Energy transfer and multicolor emission in single-phase Na₅Ln(WO₄)_4−z(MoO₄)_z:Tb³⁺,Eu³⁺ (Ln = La, Y, Gd) phosphors

Abstract

A series of Tb³⁺ and/or Eu³⁺-doped Na₅Ln(WO₄)_4−z(MoO₄)_z (0 ≤ z ≤ 4, Ln = La, Y, Gd) phosphors have been prepared via a simple Pechini method. X-ray diffraction (XRD) profile analyses have verified the formation of a single phase with scheelite-like structure. The scheelite-like tetragonal structure provides the Na₅Ln(WO₄)₄ hosts the possibility of doping with Tb³⁺/Eu³⁺ ions without substantial luminescence quenching. Under near UV light excitation, Tb³⁺ and Eu³⁺-doped Na₅Ln(WO₄)₄ phosphors exhibit characteristic emissions of Tb³⁺ (⁵D_3,4 → ⁷F_J) and Eu³⁺ (⁵D₀ → ⁷F_J), respectively. By codoping the Tb³⁺ and Eu³⁺ ions into the Na₅Ln(WO₄)₄ host and tuning their relative concentration ratio, multicolor tunable emissions are obtained by varying the ratio of Tb³⁺/Eu³⁺. The energy transfer from Tb³⁺ to Eu³⁺ in Na₅Ln(WO₄)₄ has been investigated by photoluminescence and decay measurements. The influence of W substitution by Mo on the luminescence properties of Na₅Ln(WO₄)_4−z(MoO₄)_z:Tb³⁺/Eu³⁺ has also been investigated. Both the emission intensities of Tb³⁺ and Eu³⁺ ions were found to reach a maximum when the molar ratio of W/Mo was 2 : 2. The results suggest that Na₅Ln(WO₄)_4−z(MoO₄)_z:Tb³⁺/Eu³⁺ phosphors might find potential applications in the field of solid state lighting and field emission displays.

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Introduction

Recently, the demand for novel red-emitting phosphors, especially that absorb in the near UV (350–400 nm), is gaining ever increasing importance.¹ As one of the most frequently used red emitters in rare earth ion-doped materials, Eu³⁺ mainly presents high efficiency characteristic emissions related to the large energy gap between the emitting state ⁵D_0,1,2 and the excited states ⁷F_J (J = 1, 2, 3, 4, 5) and offers an intense red composition. However, Eu³⁺-activated phosphors have narrow-line-shaped excitation peaks in the near UV region, which are not desirable for near UV LED application. In order to broaden the Eu³⁺ excitation band and improve the absorption efficiency, codoping Tb³⁺ with Eu³⁺ has been successfully employed in various hosts.^2–5 Also, the Tb³⁺ ion-doped materials are often used as green phosphors due to the ⁵D₄ → ⁷F_J (J = 6, 5, 4, and 3) transitions of Tb³⁺ ions. Therefore, it is hoped that dual emission might be realized in Eu³⁺/Tb³⁺ co-doped phosphors and the emission color is tuneable by adjusting the ratio of Tb³⁺ and Eu³⁺ ions.

Alkali metal rare-earth molybdate and tungstate compounds of the general formula, A₅Ln(MO₄)₄ (A = alkali metals; Ln = trivalent rare earth ion; M = Mo, W), have been reported to crystallize in the tetragonal crystal system with a scheelite-like structure.⁶ A₅Ln(MO₄)₄ has been regarded as one class of hosts with high quenching concentration for high luminescence output, low sintering temperature, high efficiency matrix.^7–9 These compounds containing tetrahedral anions such as MoO₄²⁻ and WO₄²⁻ were reported to be excellent phosphor hosts due to their high thermal and chemical stability, structural diversity, and capacity of hosting rare earth. The doping content of rare earth can be up to nearly 100%, in other words, the sites of Ln (Ln = La, Y, Gd) can be completely replaced, which means that this kind of matrix is unquenched. The reason is that the bond angles of O–M–O and Eu–O–M in the A₅Ln(MO₄)₄ structure are 105 and 100°, respectively, resulting in the long distance between the sensitizer ions, which hinders the energy transfer occur between rare earth ions. Recently, Eu³⁺ activated A₅Ln(WO₄)₄ phosphors with scheelite structure have gained much attention due to their good optical properties, high chemical stability, and a relatively simple preparation.^7–10 However, the luminescence properties of Tb³⁺ doped A₅Ln(WO₄)₄ have received less attention, especial for Tb³⁺/Eu³⁺ codoped A₅Ln(WO₄)₄. To the best of our knowledge, the tunable luminescence properties of Na₅Ln(WO₄)₄:Tb³⁺,Eu³⁺ phosphors have not yet been reported in the literature. Here, we synthesized Tb³⁺/Eu³⁺ singly doped or codoped Na₅Ln(WO₄)₄ phosphors, and investigated their luminescence and energy transfer properties in detail. We found that their emission color can be tuned from green to yellow and red by increasing the ratio of Eu³⁺/Tb³⁺. Energy transfer mechanism between Tb³⁺ and Eu³⁺ ions in Na₅Ln(WO₄)₄ hosts was discussed. What is more, the similar ionic radii of tetrahedral Mo⁶⁺ (0.41 Å) and W⁶⁺ (0.42 Å)¹¹ may make it possible to prepare solid solutions of the scheelite-type Na₅Ln(WO₄)_4−z(MoO₄)_z (0 ≤ z ≤ 4). Hence, we can expect that the progressive substitution of W in Na₅Ln(WO₄)₄ hosts by Mo leads to change in the luminescence property. With this in view, a series of Na₅La(WO₄)_4−z(MoO₄)_z:Tb³⁺/Eu³⁺ phosphors were synthesized and the influence of the W/Mo ratio on the structure and luminescent properties have also been studied.

