Tunable luminescence properties of the novel Tm³⁺- and Dy³⁺-codoped LiLa(MoO₄)_x(WO₄)_2−x phosphors for white light-emitting diodes

Abstract

A series of Tm³⁺- and Dy³⁺-codoped LiLa(MoO₄)_x(WO₄)_2−x phosphors were prepared via the conventional solid-state reaction method. It was discovered that the optimum single doped concentrations of both Tm³⁺ and Dy³⁺ are 0.04 for the LiLa(WO₄)₂ host; moreover, LiLa(WO₄)₂:0.04Tm³⁺ and LiLa(WO₄)₂:0.04Dy³⁺ emit blue and yellow light, respectively. On the other hand, the emission colors of Tm³⁺- and Dy³⁺-codoped LiLa(WO₄)₂ could be tuned from blue to white by tuning the energy transfer. With increase in the Dy³⁺ doped concentration, the energy transfer efficiency of LiLa(WO₄)₂:0.005Tm³⁺,yDy³⁺ increased gradually and reached as high as 88% when the concentration of Dy³⁺ was 0.05. The energy transfer from Tm³⁺ to Dy³⁺ was revealed to be resonant via the quadrupole–quadrupole mechanism. As for LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ phosphors, the emission intensities of both Tm³⁺ and Dy³⁺ were found to reach a maximum when the molar ratio of Mo/W was 0 : 2. In addition, with increase in the proportion of MoO₄²⁻, the location of the chromaticity coordinates changed from the edge to the center of the white area. When the relative ratio of Mo/W was 2 : 0, the chromaticity coordinates were much closer to the standard chromaticity coordinates for white light. The results indicate that these phosphors may have potential applications in the fields of UV-excited white light-emitting diodes.

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1. Introduction

White light-emitting diodes (WLEDs) as a more preferable replacement for fluorescent lamps and conventional incandescent bulbs have attracted considerable attention, and they have a promising application in the field of display and lighting.^1,2 To date, numerous efforts have been devoted for developing novel luminescent materials for WLEDs, especially single phase white light emitting phosphors for near ultraviolet or ultraviolet (UV) light excitation, which have lots of advantages such as no phase separation, better reproducibility and stability, and simpler fabrication processes.³ Codoping different rare earth ions as sensitizers and activators in one host to control the emission color via energy transfer processes is one current method used to realize white light emission in a single phase host, in which white light can be obtained by the combination of red, green and blue or yellow and blue.^4–6 Among the trivalent rare earth ions (RE³⁺), Tm³⁺ can provide blue light, because its major emission peak is at about 455 nm (¹D₂ → ³F₄).⁷ In general, the luminescence of Dy³⁺ mainly consists of two intense emission bands in the blue (460–500 nm, ⁴F_9/2 → ⁶H_15/2) and yellow (550–600 nm, ⁴F_9/2 → ⁶H_13/2) regions, respectively.⁸ As is well known, blue emission is a magnetic dipole transition and hardly varies with the crystal field. The yellow emission is an electric dipole transition, which is strongly influenced by the chemical environment surrounding Dy³⁺ ions.⁹ Moreover, the ⁴F_9/2 → ⁶H_13/2 transition emission will be dominant in the emission spectra when Dy³⁺ is located at a low symmetry site; therefore, the yellow emission will be much stronger than the blue emission and Dy³⁺ ions will emit yellow light. In order to obtain white light, some rare-earth ions (such as Tm³⁺), which emit blue light, are codoped in the host. Moreover, there is a conspicuous overlap between the excitation band of Dy³⁺ ions and the emission transitions of Tm³⁺ ions. On the basis of the Dexter energy pattern, the effective resonance type energy transfer from Tm³⁺ to Dy³⁺ can be expected to occur in some special crystal field environments.¹⁰ Therefore, it is possible to obtain white light at a suitable yellow-to-blue intensity ratio by codoping Tm³⁺ and Dy³⁺ into a single matrix simultaneously.

