄ crystal, we get

B₂₀ ≈ 315.20 cm⁻¹, B₂₂ ≈ 780.02 cm⁻¹, B₄₀ ≈ 1109.01 cm⁻¹, B₄₂ ≈ 2072.65 cm⁻¹,

B₄₄ ≈ −1443.31 cm⁻¹, B₆₀ ≈ −605.44 cm⁻¹, B₆₂ ≈ −468.13 cm⁻¹, B₆₄ ≈ −1824.14 cm⁻¹, B₆₆ ≈ 189.16 cm⁻¹ (6)

The comparisons between the calculated optical spectral data and the experimental values are listed in Table 1. Note that only a few low-lying multiplets are given in Table 1.

Table 1 Experimental and calculated optical spectra (cm⁻¹) as well as the error (%) of Eu³⁺ at the Mg²⁺ site of MgWO₄ phosphors

Mult.	Calc.	Expt	Error^a
a Error = |Ecalc. − Eexpt|/Eexpt.
^7F0	0	0	0
^7F1	216
	423
	430	436	1.38
^7F2	770
	830
	1045	1011	3.36
	1094
	1604
^7F3	1752
	1830
	2024
	2041	2050	0.44
	2078
	2096
	2129
^7F4	2505
	2714
	2933	2944	0.37
	2969
	3056
	3116	3107	0.29
	3151
	3184
	3508
^7F5	3695
	3710
	3922
	3945
	3948
	3996
	4023
	4168
	4539
	4615
	4657
^7F6	4892
	4903
	5110
	5141
	5272
	5376
	5376
	5384
	5518
	5545
	5604
	5860
	5860
^5D0	17 287
^5D1	19 008
	19 063
	19 072	20 408	6.54
^5D2	21 425	21 277	0.70
	21 478
	21 523	21 552	0.34
	21 549
	21 584	21 978	1.79
^5D3	24 243	24 213	0.12
	24 295
	24 343
	24 345
	24 361
	24 371
	24 388
^5L6	24 539
	24 597
	24 780
	24 803
	24 921
	25 023
	25 099
	25 146
	25 350
	25 383
	25 497	25 445	0.20
	25 550
	25 552
^5L7	26 142
	26 157
	26 188
	26 245	26 248	0.01
	26 252
	26 262
	26 282
	26 302
	26 359
	26 360
	26 405
	26 506
	26 510
	26 536
	26 689	26 667	0.08
^5D4	27 671
	27 673
	27 695	27 701	0.02

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4. Conclusions

A series of Eu³⁺-activated MgWO₄ phosphors are synthesized with the co-precipitation method. The synthesized phosphors can be efficiently excited by 393 and 464 nm which are well matched with the emission of near-UV LED and blue LED chips. The EM intensity increases with the increasing of Eu³⁺ doping concentration but concentration quenching occurs when the Eu concentration in MgWO₄:Eu³⁺ is higher than 4 mol%.

In terms of those suitable crystal-field parameters, the calculated optical spectral data show excellent agreement with the experimental values (see Table 1). The disparities (error in Table 1) between calculated and experimented spectral values are so small, which suggests that the optical spectra can be satisfactorily explained by the diagonalization (of energy matrix) method for Eu³⁺ ion at C_2v point symmetry. In addition, the good agreement between theory and experiment has also proved that the Eu³⁺ replacing Mg²⁺ site with C_2v symmetry in MgWO₄ crystal is reasonable and this is also consistent with the present experimental results.

Acknowledgements

This work was supported by the National Science Foundation of China (Grant no. 51574054), the Scientific Research Foundation for Returned Scholars, MHRSS, China (Grant no. 2015-157), and the Chongqing Research Program of Basic Research and Frontier Technology (Grant no. cstc2015jcyjA50034).

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