Experimental

Samples with the composition of Na₅Ln_1−x−yTb_xEu_y(WO₄)_4−z(MoO₄)_z (0 ≤ x ≤ 1; 0 ≤ y ≤ 1; 0 ≤ z ≤ 4; Ln = La, Y, Gd) were synthesized via a simple Pechini method as follows. NaNO₃ (Analytical Reagent, AR), H₄₀N₁₀O₄₁W₁₂·xH₂O (AR), (NH₄)₆Mo₇O₂₄·4H₂O (AR), La₂O₃ (99.99%), Y₂O₃ (99.99%), Gd₂O₃ (99.99%), Tb₄O₇ (99.99%), and Eu₂O₃ (99.99%) were used as the starting materials and were weighed in a proper stoichiometric ratio with an excess of 10% mole of W or Mo compensating for the evaporation loss. Rare earth oxides were dissolved in HNO₃. These reagents were dissolved in deionized water, and an appropriate amount of CO(NH₂)₂ (AR) was added as fuel. The solutions were introduced into a muffle furnace maintained at 500 °C for 10 min. The as obtained mixed solid powders were ground thoroughly and reheated at various temperatures for 4 h in air. After firing, the samples were cooled to room temperature and reground for further measurements.

Powder X-ray diffraction (XRD) measurement was performed on a Bruker D8 X-ray diffractometer (Bruker Co. Ltd., Karlsruhe, Germany) to check the phase purity of the as-prepared samples. The photoluminescence emission (PL) and excitation (PLE) spectra were recorded using a fluorescence spectrophotometer (F-7000, Hitachi, Japan) equipped with a 150 W xenon light source. The decay curves were collected on an Edinburgh FLS920 combined fluorescence lifetime and steady state spectrometer (Edinburgh Instruments Ltd, Edinburgh, England) with a 450 W xenon lamp and 60 μF flash lamp, respectively. The quantum yield (QY) was measured using the integrating sphere on the FLS920 fluorescence spectrophotometer. For comparison, all measurements were carried out at room temperature with the identical instrumental parameters.

Results and discussion

Crystal structure

The most suitable sintering temperature for the synthesis of single phase crystalline powders was investigated. Fig. 1a presents the XRD patterns of the sample Na₅Tb_0.5Eu_0.5(WO₄)₄ sintered at different temperatures. The increase in the sintering temperature from 550 to 800 °C resulted in some differences of XRD intensity, which indicated that the sintering temperature can bring about significant influence on the crystallization of the samples. As shown in Fig. 1a, the samples obtained at 550, 600, 650, and 700 °C show the very similar XRD patterns. All the diffractions can be well indexed to pure tetragonal phase Na₅La(WO₄)₄ (JCPDS no. 72-2157). With the increase in the sintering temperature, the intensity of the diffraction peaks increases. The intensity of peak at 29.13° of 700 °C is about 1.5 times that of 550 °C. The stronger diffraction peaks exist by virtue of the higher crystallinity. However, extra diffraction peaks centered at around 16.78°, 27.59°, and 32.50° form the impurity phases, which can be identified as Na₂WO₄, appear after annealing at 750 and 800 °C. Moreover, the XRD peak intensity of the tetragonal phase decreases as the sintering temperature was increased from 700 to 800 °C, while the intensities of the peaks for the Na₂WO₄ phase increase. This implies that the Na₂WO₄ crystalline phase formation may result from the dissolution of the Na₅Tb_0.5Eu_0.5(WO₄)₄ crystalline phase, which is similar to that reported in the hosts such as Na₅Ln(MoO₄)₄ (Ln = La, Y, Gd).^7,10 We can conclude that the 700 °C is enough for the crystallization of as-prepared samples.

Fig. 1 (a) XRD patterns of Na₅Tb_0.5Eu_0.5(WO₄)₄ sintered at different temperature; (b) comparison of XRD profiles of typical Na₅Ln_1−x−yTb_xEu_y(WO₄)_4−z(MoO₄)_z (Ln = La, Y, Gd) samples and the evolution on reflections near 2θ = 29°.