At present, there are many hosts, such as borates,¹¹ phosphates,¹² silicates,¹³ molybdates,¹⁴ tungstates,¹⁵ and vanadates,¹⁶ which can be doped by rare earth ions. Among all these hosts, molybdates and tungstates with similar chemical properties have attracted great interest and attention due to their excellent thermal and hydrolytic stabilities as well as their spectroscopic properties.¹⁷ Scheelite structure double molybdate LiLa(MoO₄)₂ and double tungstate LiLa(WO₄)₂ exhibit a number of fascinating, peculiar features associated with the low symmetry of their crystal lattice. In these compounds, Mo⁶⁺ (or W⁶⁺) is coordinated by four O²⁻ at a tetrahedral site, which makes MoO₄²⁻ (or WO₄²⁻) relatively stable. Li⁺ and La³⁺ cations are randomly distributed over the same sites, and they are coordinated by eight O²⁻ ions from four nearby MoO₄²⁻ (or WO₄²⁻) groups with symmetry S₄ (without an inversion center).¹⁸ The random distribution of La³⁺ will induce the inhomogeneous broadening of optical spectra when the rare earth ions are doped in these crystals and occupy the positions of La³⁺ ions.¹⁹ Moreover, MoO₄²⁻ and WO₄²⁻ groups have strong absorption in the UV region, and the energy can be transferred from MoO₄²⁻ and WO₄²⁻ to rare earth ions, which can greatly enhance the external quantum efficiency of rare earth ions doped materials.²⁰

Complex molybdate–tungstates have attracted considerable attention because Mo⁶⁺ and W⁶⁺ have similar ionic radii and can be substituted for each other. The resulting sub-lattice structures around the luminescent center ions will be expected to be somewhat diverse; therefore, the luminescent properties are expected to be tunable. Recently, most of the investigations have been about the luminescence properties of Eu³⁺ doped mixed molybdate–tungstate phosphors. Sivakumar et al.²¹ and Chiu et al.²² reported the spectroscopic properties of Eu³⁺ ions in AgGd_0.95Eu_0.05(WO₄)_2−x(MoO₄)_x and M₅Eu(WO₄)_4−x(MoO₄)_x crystals, respectively. The luminescence properties of LiEu(WO₄)_2−x(MoO₄)_x were also described by Chiu's group and the maximum luminescence intensity was observed when the molar ratio of Mo/W was 2 : 0.²³ Lu et al.²⁴ prepared the NaLa_1−y(MoO₄)_2−x(WO₄)_x:yEu³⁺ series of red-emitting phosphors. For this composition, the intensity of ⁵D₀ → ⁷F₂ emission of Eu³⁺ was found to increase and reach a maximum when the relative ratio of Mo/W was 1 : 1. For the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions, the ¹D₂ → ³F₄ of Tm³⁺ and ⁴F_9/2 → ⁶H_13/2 of Dy³⁺ are also strongly influenced by the crystal field; thus, the luminescent properties of Tm³⁺ and Dy³⁺ doped mixed molybdate–tungstate phosphors are valuable to study. However, as far as we know, there have been no reports on this subject. Among the many synthetic methods, the solid-state reaction method is the most typical and common. The direct advantages of this method include more convenient preparation routes, high crystallinity and purity, excellent productivity and high emission intensity, which make it ideally suitable for the preparation of luminescent materials.

Based on the above investigations and motivated by the attempt to develop novel efficient phosphors for white light-emitting diodes, a series of Tm³⁺ and Dy³⁺ codoped LiLa(MoO₄)_x(WO₄)_2−x phosphors were prepared via a conventional solid-state reaction technique. Their crystal structures, luminescence and color chromaticity properties were investigated in detail. The emission colors of LiLa(WO₄)₂:0.005Tm³⁺,yDy³⁺ can be tuned from blue to white through tuning the energy transfer process. Their energy transfer efficiency increases gradually with increase in Dy³⁺ doped concentration. For the LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ phosphors, the emission intensities of both Tm³⁺ and Dy³⁺ were found to reach a maximum when the relative ratio of Mo/W is 0 : 2. In addition, the location of the chromaticity coordinates changed from the edge to the center of the white area with increase in the content of MoO₄²⁻. When the molar ratio of Mo/W was 2 : 0, the chromaticity coordinates were closest to the standard chromaticity coordinates for white. These phosphors are expected to show great promise for applications in the fields of UV-excited white light-emitting diodes.

2. Experimental

LiLa(MoO₄)_x(WO₄)_2−x:Tm³⁺,Dy³⁺ (x = 0, 0.4, 0.8, 1.2, 1.6, and 2.0) phosphors were synthesized via the solid-state reaction technique. Li₂CO₃ (A.R.), MoO₃ (A.R.), WO₃ (A.R.), La₂O₃ (99.99%), Tm₂O₃ (99.99%) and Dy₂O₃ (99.99%) were used as starting materials. All the doped Tm³⁺ and Dy³⁺ ions were in the molar percentage in our experiments. For the synthesis of LiLa(WO₄)₂:0.005Tm³⁺,0.03Dy³⁺, 1.0 mmol Li₂CO₃, 4.0 mmol WO₃, 0.965 mmol La₂O₃, 0.005 mmol Tm₂O₃ and 0.03 mmol Dy₂O₃ were weighed and thoroughly mixed in an agate mortar by grinding for 1 h. Then, the mixture was transferred to an alumina crucible and sintered in a furnace at 800 °C for 4 h. After that, the alumina crucible was allowed to cool down to room temperature naturally. The sintered cake was ground and then the LiLa(WO₄)₂:0.005Tm³⁺,0.03Dy³⁺ phosphor was obtained. The other samples were prepared in a similar manner except using different amounts of raw materials.

