Effect of anionic group [SiO4]4−/[PO4]3− on the luminescence properties of Dy3+-doped tungstate structural compounds

Novel scheelite structures of Li2Ca(WO4)2, Li2Ca2(WO4)(SiO4), and LiCa2(WO4)(PO4) fluorescent materials were successfully prepared using a high-temperature solid-phase process. The compounds were characterized by X-ray diffraction and energy dispersive spectroscopy. The tests revealed that the substitution of [WO4]2− by [SiO4]4− or [PO4]3− tetrahedron in tungstate had no significant influence on the crystal structure of the Li2Ca(WO4)2. When Dy3+ ions were introduced as an activator at an optimum doping concentration of 0.08 mol%, all of the as-prepared phosphors generated yellow light emissions, and the emission peak was located close to 576 nm. Replacing [WO4]2− with [SiO4]4− or [PO4]3− tetrahedron significantly increased the luminescence of the Li2Ca(WO4)2 phosphors. Among them, the LiCa2(WO4)(PO4):0.08Dy3+ phosphor had the best luminescence properties, decay life (τ = 0.049 ms), and thermal stability (87.8%). In addition, the as-prepared yellow Li2Ca(WO4)2:0.08Dy3+, Li2Ca2(WO4)(SiO4):0.08Dy3+, and LiCa2(WO4)(PO4):0.08Dy3+ phosphor can be used to fabricate white light emitting diode (LED) devices.


Introduction 
Inorganic luminescent materials have historically attracted considerable attention due to their characteristics such as long decay lifetime [1], easy preparation, low toxicity, and high luminous efficiency [2][3][4]. Rare earth luminescent materials have considerable application potential because of their unique 4f orbit, which can be pumped to excited energy levels to emit colorful light The luminescent properties of phosphors are affected by the doped rare earth ions, as well as the structure and composition of the substance host [11,12]. Tungstate materials are considered to be effective self-activated fluorescent materials with high-absorption cross-sections [13,14], broadband emissions [15], and high quantum efficiency [16,17]. To fix the weak f-f transitions in trivalent rare earth ions, self-activating compounds may also serve as matrix sensitizers. The lattice of matrix material not only affects the dopant's optical transition, but also reduces its excitation energy [18]. Among various matrix materials, phosphate has a wide band gap and high efficiency, with high thermal stability and chemical durability [19,20]. Silicate has been widely studied as a satisfactory matrix for fluorescent materials due to its excellent optical properties, chemical stability, and structural diversity [21,22]. On the basis of these advantages, [SiO 4 ] 4or [PO 4 ] 3ion substitution of [WO 4 ] 2ion to synthesize a novel scheelite structural matrix in tungstate is considered to be a viable and suitable luminescent material carrier [23]. Combining the stable physical and chemical properties of the matrix materials, Li 2 Ca(WO 4 ) 2 :xDy 3+ , Li 2 Ca 2 (WO 4 )(SiO 4 ):yDy 3+ , and LiCa 2 (WO 4 )(PO 4 ):zDy 3+ phosphors with low energy consumption and high thermal stability were synthesized using a high-temperature solid-phase process, and the luminescence behaviors of Dy 3+ in the three hosts were studied. To verify the effect of [SiO 4 ] 4or [PO 4 ] 3substitution on the properties of the tungstate matrix fluorescent materials and elucidate the effect of anion substitution on their crystal structures, a series of comparative experiments were conducted to evaluate the different properties after the replacement of the crystal phase and anions in the samples. The luminescence properties of the doped samples were discussed in the three modified matrices, including excitation spectrum, emission spectrum, decay time, chromaticity diagram, and luminescence mechanism of the doped ions in them. For their future practical applications, the thermal stability has also been investigated.

1 Material preparation
All of the as-prepared solid-solution samples were synthesized using a typical high-temperature solid-phase reaction method under air atmosphere. The initial raw materials were WO 3 (analytical reagent, AR), SiO 2 (AR), Dy 2 O 3 (99.99%), CaCO 3 (AR), Li 2 CO 3 (AR), and (NH 4 ) 2 HPO 4 (AR) with excessive H 3 BO 3 (AR) as flux, which were all commercially bought from Beijing Chemical Co., China. All of the initial reagents were weighed according to the stoichiometric ratios and mixed by grinding in an agate mortar for more than 10 min. The mixed powders were then calcined in a muffle furnace at 550 ℃ for 2 h, cooled to room temperature naturally, and thoroughly ground again for another 5 min. Afterwards, different anion-regulated compounds were synthesized under varying conditions. The Li 2 Ca(WO 4 ) 2 :xDy 3+ samples were prepared at 1000 ℃ for 3 h in a high-temperature tube furnace at a heating rate of 5 ℃/min. The Li 2 Ca 2 (WO 4 )(SiO 4 ):yDy 3+ samples were synthesized in the high-temperature tube furnace by calcining at the same heating rate to 850 ℃ for 3 h, but the LiCa 2 (WO 4 )(PO 4 ):zDy 3+ samples were calcined at 1400 ℃ for 3 h. All the three kinds of samples were then naturally cooled to room temperature and thoroughly ground in an agate mortar into fine powders for further measurements.