Fig. 1b presents the XRD patterns of Na₅Eu(WO₄)₄, Na₅Tb(WO₄)₄, Na₅Tb_0.5Eu_0.5(WO₄)₄, Na₅La_0.4Tb_0.5Eu_0.1(WO₄)₂(MoO₄)₂ and Na₅Ln_0.7Tb_0.2Eu_0.1(WO₄)₄ (Ln = La, Y, Gd) samples obtained after sintering at 700 °C. These samples show the very similar XRD patterns. Obviously, all the diffraction peaks can be well assigned to the corresponding standard data for the tetragonal phase of Na₅La(WO₄)₄, indicating no phase transformation or structural variation occurs in the host upon Eu/Tb doping or W substitution by Mo. The strong and sharp diffraction peaks of the samples indicate that the as-obtained samples are well crystallized. The results show that different rare earth concentrations do not result in any other phase except for the main phase. The panel on the right in Fig. 1b shows the reflection peak near 29° (2θ). We find that the characteristic diffraction peak (132) of Na₅Gd_0.7Tb_0.2Eu_0.1(WO₄)₄ and Na₅Y_0.7Tb_0.2Eu_0.1(WO₄)₄ shifts obviously to higher angles with that of Na₅La_0.7Tb_0.2Eu_0.1(WO₄)₄. The effective ionic radii for eight-coordinated La³⁺, Y³⁺, Gd³⁺, Eu³⁺, and Tb³⁺ are 1.16, 1.019, 1.053, 1.066, and 1.04 Å,¹¹ respectively. According to Bragg's law, 2d sin θ = nλ, where n is an integer, λ is the wavelength of X-ray, d is the spacing between the planes in the atomic lattice, and θ is the angle between the incident ray and the scattering planes. Shrinkage of the lattice volume (d) will lead to the increase of the 2θ value. As shown in Fig. 1b, the 2θ value of the (132) peak is 28.78° for the Na₅La_0.7Tb_0.2Eu_0.1(WO₄)₄ sample. When La³⁺ is replaced by the smaller Gd³⁺ and Y³⁺ ions, the volume shrinks and the 2θ value increases to 29.11° and 29.25°, respectively. In addition, the 2θ value of the (132) increases from 29.06° (x = 0) to 29.07° (x = 0.5) and further to 29.12° (x = 1) for Na₅Eu_1−xTb_x(WO₄)₄ samples. This variation could also be due to the different ionic radii for Eu³⁺ and Tb³⁺. The Na₅La_0.4Tb_0.5Eu_0.1(WO₄)₂(MoO₄)₂ sample is still single phase with good crystalline nature, suggesting that Na₅Ln(WO₄)_4−z(MoO₄)_z samples with different W/Mo ratios can form a single phase due to the almost identical ionic radius of Mo⁶⁺ (0.41 Å) and W⁶⁺ (0.42 Å). Therefore, the as-prepared Na₅Ln(WO₄)_4−z(MoO₄)_z (Ln = La, Y, Gd, Tb, Eu; 0 ≤ z ≤ 4) phosphors share the same scheelite structure with the space group I4₁/a.

Fig. 2 depicts the crystal structure of tetragonal Na₅Ln(WO₄)₄ (along the c axis), as well as the coordination condition of metal cations. There are two nonequivalent Na⁺ lattice sites. One Na⁺ lattice sites (Na1) is coordinated by four O²⁻ ions. The other one (Na2) is six-fold coordinated by six O²⁻ ions. The W⁶⁺ ions are surrounded by four oxygen ions and form WO₄ tetrahedron. The Ln³⁺ lattice sites, isolated from each other, are eight-coordinated to O²⁻ ions to form LnO₈ dodecahedron. In view of the similar ion radius and identical valence, the Eu³⁺/Tb³⁺ and Mo⁶⁺ ions are expected to occupy the crystallographic sites of Ln³⁺ and W⁶⁺ ions, respectively, in the Na₅Ln(WO₄)₄ crystal host.

Fig. 2 The crystal structure of Na₅Ln(WO₄)₄ (Ln = La, Y, Gd) (along the c axis), as well as the coordination condition of metal cations.

Luminescence properties of Na₅Ln(WO₄)₄:Eu³⁺ and Na₅Ln(WO₄)₄:Tb³⁺ (Ln = La, Y, Gd)

The photoluminescence excitation (PLE) and emission (PL) spectra of Na₅Tb(WO₄)₄ and Na₅Eu(WO₄)₄ phosphors are shown in Fig. 3. The PLE spectrum (Fig. 3a, left) of Na₅Tb(WO₄)₄ was recorded by monitored the green emission of Tb^{3+ 5}D₄ → ⁷F₅ transition at 547 nm. The PLE spectrum consists of a strong and broad excitation band centred at 250 nm in the range of 200–300 nm, which corresponds to the charge transfer bands (CTB) of O²⁻ → W⁶⁺ transitions,¹² and spin-allowed 4f⁸ → 4f⁷5d¹ transitions of Tb³⁺ ions. In the range from 300 to 380 nm, Na₅Tb(WO₄)₄ sample shows the characteristic f → f transitions of Tb³⁺: ⁷F₆ → ⁵D₂ transition for 353 nm, ⁷F₆ → ⁵L₁₀ transition for 360 nm, ⁷F₆ → ⁵G₅ transition for 371 nm, and ⁷F₆ → ⁵G₆ transition for 378 nm.¹³ Excited at 378 nm, the emission spectrum (Fig. 3a, right) shows typical emission lines of Tb³⁺ ion in the range of 480–700 nm. The emission lines are located at 491, 547, 585, and 624 nm, which are ascribed to the ⁵D₄ → ⁷F_J (J = 6, 5, 4, and 3) transitions of Tb³⁺ ions, respectively. The green emission of ⁵D₄ → ⁷F₅ transition was found to be much stronger than that of ⁵D₄ → ⁷F_6,4,3. The energy absorbed by the WO₄²⁻ group is transferred to Tb³⁺ levels nonradiatively, which is known as “host-sensitized”.¹⁴ Although the intensity of the CTBs is much stronger than that of Tb^{3+ 7}F₆ → ⁵G₆ transition, the characteristic emission of WO₄²⁻ is quenched completely and only the green-light emission of Tb³⁺ ions appears, revealing that the energy transfer from the WO₄²⁻ group to Tb³⁺ is quite efficient in Na₅Tb(WO₄)₄.

Fig. 3 The PL excitation and emission spectra of the as prepared Na₅Tb(WO₄)₄ (a) and Na₅Eu(WO₄)₄ (b).