The obtained samples were examined by X-ray diffraction (XRD) measurements performed on a Rigaku D/max-II B X-ray diffractometer at a scanning rate of 10° min⁻¹ in the 2θ range from 10° to 80° with graphite-monochromatized Cu K_α radiation (λ = 0.15406 nm). The morphologies of the as-synthesized products were characterized by field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi), employing the accelerating voltage of 5 kV. High-resolution transmission electron microscopy (HRTEM) was performed using a transmission electron microscope (JEM-2010). Images were acquired digitally on a Gatan multiple CCD camera. The constituent elements were detected by energy-dispersive X-ray spectroscopy (EDX) using an X-ray detector attached to the JEM-2010 instrument. The photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W Xe lamp as the excitation source. The quantum yield was measured using the integrating sphere on the FLS920 time resolved and steady state fluorescence spectrometers (Edinburgh Instruments Ltd, Edinburgh, England), and a Xe900 lamp was used as an excitation source. All the measurements were performed at room temperature.

3. Results and discussion

3.1. Phase identification and morphology

On the basis of the carefully controlled exploration of solid-state reaction conditions, the suitable annealing temperature for the formation of LiLa(WO₄)₂ is 800 °C, as given in the TGA-DSC curve in Fig. S1 (in the ESI†). The XRD patterns of LiLa(WO₄)₂ samples that reacted at 800 °C for 4 h, 8 h, 12 h and 500 °C for 12 h, and then 800 °C for 12 h, are shown in Fig. 1a, as well as the standard data of LiLa(WO₄)₂ powder (JCPDS no. 04-008-0385) and Li₂W₂O₇ powder (JCPDS no. 00-024-0664). As shown in Fig. 1a, most of the diffraction patterns of both products are consistent with the standard data of tetragonal phase LiLa(WO₄)₂ (space group: I4₁/a), but there are some weak impurity peaks at 13.4°, 14.7°, 19.3°–27.2°, 29.8°, which are assigned to the intermediate product Li₂W₂O₇. The weight fractions of the impure phase were calculated to be 6.5%, 7.4%, 8.6% and 7.2% of samples wich reacted at 800 °C for 4 h, 8 h, 12 h and 500 °C for 12 h then 800 °C for 12 h, respectively. The above results also reveal that the impurity peaks cannot be reduced even after prolonging the reaction time to 24 h. We tried to increase the reaction temperature to 900 °C; however, the desired product was not obtained because the materials fused under the higher temperature. The host LiLa(WO₄)₂ can be very difficult to synthesize, and its standard data can only be found in the newest powder diffraction file database (PDF4-2009). There are also some articles investigating the luminescent properties of rare earth ions doped phosphors, which contain impurity phases.^17,25,26 Therefore, we determined that the reaction conditions were 800 °C and 4 h. Fig. 1b shows the schematic illustration of the tetragonal phase structure of LiLa(WO₄)₂. It can be seen that the central W⁶⁺ is coordinated by four equivalent O²⁻ at a tetrahedral site. La³⁺ and Li⁺ are randomly distributed over the same cationic sublattice, and they share corners with eight adjacent O²⁻ from four nearby (WO₄)²⁻ groups with symmetry S₄ (without an inversion center).

Fig. 1 XRD patterns of LiLa(WO₄)₂ samples that reacted at 800 °C for 4 h, 8 h, and 12 h; 500 °C for 12 h; and then 800 °C for 12 h, as well as the standard data of LiLa(WO₄)₂ (JCPDS no. 04-008-0385) and Li₂W₂O₇ (JCPDS no. 00-024-0664) (a), and crystal structure of tetragonal phase LiLa(WO₄)₂ (b).