Monitored at 616 nm, the PLE spectrum of (Fig. 3b, left) of Na₅Eu(WO₄)₄ comprises of a broad band at 220–300 nm and some sharp lines between 360 nm and 480 nm. The weak broad excitation band exhibits two absorption peaks centred at 228 and 260 nm, respectively. The excitation band peaked at about 228 nm is ascribed to a charge-transfer band (CTB) of O²⁻ → W⁶⁺ transition within the WO₄²⁻ group. The excitation band peaked at longer wavelength (260 nm) correspond to the CTB of O²⁻ → Eu³⁺ transition. This phenomenon is similar to CTBs of O²⁻ → W⁶⁺ and O²⁻ → Eu³⁺ transitions in Eu₂(WO₄)₃ (ref. 15) and (Ca,Eu)WO₄.¹⁶ In the spectral region from 360 to 480 nm, the PLE spectrum shows characteristic intra-configurational 4f → 4f transition of Eu³⁺: 364 nm (⁵F₀ → ⁵D₄), 384 nm (⁵F₀ → ⁵L₇), 394 nm (⁵F₀ → ⁵L₆), 418 nm (⁵F₀ → ⁵D₃), and 465 nm (⁵F₀ → ⁵D₂). The emission spectrum (Fig. 3b, right) consists of typical 4f levels specific transitions of Eu³⁺, that is, ⁵D₀ → ⁵F_J (J = 0, 1, 2, 3, 4), which is ascribed to the emission peaks at 537, 592, 616, 655, and 704 nm, respectively. As well known, the emission at 593 nm is due to the ⁵D₀ → ⁷F₁ magnetic dipole transition of the Eu³⁺ ions, which is insensitive to the site symmetry. By contrast, the intensity of the ⁵D₀ → ⁷F₂ transition (616 nm) is strongly dependent on the surroundings of the Eu³⁺ ion because of its magnetic dipole character. Thus, the R/O ratio (emission intensity ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁) strongly depends on the local symmetry of the Eu³⁺ ions in host lattice. The R/O ratio was calculated to be about 8.5, indicating that Eu³⁺ occupied the lattice site of noncentrosymmetric environment in the scheelite phases. Although no emission corresponding to tungstates is observed, the presence of CTB resulting from oxygen to tungstates in Fig. 3b has indicated that the energy absorbed by the WO₄²⁻ group is transferred to Eu³⁺ levels nonradiatively, which is also known as “host-sensitized”.¹⁴

Since the properties are similar for La, Y, Gd, only the optical properties of Na₅La_1−xTb_x(WO₄)₄ (0 ≤ x ≤ 1) and Na₅La_1−yEu_y(WO₄)₄ (0 ≤ y ≤ 1) will be discussed in detail as a representative when Tb³⁺ or Eu³⁺ is doped in Na₅Ln(WO₄)₄ host. Fig. 4a presents the PL spectra of Na₅La_1−xTb_x(WO₄)₄ with different Tb³⁺ doping concentrations (x). Under 378 nm UV excitation, the PL spectra of the as-prepared Na₅La_1−xTb_x(WO₄)₄ phosphors consist of the ⁵D_3,4 → ⁷F_J transitions of Tb³⁺ ions. The emission peaks at 414 and 438 nm are attributed to the ⁵D₃ → ⁷F_5,4 transitions, while the emission peaks between 490 and 630 nm are ascribed to the ⁵D₄ → ⁷F_6,5,4,3 transitions. The PL spectra exhibit various ratios between the ⁵D₃ and ⁵D₄ emissions with changing Tb³⁺ concentrations (x). The emission intensity of ⁵D₃ is continuously reduced, and the emission from the ⁵D₄ level becomes dominant with increasing the Tb³⁺ concentration. The quenching of the emission from the ⁵D₃ level is due to the cross-relaxation via the resonant energy transfer process: Tb³⁺ (⁵D₃) + Tb³⁺ (⁷F₆) → Tb³⁺ (⁵D₄) + Tb³⁺ (⁷F₀). The cross-relaxation effect becomes stronger with higher doping concentration (x), and the emission of ⁵D₃ → ⁷F_J transitions quenches, resulting in the enhancement of the emission from the ⁵D₄ to the ⁷F_J levels.¹⁷ Fig. 4b exhibits the PL spectra of Na₅La_1−yEu_y(WO₄)₄ obtained by exciting with 394 nm light. The increasing concentrations of the Eu³⁺ ion bring no alteration in the shape of the PL spectra of the samples. No additional peaks or shifts in peak positions are observed, only the relative intensity changes.

Fig. 4 The PL emission spectra of Na₅La_1−xTb_x(WO₄)₄ (a) and Na₅La_1−yEu_y(WO₄)₄ (b) samples under UV excitation of 378 and 394 nm, respectively.

The dependences of the normalized PL intensities (⁵D₄ → ⁷F₅ transition of Tb³⁺, λ_ex = 378 nm; ⁵D₀ → ⁷F₂ transition of Eu³⁺, λ_ex = 394 nm) on the Tb³⁺ and Eu³⁺ concentrations are shown in Fig. 5a and b, respectively. The emission intensities of Na₅Ln_0.8Eu_0.2(WO₄)₄ samples increase in the order of I_La < I_Gd < I_Y under the excitation of 394 nm. Considering the ionic radii of Y³⁺, Gd³⁺, La³⁺, and Eu³⁺,¹⁰ the ionic radii of Gd³⁺ and Eu³⁺ are closer than those of Y³⁺/La³⁺ and Eu³⁺, which induces a weaker local distortion in the lattice. The larger degree of disorder and the lower local symmetry of Eu³⁺ ions lead to an enhancement of the ⁵D₀ → ⁷F₂ transitions.¹⁸ This is the reason that the Eu³⁺ luminescence in Na₅Gd_0.8Eu_0.2(WO₄)₄ is weaker than that in Na₅Y_0.8Eu_0.2(WO₄)₄. The ionic radii increase in the order of Y³⁺ < Eu³⁺ < La³⁺, thus the Eu³⁺ ions have a stiff lattice environment in Na₅Y_0.8Eu_0.2(WO₄)₄, but a loose one in Na₅La_0.8Eu_0.2(WO₄)₄. The Eu³⁺ ion is strongly restricted by the surrounding in Na₅Y(WO₄)₄ in comparison with that of Eu³⁺ in Na₅La(WO₄)₄. More nonradiative relaxation occurs in Na₅La(WO₄)₄ than that in Na₅Y(WO₄)₄, thus the intensity of Na₅Y(WO₄)₄ is higher than that of Na₅La(WO₄)₄.¹⁹ In the samples with high Eu³⁺ ion doping concentration, the effect of ionic radii difference on the Eu³⁺ luminescence becomes unobvious.