In order to prove the presence of the impurity, high-resolution transmission electron microscopy (HRTEM) was performed on the sample, which reacted at 800 °C for 4 h. From Fig. 2a and b, it can be observed that the sample has clear lattice fringes that reveal its highly crystalline nature. The resolved interplanar distances between the adjacent lattice fringes are found to be about 0.314 and 0.342 nm, which are consistent with the d-spacing of the (112) plane of the tetragonal LiLa(WO₄)₂ (JCPDS no. 04-008-0385) and on the plane of Li₂W₂O₇ (JCPDS no. 00-024-0664) at 26.11° (there are no detailed indices of the crystal face of Li₂W₂O₇ in the standard PDF card), respectively. In Fig. 2c, the EDX spectrum confirms the strong signals of lanthanum (La), tungsten (W) and oxygen (O). The copper (Cu) signal is due to the copper grid and the silicon (Si) signal is from the detector. On the basis of the above results, it can be deduced that the sample contains two phases, i.e. LiLa(WO₄)₂ and Li₂W₂O₇, which is in agreement with the XRD study.

Fig. 2 HRTEM images (a and b) and EDX (c) spectrum of the sample that reacted at 800 °C for 4 h. The inset of (a and b) is the corresponding fast Fourier transform (FFT) pattern.

The XRD patterns of LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ (x = 0, 0.4, 0.8, 1.2, 1.6, and 2.0) and the standard data of LiLa(MoO₄)₂ powder (JCPDS no. 00-018-0734) and LiLa(WO₄)₂ powder are presented in Fig. 3. With increase in the proportion of MoO₄²⁻, the transformation from LiLa(WO₄)₂ to the tetragonal phase LiLa(MoO₄)₂ is observed. At the same time, the impurity phase changes from Li₂W₂O₇ to Li₂Mo₂O₇. The weight fractions of the impure phase were calculated to be 6.4%, 6.1%, 5.5%, 4.9%, 2.9% and 2.9% for x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0, respectively. The doped Tm³⁺ and Dy³⁺ ions are expected to randomly occupy the La³⁺ sites in the host lattice, which is due to the similar ionic size (CN = 8) of Tm³⁺ (0.994 Å), Dy³⁺ (1.027 Å) and La³⁺ (1.160 Å).²⁷ Thus, no other impurity lines were observed, clearly indicating that Tm³⁺ and Dy³⁺ ions have been effectively incorporated into the host lattice.

Fig. 3 XRD patterns of LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ and the standard data of LiLa(MoO₄)₂ (JCPDS no. 00-018-0734) and LiLa(WO₄)₂.

The morphologies of LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ (x = 0, 0.4, 0.8, 1.2, 1.6, and 2.0) were inspected using a scanning electron microscope (SEM). The obtained SEM images are depicted in Fig. 4. It can be seen clearly that the samples are composed of agglomerated particles ranging from 0.5 to 5 μm with irregular shape and surrounded by smaller particles. However, no significant differences are observed in the SEM images for the samples with different Mo/W ratio. The obtained relatively large particles at a relatively low sintering temperature can be explained by the high Li content in LiLa(MoO₄)_x(WO₄)_2−x, which promotes particle growth and improves crystallinity.^28,29

Fig. 4 SEM images of LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺: x = 0 (a), x = 0.4 (b), x = 0.8 (c), x = 1.2 (d), x = 1.8 (e) and x = 2.0 (f).

3.2. Luminescence properties of Tm³⁺- or Dy³⁺-single doped LiLa(WO₄)₂ phosphors

Fig. 5 illustrates the photoluminescence excitation (EX) and emission (EM) spectra of the purely Tm³⁺-activated LiLa(WO₄)₂ phosphor. Monitored at 453 nm, the excitation spectrum consists of a broad band and an intense excitation peak. The wide band ranging from 220 to 320 nm corresponds to the charge transfer band (CTB) of O²⁻ → W⁶⁺ within the WO₄²⁻ group. The sharp excitation peak, which is located at 361 nm, is responsible for the ³H₆ → ¹D₂ transition of Tm³⁺.³⁰ This excitation peak is coupled well with the emission of commercial UV light-emitting diode chips. Excitation into the strongest ³H₆ → ¹D₂ transition of Tm³⁺ yields the corresponding emission spectrum, in which the main emission peak at 453 nm is assigned to the ¹D₂ → ³F₄ electronic dipole transition, and the weak emission peak at about 473 nm is attributed to the ¹G₄ → ³H₆ transition.³¹

Fig. 5 Excitation (EX) and emission (EM) spectra of the LiLa(WO₄)₂:0.04Tm³⁺ phosphor.

The measured emission spectra of LiLa(WO₄)₂:xTm³⁺ (x = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) phosphors upon 361 nm excitation are shown in Fig. 6. By comparing the curves, it can be observed that the intensities of the emission transitions are found to increase with increase in Tm³⁺ ion concentration and reach a maximum at x = 0.04, and then decrease because of concentration quenching. Such a variation indicates that the optimum doped concentration of Tm³⁺ is estimated to be 0.04 for the LiLa(WO₄)₂ host. The CIE color chromaticity is considered to be a critical parameter for evaluating the performance of the prepared phosphors from the aspect of application. The CIE chromaticity coordinates of LiLa(WO₄)₂:0.04Tm³⁺ under 361 nm light irradiation have been calculated based on the corresponding emission spectrum and using the CIE 1931 color matching functions. The obtained chromaticity coordinates are (0.228, 0.160), which are located in the blue region.

Fig. 6 Emission spectra of LiLa(WO₄)₂:xTm³⁺ (x = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) phosphors upon 361 nm excitation.

Fig. 7 shows the excitation (EX) and emission (EM) spectra of the single Dy³⁺-doped LiLa(WO₄)₂ phosphor. The excitation spectrum is composed of two parts. Similar to Tm³⁺ ions, the broad band ranging from 220 to 320 nm is responsible for the charge transfer band (CTB) of O²⁻ → W⁶⁺. The other part is composed of some sharp peaks located at about 354, 369, 390, 429, 455 and 477 nm. These sharp lines are assigned to the f–f transitions of the Dy³⁺ ion from the ground state ⁶H_15/2 to the various excited states ⁶P_7/2, ⁶P_5/2, ⁴F_7/2 + ⁴I_13/2, ⁴G_11/2, ⁴I_15/2 and ⁴F_9/2, respectively.³² The corresponding emission spectrum of the LiLa(WO₄)₂:0.04Dy³⁺ phosphor excited at 390 nm contains two main bands in the blue and yellow regions. The blue emissions at 478 nm and 485 nm correspond to the ⁴F_9/2 → ⁶H_15/2 transition, and the yellow emission at about 574 nm is assigned to the ⁴F_9/2 → ⁶H_13/2 transition.³³ It is well known that the ⁴F_9/2 → ⁶H_15/2 transition belongs to the magnetic dipole transition, and it is hardly influenced by the crystal field around the Dy³⁺ ions, while the ⁴F_9/2 → ⁶H_13/2 transition corresponds to the forced electric dipole transition (ΔJ = 2), which strongly varies with the chemical environment surrounding Dy³⁺. When Dy³⁺ is located at a low symmetry site (without inversion symmetry), the ⁴F_9/2 → ⁶H_13/2 transition emission will be dominant in the emission spectra.³⁴ In the tetragonal Scheelite structure, LiLa(WO₄)₂, Dy³⁺ ions are expected to occupy the La³⁺ sites (without an inversion center); therefore, the 574 nm emission peak corresponding to the ⁴F_9/2 → ⁶H_13/2 transition is much stronger than the other emissions, which is consistent with the structural study.

Fig. 7 Excitation (EX) and emission (EM) spectra of the LiLa(WO₄)₂:0.04Dy³⁺ phosphor.

The emission spectra of LiLa(WO₄)₂:yDy³⁺ (y = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) phosphors under excitation at 390 nm are displayed in Fig. 8. Firstly, with increase in the concentration of Dy³⁺ ions, the emission intensities are found to increase and reach a maximum at y = 0.04, and then there is a decrease because of concentration quenching. Based on the energy match rule, the cross relaxation between the energy level pairs of ⁴F_9/2–⁶F_9/2 and ⁶H_15/2–⁶F_11/2 is considered to be the main reason for the concentration quenching.³ The CIE chromaticity coordinates of LiLa(WO₄)₂:0.04Dy³⁺ upon 390 nm excitation are (0.360, 0.395), which are located in the yellow region.

Fig. 8 Emission spectra of LiLa(WO₄)₂:yDy³⁺ (y = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) phosphors excited by 390 nm light.