Fig. 5 The concentration dependence of the peak intensity ⁵D₄ → ⁷F₅ of Tb³⁺ (a) and ⁵D₀ → ⁷F₂ of Eu³⁺ (b) in Na₅Ln(WO₄)₄ (Ln = La, Y, Gd).

With increasing the doping content, the variation trend of emission intensity for Tb³⁺ or Eu³⁺ is similar. The emission intensity of Tb³⁺ or Eu³⁺ increases gradually with its concentration, and reaches a maximum at which the Ln³⁺ sites are replaced by Tb³⁺ or Eu³⁺ completely. This observation confirms that the concentration quenching of Tb³⁺ or Eu³⁺ never occurs in Na₅Ln(WO₄)₄ host, so highly doping concentration samples are performed here. Usually, concentration quenching occurs because of pairing or aggregation of the identical luminescent centers at high concentrations, which lead to a small average distance and efficient resonant energy transfer between Eu³⁺–Eu³⁺ or Tb³⁺–Tb³⁺ ion pairs. In the compounds Na₅Tb(WO₄)₄ and Na₅Eu(WO₄)₄, the bond angles of O–W–O and Eu/Tb–O–W are 105° and 100°,²⁰ respectively, resulting in the long distance between Eu³⁺–Eu³⁺ or Tb³⁺–Tb³⁺ ion pairs, which blocks energy transfer between ion pairs. Thus, the special scheelite-like tetragonal structure provides these Na₅Ln(WO₄)₄ hosts for the possibility of doping with a high concentration of Tb³⁺/Eu³⁺ ions without concentration quenching.

Multicolor tunable luminescence of Na₅Ln(WO₄)₄:Eu³⁺/Tb³⁺ (Ln = La, Y, Gd)

Tb³⁺ ion can be used as green-emitting activator in commercial phosphors due to its predominant ⁵D₄ → ⁷F₅ transition, while the Eu³⁺ ion can be used as an activator of red emitting luminescent materials because the emission of the Eu³⁺ ion usually consists of lines in the red spectral area due to the ⁵D₀ → 7F_J (J = 0, 1, 2, 3, and 4) transitions. In order to realize the multicolor tunable luminescence, Tb³⁺ and Eu³⁺ ions co-doped Na₅Ln(WO₄)₄ (Ln = La, Y, Gd) phosphors were also prepared. Fig. 6 illustrates the PL spectra of Tb³⁺–Eu³⁺ doubly doped Na₅La(WO₄)₄. Excited at 378 nm (⁷F₆ → ⁵G₆ of Tb³⁺), the characteristic sharp line emissions of Tb³⁺ and Eu³⁺ are both observed. Fig. 6a displays the variation of the PL spectra of Tb³⁺ and Eu³⁺ in the Na₅La_0.5−yTb_0.5Eu_y(WO₄)₄ system with changing Eu³⁺ doping content (y). Here the Tb³⁺ content is fixed at 0.5. As the Eu³⁺ content increases, the PL intensity of Eu³⁺ increases at the expense of that of the Tb³⁺ ions. Therefore, we can speculate that the energy transfer from Tb³⁺ to Eu³⁺ occurs in the Na₅La(WO₄)₄ host.

Fig. 6 The PL spectra of the phosphors Na₅La_0.5−yTb_0.5Eu_y(WO₄)₄ (y = 0–0.5) (a) and Na₅La_0.9−xTb_xEu_0.1(WO₄)₄ (x = 0–0.9) (b) under 378 nm excitation.

Fig. 6b shows the PL spectra of Na₅La_0.9−xTb_xEu_0.1(WO₄)₄ with different Tb³⁺-doping concentrations. Excited at 378 nm, the intense characteristic emission peaks of Tb³⁺ and Eu³⁺ can both be observed in the emission spectra of all samples. The PL intensity of Eu³⁺ dramatically increases with increasing Tb³⁺ concentration (x). And both emission intensities of Tb³⁺ and Eu³⁺ are increased with increasing the Tb³⁺ content when x = 0.9. All these results can validate the efficient energy transfer from Tb³⁺ to Eu³⁺.