3.3. Energy transfer mechanism of LiLa(WO₄)₂:Tm³⁺,Dy³⁺ phosphors

The comparison of the excitation spectrum of LiLa(WO₄)₂:0.04Dy³⁺ with the emission spectrum of LiLa(WO₄)₂:0.04Tm³⁺ (Fig. S2†) reveals a conspicuous spectral overlap. As a result, efficient resonance type energy transfer from Tm³⁺ to Dy³⁺ can be expected to occur in the LiLa(WO₄)₂ host. Fig. 9a illustrates the emission spectra of LiLa(WO₄)₂:0.005Tm³⁺,yDy³⁺ (y = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) phosphors. Moreover, Fig. 9b shows the variation trend of emission intensities at 453 nm of Tm³⁺ ions and 574 nm of Dy³⁺ ions. It can be seen that after excitation of Tm³⁺ at 361 nm, the Tm³⁺/Dy³⁺ codoped samples show the characteristic emission peaks of both Tm³⁺ and Dy³⁺. Moreover, the emission intensity of the activator Dy³⁺ is at first observed to increase and becomes strongest at y = 0.03, and then it decreases with further increase in the Dy³⁺ doping concentration due to concentration quenching. However, the emission intensity of the sensitizer Tm³⁺ is simultaneously found to decrease monotonically with increase in the Dy³⁺ doping concentration. The abovementioned results give direct evidence to demonstrate that the Tm³⁺ ions can transfer the energy to Dy³⁺ ions efficiently.

Fig. 9 Emission spectra of LiLa(WO₄)₂:0.005Tm³⁺,yDy³⁺ (y = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) phosphors (a), the variation trend of emission intensities at 453 nm and 574 nm (b), the excitation, possible energy transfer, and emission processes of Tm³⁺ and Dy³⁺ ions (c), and CIE chromaticity coordinates of LiLa(WO₄)₂:0.005Tm³⁺,yDy³⁺ (d).

To help explain the paths of energy transfer from Tm³⁺ to Dy³⁺ during luminescence, a schematic diagram is shown in Fig. 9c. Under 361 nm excitation, corresponding to the ³H₆ → ¹D₂ transition of Tm³⁺, Tm³⁺ ions absorb the energy, and then a part of the energy shifts to the lower excited energy level ¹G₄ through relaxation, and eventually the blue emissions occur through the ¹D₂ → ³F₄ and ¹G₄ → ³H₆ transitions. On the other hand, Tm³⁺ also can transfer a part of its energy to the ⁴I_11/2 state of Dy³⁺, and then the energy shifts to the lower excited energy level ⁴F_9/2, at last the energy falls to the ⁶H_13/2 and ⁶H_15/2 energy levels of Dy³⁺. The energy transfer from Tm³⁺ to Dy³⁺ is based on the fact that the ¹D₂ level of Tm³⁺ is close to the ⁴I_11/2 level of Dy³⁺, as demonstrated in Fig. 9c. The CIE chromaticity coordinates of LiLa(WO₄)₂:0.005Tm³⁺,yDy³⁺ (y = 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) samples excited by 361 nm light are listed in Table 1 and presented in Fig. 9d. It can be clearly seen that with increase in Dy³⁺ ions concentration, the emission colors change from blue to white. However, the chromaticity coordinates are located on the edge of the white light region even when y = 0.05.

Table 1 The chromaticity coordinates and energy transfer efficiency (η_T) of LiLa(WO₄)₂:0.005Tm³⁺,yDy³⁺ phosphors excited by 361 nm light

Compound	CIE		ηT
	x	y
LiLa(WO4)2:0.005Tm^3+,0.005Dy^3+	0.211	0.144	0.03
LiLa(WO4)2:0.005Tm^3+,0.01Dy^3+	0.226	0.168	0.19
LiLa(WO4)2:0.005Tm^3+,0.02Dy^3+	0.235	0.184	0.32
LiLa(WO4)2:0.005Tm^3+,0.03Dy^3+	0.259	0.232	0.63
LiLa(WO4)2:0.005Tm^3+,0.04Dy^3+	0.245	0.211	0.68
LiLa(WO4)2:0.005Tm^3+,0.05Dy^3+	0.263	0.243	0.88

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In addition, the energy transfer efficiency (η_T) from Tm³⁺ to Dy³⁺ was also investigated as a function of Dy³⁺ ions doped concentration, which is shown in Fig. 10. A simple operational definition of the energy transfer efficiency (η_T) from the sensitizer to the activator can be expressed according to the following formula:³⁵

where I_S0 is the intrinsic luminescence intensity of the sensitizer (Tm³⁺), and I_S is the luminescence intensity of the sensitizer with the activator (Dy³⁺) that is present. With increase in Dy³⁺ doped concentration, the energy transfer efficiency increases gradually and reaches as high as 88% when the concentration of Dy³⁺ is 0.05, as shown in Table 1.

Fig. 10 The dependence of energy transfer efficiency η_T of LiLa(WO₄)₂:0.005Tm³⁺,yDy³⁺ on Dy³⁺ ions doping concentration.

Concentration quenching may occur because the excitation energy migrates about a large number of centers before being emitted. With increase in RE³⁺ ion concentration, the average distance between RE³⁺ ions will decrease. The excitation energy may be transferred between the close RE³⁺ ions. For this reason, it is necessary to obtain the critical distance (R_C), which is the critical separation between the sensitizer and activator. The critical distance R_C between Tm³⁺ and Dy³⁺ in the LiLa(WO₄)₂ host can be estimated by the following equation:³⁶

where χ_c is the total critical concentration of Tm³⁺ and Dy³⁺, N is the number of formula units in the LiLa(WO₄)₂ unit cell and V is the volume of the unit cell. By taking the experimental and analytical values of χ_c, N and V (0.035, 4 and 323.97 ų, respectively), the critical transfer distance of Tm³⁺ and Dy³⁺ in LiLa(WO₄)₂ is found to be about 16.41 Å. There are two main aspects that are responsible for the resonant energy transfer mechanism.^37,38 The first one is exchange interaction and the other one is multipolar interaction. The exchange interaction needs a large overlap between sensitizer and activator orbitals, which are responsible for the energy transfer of forbidden transitions and shorter critical distances of less than 5 Å. The estimated critical distance of LiLa(WO₄)₂:Tm³⁺,Dy³⁺ is 16.41 Å, which is much longer than the restriction of the critical distance, indicating little possibility of energy transfer via the exchange interaction mechanism. As a result, the process of energy transfer would be due to multipolar interaction; moreover, there are three multipolar interactions: dipole–dipole, dipole–quadrupole and quadrupole–quadrupole, respectively.³⁹

Based on Dexter's energy transfer expressions of multipolar interaction and Reisfeld's approximation, the following relation can be obtained:⁶

where η₀ and η are the luminescence quantum efficiency of the sensitizer Tm³⁺ with and without the activator Dy³⁺. The values of η₀/η can be approximately calculated by the ratio of related luminescence intensities (I_S0/I_S). C is the total doped concentration of Tm³⁺ and Dy³⁺. n represents the interaction mechanism between rare earth ions, and n = 6, 8 or 10 for dipole–dipole, dipole–quadrupole, or quadrupole–quadrupole interactions, respectively. The relationship of I_S0/I_S − C^n/3 is illustrated in Fig. 11, and the curve when n = 10 exhibits the best linear relation of the three plots. This clearly indicates that the quadrupole–quadrupole interaction is the dominant energy transfer mechanism for LiLa(WO₄)₂:Tm³⁺,Dy³⁺ phosphors.

Fig. 11 The dependence of I_S0/I_S of Tm³⁺ on C^6/3, C^8/3, and C^10/3.

3.4. Tunable luminescence properties of LiLa(MoO₄)_x(WO₄)_2−x:Tm³⁺,Dy³⁺ phosphors

The emission spectra of the LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ (x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0) phosphors after excitation at 361 nm are shown in Fig. 12a. Moreover, Fig. 12b displays the variation trend of emission intensities of Tm³⁺ ions at 453 nm and Dy³⁺ ions at 574 nm. When the content of WO₄²⁻ is greater than that of MoO₄²⁻, the intensities of the ¹D₂ → ³F₄ transition of Tm³⁺ and the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ are both found to decrease with increase in the ratio of Mo/W. When the content of WO₄²⁻ is less than that of MoO₄²⁻, the intensity of the ¹D₂ → ³F₄ transition is observed to decrease slightly with increase in the Mo/W ratio, while the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ is found to increase mildly. The maximum emission intensities of both Tm³⁺ and Dy³⁺ were found when the relative ratio of Mo/W was 0 : 2. The reason for this observation may be due to the different degrees of disorder and the local symmetry around Tm³⁺ and Dy³⁺ ions, as well as the different energy transfer efficiencies from Tm³⁺ to Dy³⁺. Both the ¹D₂ → ³F₄ transition of Tm³⁺ and the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ ions correspond to the forced electric dipole transition (ΔJ = 2), which is strongly influenced by the crystal field around Tm³⁺ and Dy³⁺ ions. The higher degree of disorder and lower symmetry help to improve the intensity of these transitions.⁴⁰ The mole fraction of the main phase LiLa(MoO₄)_x(WO₄)_2−x is more than 70%, and the doped concentration of Tm³⁺ and Dy³⁺ ions is only 3.5%; therefore, there are sufficient La³⁺ sites for Tm³⁺ and Dy³⁺ ions to occupy. However, the mole fractions of the impurity phase were found to decrease with increase in the ratio of Mo/W. The impurity phase is expected to distort the crystal structure of LiLa(MoO₄)_x(WO₄)_2−x and make more sub-lattice variations; therefore, the local distortion induced by the impurity phase will decrease with increase in the ratio of Mo/W. The lower degree of disorder will lead to higher local symmetry of Tm³⁺ and Dy³⁺ ions. Furthermore, the lower degree of disorder and the higher local symmetry are not beneficial for the ¹D₂ → ³F₄ transition of Tm³⁺ and the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺; therefore, the emission intensity of Tm³⁺ and Dy³⁺ ions will decrease with increase in Mo/W ratio. However, the energy transfer efficiency from Tm³⁺ to Dy³⁺ can affect the emission intensity. When the content of WO₄²⁻ is less than that of MoO₄²⁻, the energy transfer efficiency is increased; thus, the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ is found to increase mildly. In addition, the advent of ion pair interactions between rare earth ions is expected to be stronger in the molybdate crystal than in the tungstate crystal because of the differences in the ionic size (CN = 4) of Mo⁶⁺ (0.41 Å) and W⁶⁺ (0.42 Å).^23,24 Eventually, as a comprehensive result of the above factors, the emission intensities reveal the above variation trend.