The Na₅Ln(WO₄)₄ samples in which all of the Ln sites are substituted by La³⁺, Y³⁺ and Gd³⁺ cations are investigated. The PL spectra of the Na₅La_0.5−yTb_0.5Eu_y(WO₄)₄ and Na₅La_0.9−xTb_xEu_0.1(WO₄)₄ (Ln = La, Y, Gd) phosphors are illustrated in Fig. S1–S6 (in the ESI†), respectively. The variation of the normalized peak intensity ⁵D₀ → ⁷F₂ of Eu³⁺ at 616 nm and ⁵D₄ → ⁷F₅ of Tb³⁺ at 547 nm in Na₅Ln_0.5−yTb_0.5Eu_y(WO₄)₄ system with various Eu³⁺ doping concentrations (y) are shown in Fig. 7a. Here the Tb³⁺ concentration is fixed at 0.5 as the Eu³⁺ concentration (y) increases; the emission intensity of Eu³⁺ is enhanced clearly whereas the section of Tb³⁺ is decreased. As shown in Fig. 7b, the variation trends of normalized intensity for Tb³⁺ and Eu³⁺ in the Na₅Ln_0.9−xEu_0.1Tb_x(WO₄)₄ system are similar. Both the Tb³⁺ and Eu³⁺ emissions are increased progressively with increasing the Tb³⁺ concentration (x). The concentration quenching effect is unobvious, which could be attributed to the weak interaction among the activators.

Fig. 7 The emission intensity of Tb³⁺ (⁵D₄ → ⁷F₅) and of Eu³⁺ (⁵D₀ → ⁷F₂) in phosphors Na₅Ln_0.5−yTb_0.5Eu_y(WO₄)₄ (a) and Na₅Ln_0.9−xTb_xEu_0.1(WO₄)₄ (b) as a function of Eu³⁺ or Tb³⁺ concentration (λ_ex = 378 nm).

Energy transfer mechanism in Na₅Ln(WO₄)₄:Tb³⁺,Eu³⁺

An efficient energy transfer from Tb³⁺ to Eu³⁺ has been witnessed in various Na₅Ln(WO₄)₄ hosts. In order to further demonstrate the ET_Tb→Eu process, the PL decay curves of Tb³⁺ emission of Na₅La_0.5−yTb_0.5(WO₄)₄:yEu (y = 0–0.5) excited at 378 nm and monitored at 547 nm are shown in Fig. 8a. All decay curves can be well fitted by the first order exponential decay method using the formula:^21,22

I(t) = I₀ exp(−t/τ) (1)

where I(t) and I₀ are the luminescence intensities at time t and 0, and τ is the luminescence lifetime. The effective lifetime values were determined to be 0.740, 0.683, 0.436, 0.230, and 0.142 ms for Na₅La(WO₄)₄:0.5Tb³⁺,yEu³⁺ with y = 0, 0.01, 0.1, 0.3, and 0.5, respectively. The lifetime of the Tb³⁺ ions decreases monotonically with an increase in the Eu³⁺ concentration, which can further confirm the existence of energy transfer from Tb³⁺ to Eu³⁺ ions. Therefore, the energy transfer efficiency η_ET from Tb³⁺ to Eu³⁺ in Na₅Ln(WO₄)₄ host can be calculated using the following expression:

η_ET = 1 − τ/τ₀ (2)

here τ₀ and τ are the corresponding lifetimes of donor Tb³⁺ in the absence and presence of the acceptor Eu³⁺, and η_ET is the calculation of energy transfer efficiency. As shown in Fig. 8b, the η_ET value increases with increasing Eu³⁺ concentration, while the effective lifetime of Tb³⁺ decreases. The maximum energy transfer efficiency is about 91.7%, indicating that the ET_Tb→Eu process is quite efficient in the Na₅La(WO₄)₄ host.

Fig. 8 Decay time curves of Tb³⁺ in Na₅La_0.5−yEu_yTb_0.5(WO₄)₄ excited at 378 nm and monitored at 547 nm (a), and energy transfer efficiency from Tb³⁺ to Eu³⁺ and the lifetime of Tb³⁺ in Na₅La_0.5−yEu_yTb_0.5(WO₄)₄ (b).

It is well known that the energy transfer takes place via exchange interaction and/or multipolar interaction. The exchange interaction needs a large overlap between sensitizer and activator orbitals and the distances R_c between the sensitizer and activator is less than 5 Å. Otherwise, the electric multipolar interaction may dominate.²³ As suggested by Blasse,²⁴ the distance R_c between Tb³⁺ and Eu³⁺ ions in Na₅Ln(WO₄)₄ hosts can be estimated by the following formula:

where V is the volume of the unit cell, χ_c is the total concentration of Eu³⁺ and Tb³⁺, and N is the number of host cations in the Na₅Ln(WO₄)₄ unit cell. In this case, V = 1156.6 ų and N = 4. Since no concentration quenching exists in the compound Na₅Ln(WO₄)₄ (Ln = La, Gd and Y), χ_c = 1. The distance R_Tb–Eu is determined to be about 8.2 Å, which is much larger than 5 Å, the critical distance of the exchange interaction.

According to Dexter's energy transfer formula and Reisfeld's approximation,²⁵ the following relation can be obtained:

ln(τ₀/τ) ∝ c and τ₀/τ ∝ c^α/3 (4)

where ln(τ₀/τ) ∝ c is corresponding to the exchange interaction; whereas α = 6, 8, 10, are dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The relationships are illustrated in Fig. 9a–d, respectively. The optimal linear relationship was obtained when α = 6, indicating that the energy transfer from Tb³⁺ to Eu³⁺ in Na₅La_0.5−yEu_yTb_0.5(WO₄)₄ occurs following a dipole–dipole interaction. This result agrees well with that reported on Tb³⁺ to Eu³⁺ energy transfer.^26,27

Fig. 9 Dependence of ln(τ₀/τ) on c_Tb+Eu (a) and the dependence of τ₀/τ on (b) c_Tb+Eu^6/3, (c) c_Tb+Eu^8/3 and (d) c_Tb+Eu^10/3.

Tunable luminescence properties of Na₅Ln(WO₄)₄:Tb³⁺,Eu³⁺

Fig. 10a shows the luminescence photographs of Na₅Ln_0.5−yTb_0.5Eu_y(WO₄)₄ (Ln = La, Y, Gd) powders with increasing the Eu³⁺ content (y) upon excitation with 365 nm from a UV lamp. When the Eu³⁺ content (y) is 0, the emission color of the phosphors is green. With increasing the Eu³⁺ content (y), the emission color can be tuned from green, green-yellow, and yellow, to red. This fact indicates that the tuning of the emission color from green to red can be realized in the Na₅Ln(WO₄)₄:Tb³⁺,Eu³⁺ phosphors based on energy transfer. The CIE chromaticity diagram of the Na₅La_0.5−yTb_0.5Eu_y(WO₄)₄ samples presented in Fig. 10b also support this result. The calculated CIE chromaticity coordinates (x,y) for Na₅Ln_0.5−yTb_0.5Eu_y(WO₄)₄ samples are also listed in Table 1. The CIE coordinates vary systematically from green (x = 0.2935, y = 0.5994) to red (x = 0.6038, y = 0.3566) for Na₅La_0.5−yTb_0.5Eu_y(WO₄)₄ samples with increase in the Eu³⁺ content (y). Table 1 also presents the CIE color coordinates (x,y) on its doping Eu³⁺ concentration in the Na₅Ln_0.5−yTb_0.5Eu_y(WO₄)₄ (Ln = Y, Gd) samples under 378 nm excitation. Thus, for the series of Na₅Ln_0.5−yTb_0.5Eu_y(WO₄)₄ (Ln = La, Y, Gd) phosphors, the emission color can be tuned from green to red by adjusting the ratio of Tb³⁺ and Eu³⁺. As given in Table 1, the CIE chromaticity coordinates (x,y) for Na₅Ln_0.9−xTb_xEu_0.1(WO₄)₄ samples with the changes of the Tb³⁺ concentration (y) are too close to be distinguished from each other in a chromaticity diagram. We measured the absolute quantum yields (QYs) of Na₅La_0.5Tb_0.5(WO₄)₄ and Na₅Tb_0.5Eu_0.5(WO₄)₄ samples. Under the excitation of 378 nm UV, the obtained values of absolute QYs are determined to be 76% and 91%, respectively. The luminescence properties of Na₅Ln(WO₄)₄:Tb³⁺,Eu³⁺ (Ln = La, Y, Gd) phosphors indicate that the prepared samples may serve as single-component color-tunable phosphor in the fields of lighting and display devices.

Fig. 10 Luminescence photographs of Na₅Ln_0.5−yTb_0.5Eu_y(WO₄)₄ (Ln = La, Y, Gd; y = 0, 0.05, 0.1, 0.2, 0.4, 0.5) phosphors under 365 nm UV lamp (a); and CIE chromaticity diagram of the selected Na₅La_0.5−yTb_0.5Eu_y(WO₄)₄ phosphors under 378 nm excitation (b).

Table 1 The CIE chromaticity coordinates (x,y) for Na₅Ln(WO₄)₄:0.5Tb,yEu (0 ≤ y ≤ 0.5), Na₅Ln(WO₄)₄:0.1Eu,xTb (0 ≤ x ≤ 0.9) and Na₅La_0.4Tb_0.5Eu_0.1(WO₄)_4−z(MoO₄)_z (0 ≤ z ≤ 4) samples excited at 378 nm

Composition	CIE coordinates (x,y)
	Ln = La	Ln = Y	Ln = Gd
Na5La0.5Tb0.5(WO4)4	(0.2935,0.5994)	(0.2914,0.5953)	(0.3064,0.5892)
Na5La0.45Tb0.5Eu0.05(WO4)4	(0.3637,0.5395)	(0.3712,0.5277)	(0.3842,0.5192)
Na5La0.4Tb0.5Eu0.1(WO4)4	(0.4365,0.4769)	(0.4294,0.4768)	(0.4192,0.4859)
Na5La0.3Tb0.5Eu0.2(WO4)4	(0.5113,0.4258)	(0.5087,0.4280)	(0.5091,0.4271)
Na5La0.1Tb0.5Eu0.4(WO4)4	(0.5841,0.3746)	(0.5889,0.3703)	(0.5809,0.3715)
Na5Tb0.5Eu0.5(WO4)4	(0.6038,0.3566)	(0.6054,0.3566)	(0.6015,0.3562)
Na5La0.9Eu0.1(WO4)4	(0.4539,0.2765)	(0.5174,0.2888)	(0.5056,0.2888)
Na5La0.8Tb0.1Eu0.1(WO4)4	(0.3989,0.3300)	(0.3966,0.3381)	(0.3907,0.3363)
Na5La0.7Tb0.2Eu0.1(WO4)4	(0.3968,0.4166)	(0.3957,0.4171)	(0.3931,0.4220)
Na5La0.1Tb0.8Eu0.1(WO4)4	(0.4284,0.4988)	(0.4328,0.4964)	(0.4316,0.4865)
Na5Tb0.9Eu0.1(WO4)4	(0.4297,0.5011)	(0.4324,0.5009)	(0.4283,0.4877)
Na5La0.4Tb0.5Eu0.1(WO4)3(MoO4)	(0.4467,0.4754)	—	—
Na5La0.4Tb0.5Eu0.1(WO4)2(MoO4)2	(0.4533,0.4706)	—	—
Na5La0.4Tb0.5Eu0.1(WO4)(MoO4)3	(0.4761,0.4453)	—	—
Na5La0.4Tb0.5Eu0.1(MoO4)4	(0.5469,0.3427)	—	—