Fig. 12 Emission spectra of LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ (x = 0, 0.4, 0.8, 1.2, 1.6, and 2.0) phosphors (a), the variation trend of emission intensities at 453 nm and 574 nm (b), and CIE chromaticity coordinates of LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ (1: x = 0, 2: x = 0.4, 3: x = 0.8, 4: x = 1.2, 5: x = 1.6, 6: x = 2.0) (c).

The calculated CIE chromaticity coordinates of LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ (x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0) samples at the excitation of 361 nm are listed in Table 2 and presented in Fig. 12c. With increase in the content of MoO₄²⁻, the location of the chromaticity coordinates change from the edge to the center of the white area. Moreover, the chromaticity coordinates when the molar ratio of Mo/W is 2 : 0 is closest to the National Television Standard Committee (NTSC) standard CIE chromaticity coordinate values for white light (0.33, 0.33). In order to further investigate the possible practical application of these phosphors in WLEDs, the quantum yield has also been studied and measured; moreover, the quantum yield of LiLa(WO₄)₂:0.005Tm³⁺,0.03Dy³⁺ is 30.59%.

Table 2 The chromaticity coordinates of LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ samples upon 361 nm excitation

Host compound	CIE chromaticity coordinates
	x	y
LiLa(WO4)2	0.258	0.248
LiLa(MoO4)0.4(WO4)1.6	0.284	0.279
LiLa(MoO4)0.8(WO4)1.2	0.287	0.286
LiLa(MoO4)1.2(WO4)0.8	0.291	0.293
LiLa(MoO4)1.6(WO4)0.4	0.302	0.306
LiLa(MoO4)2	0.310	0.310
White	0.33	0.33

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4. Conclusions

In summary, LiLa(MoO₄)_x(WO₄)_2−x:Tm³⁺,Dy³⁺ phosphors have been successfully prepared via the conventional solid-state reaction method. The emission colors of LiLa(WO₄)₂:0.005Tm³⁺,Dy³⁺ could be tuned from blue to white by tuning the energy transfer, and the energy transfer efficiency increases gradually with increase in the Dy³⁺ doped concentration. The energy transfer from Tm³⁺ to Dy³⁺ was revealed to be resonant via the quadrupole–quadrupole mechanism. As for the complex molybdate–tungstate LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ phosphors, both the emission intensities of Tm³⁺ and Dy³⁺ were found to reach a maximum when the molar ratio of Mo/W was 0 : 2. In addition, with increase in the content of MoO₄²⁻, the location of the chromaticity coordinates for the LiLa(MoO₄)_x(WO₄)_2−x:0.005Tm³⁺,0.03Dy³⁺ phosphors changed from the edge to the center of the white area. The chromaticity coordinates when the relative ratio of Mo/W was 2 : 0 were much closer to the standard chromaticity coordinates for white light. This study revealed that the prepared phosphors exhibited a high potential for application in the fields of UV-excited white light-emitting diodes.

Acknowledgements

This present work was financially supported by the Mineral and Ore Resources Comprehensive Utilization of Advanced Technology Popularization and Practical Research (MORCUATPPR) funded by China Geological Survey (Grant no. 12120113088300), and the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources, no. OSR-5.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15643a

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