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The influence of W/Mo on the luminescence properties

Mo⁶⁺ and W⁶⁺ have similar ionic radius and can be substituted for each other. The sub-lattice structures around activators such as Tb³⁺ and Eu³⁺ will be somewhat changed. Therefore, the ratio of W to Mo is expected to have an impact on the luminescence properties. The emission spectra of as-prepared molybdotungstate samples with selected compositions of Na₅La_0.4Tb_0.5Eu_0.1(WO₄)_4−z(MoO₄)_z (z = 0, 1, 2, 3, 4) are recorded after excitation at 378 nm and are shown in Fig. 11a. Moreover, the variation trend of emission intensities of Tb³⁺ ions at 547 nm and Eu³⁺ ions at 593 and 616 nm are displayed in Fig. 11b. When W is gradually replaced by Mo, the emission intensities of Tb³⁺ and Eu³⁺ are both found to increase, reaching a peak at z = 2. The reason for the luminescence enhancement may be due to the advent of ion pair interaction between Tb³⁺ and Eu³⁺ ions, which is expected to be much stronger when Mo/W occupied bisection of the lattice sites separately. When the content of MoO₄²⁻ is greater than that of WO₄²⁻, the emission intensities of Eu³⁺ and Tb³⁺ decrease simultaneously with increasing in the Mo/W ratio. The Mo⁶⁺ substitution of W⁶⁺ results in changing the sub-lattice structures around Tb³⁺ or Eu³⁺ ions, therefore, the luminescent properties are expected to be tunable. For the series of Na₅La_0.4Tb_0.5Eu_0.1(WO₄)_4−z(MoO₄)_z samples with different Mo/W ratios, the CIE (x,y) coordinates listed in Table 1 indicate that as the concentration of Mo increases, the value of x coordinate increases while y-value decreases and the chromaticity is shift slightly from red toward deep-red.

Fig. 11 PL spectra of Na₅La_0.4Tb_0.5Eu_0.1(WO₄)_4−z(MoO₄)_z (z = 0, 1, 2, 3, 4) under 378 nm excitation (a), and the normalized intensities at 547, 593, and 616 nm as a function of the Mo content (z), respectively (b).

For Na₅La_0.4Tb_0.5Eu_0.1(WO₄)_4−z(MoO₄)_z samples, the values for R/O ratio vary from 5.84 for z = 0 to 8.61 for z = 4, which is in line with that reported for other scheelite related phosphors.²⁸ Based on these values, we can expect that the degree of the local symmetry around Eu³⁺ ions decreases with increasing Mo proportion. Similar observation to change W/Mo relative ratio has also been witnessed in some systems such as M₅Eu(WO₄)_4−x(MoO₄)_x, (M = Li, Na, K) (x = 0−4),⁹ CaGd_2(1−x)Eu_2x(MoO₄)_4(1−y)(WO₄)_4y (0 ≤ x ≤ 1, 0 ≤ y ≤ 1),²⁹ and AgLa_0.95Eu_0.05(WO₄)_2−x(MoO₄)_x (x = 0–2).³⁰

Conclusions

In summary, Na₅Ln(MO₄)₄:Tb³⁺,Eu³⁺ (Ln = La, Y, Gd; M = W, Mo) solid solutions with scheelite-type structure were prepared using a simple Pechini method. The introduction of different rare-earth ions or replacement of W⁶⁺ by Mo⁶⁺ has no effect on the crystal structure. The concentration quenching of Tb³⁺ or Eu³⁺ ions does not occur, which is ascribed to the special structure of Na₅Ln(MO₄)₄ hosts. The photoluminescence spectra, the decay curves, and the effect of the Eu³⁺/Tb³⁺ ratio have been used to investigated the energy transfer from Tb³⁺ → Eu³⁺. It is demonstrated that the luminescence of Eu³⁺ can efficiently sensitized by Tb³⁺ ions under NUV. Under 378 nm excitation, the emission color of Na₅Ln_0.5−yTb_0.5Eu_y(WO₄)₄ (y = 0–0.5) phosphors can be tunable from green through yellow and eventually to red region by adjusting the ratio of Eu³⁺ and Tb³⁺. Moreover, the W/Mo ratio-dependence of luminescence shows that the emission intensities of both Tb³⁺ and Eu³⁺ were found to reach a maximum when the molar ratio of Mo/W was 2 : 2. These results indicate that Tb³⁺ and Eu³⁺-codoped Na₅Ln(WO₄)₂(MoO₄)₂ phosphor can find application in the field of NUV-based WLEDs.

Acknowledgements

This research was financially supported by “the Fundamental Research Funds for the Central Universities”, South-Central University for Nationalities (CZY15002).

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 PL spectra of Na₅La_0.5−yTb_0.5Eu_y(WO₄)₄; Fig. S2 PL spectra of Na₅Y_0.5−yTb_0.5Eu_y(WO₄)₄; Fig. S3 PL spectra of Na₅Gd_0.5−yTb_0.5Eu_y(WO₄)₄; Fig. S4 PL spectra of Na₅La_0.9−xTb_xEu_0.1(WO₄)₄; Fig. S5 PL spectra of Na₅Y_0.9−xTb_xEu_0.1(WO₄)₄; Fig. S6 PL spectra of Na₅Gd_0.9−xTb_xEu_0.1(WO₄)₄. See DOI: 10.1039/c6ra10763j

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