2 Material characterization
The structural properties of the as-prepared phosphors were determined using a Bruker D8 powder X-ray diffractometer (XRD, Germany) with Cu Kα radiation (λ = 0.15406 nm) working at 40 kV and 40 mA. The step scanning rate (2θ ranging from 5° to 130°) was 2.5 s/step with a step size of 0.02°. The excitation and emission spectra of the as-prepared samples were measured using a spectrophotometer (F-4600, HITACHI, Japan) equipped with a 150 W Xe lamp operating at 400 V. The field emission scanning electron microscope (FE-SEM, Hitachi, Japan) equipped with an energy dispersive X-ray spectroscopy (EDS) system was applied to observe the morphology and measure the elemental composition of the as-prepared powders. The decay curves of the samples were recorded by a spectrofluorometer (HORIBA, Jobin Yvon FL3-21, France). A Hitachi F-4600 spectrophotometer with an automatic heating system and a self-controlled electric incinerator was used to determine the temperaturedependent photoluminescence (PL) spectrum.

3 Fabrication of white light emitting diode (LED) devices
To fabricate white LED devices, the commercially available blue BaMgAl 10  coincided diffraction peaks are consistent with previous reports [24], revealing that the investigated three samples possess the same phase as tungstate-structural CaWO 4 (cubic structure, I41/a; space group No. 88) and the doping ions are successfully incorporated into the host lattices. Additionally, the few of peaks in the range of 15°-20° and 25°-30° could be attributed to the Li 2 WO 4 phase (JCPDS Card No. 12-0772), but it would not affect the luminous performance of the main phase [25].

2 Morphology and elemental analysis
SEM imaging and EDS analysis were carried out to observe the morphology and measure the chemical composition of the compounds prepared in this work. Figure 2(a) displays the typical SEM image of the designed Li 2 Ca 2 (WO 4 )(SiO 4 ) sample. As seen from Fig. 2(a), the particles in the sample could be relatively large due to their sintering during calcining. On a randomly selected large particle, EDS analysis was performed. Figure 2(b) shows the EDS spectrum by surface-point elemental analysis, and the measured corresponding elemental composition is presented in

4 Effect of [SiO 4 ] 4and [PO 4 ] 3substituting [WO 4 ] 2ions on the thermal stability of phosphors
In the practical applications of high-power solid state lighting, thermal stability is an important parameter to be considered [30][31][32]. So the thermal stability of the as-obtained Li 2 Ca(WO 4 ) 2 :0.08Dy 3+ , Li 2 Ca 2 (WO 4 )(SiO 4 ): 0.08Dy 3+ , and LiCa 2 (WO 4 )(PO 4 ):0.08Dy 3+ samples was evaluated according to their temperature dependence. The emission spectrum obtained by using 353 nm wavelength light was applied to produce excitation light in a temperature range of 30-300 ℃. Figure 5 clearly shows the dependence of the luminescence intensity of the Dy 3+ ions (576 nm) on temperatures.
As the temperature increased, the luminescence intensity gradually decreased and thermal quenching occurred.
The changes in the luminescence intensity of the three phosphors differed from each other, indicating that the thermal stability of tungsten fluorescent materials could be significantly influenced by the substitution of anions [33]. When the temperature increased to 150 ℃, the luminescence intensity of the Dy 3+ ions in the obtained Li

5 Fluorescence lifetime analysis
We further investigated the effect of anion substitution on the luminescence properties and fluorescence lifetime of the scheelite structural compounds [34,35]. According to the recorded experimental data, the attenuation curves of the three compounds exhibited a second-order exponential decay that can be fitted into the following equation [36]: where I (t) is the luminescence intensity, I 0 denotes the initial integrated emission intensity, A 1 and A 2 are the fitting parameters, t is the time, and τ 1 and τ 2 are the lifetime of the exponential components, respectively. The average decay time τ* is calculated by using the following equation: In this work, the as-obtained Li 2 Ca(WO 4 ) 2 :0.08Dy 3+ , Li 2 Ca 2 (WO 4 )(SiO 4 ):0.08Dy 3+ , and LiCa 2 (WO 4 )(PO 4 ): 0.08Dy 3+ samples (λ ex = 353 nm and λ em = 576 nm) were tested for fluorescence decay life. Figure 6 shows that the τ* of the Li 2  0.08Dy 3+ samples were 0.024 and 0.049 ms, respectively. As expected, the LiCa 2 (WO 4 )(PO 4 ):0.08Dy 3+ sample has the longest fluorescence lifetime, which corresponds to the variations in PL spectrum and thermo-stability previously described in Ref. [37].   ] 2-, the CIE chromaticity coordinates changed slightly, which was not large because the luminescent color of the fluorescent materials was primarily determined by the type of the doped rare earth ions [38]. Therefore, yellow light-emitting phosphors that are effectively excited by the near-ultraviolet light chip could be obtained by doping the aforementioned matrix material with Dy 3+ ions.

6 CIE spectrum coordination
To evaluate the potential applications of the present three kinds of fluorescent materials as yellow phosphors for white LEDs, the as-obtained Li 2 Ca(WO 4 ) 2 :0.08Dy 3+ , Li 2 Ca 2 (WO 4 )(SiO 4 ):0.08Dy 3+ , and LiCa 2 (WO 4 )(PO 4 ): 0.08Dy 3+ phosphors were used to fabricate white light LED prototypes, which are named as LED1, LED2, and LED3, respectively. As seen from Fig. 7(b), bright white light can be clearly observed with the as-fabricated devices. Among them, the LED1 exhibited a bright white light with a CCT = 8095 K, Ra = 47, and the CIE coordinates of (0.2776, 0.3536); the LED2 displayed a bright white light with a CCT =7766 K, Ra = 55, and the CIE coordinates of (0.2848, 0.3495); and the LED3 also presented a bright white light with a CCT = 6291 K, Ra = 50, and the CIE coordinates of (0.3145, 0.3477). All these results reveal that the as-prepared yellow phosphors might be promising candidates for white LED luminescent materials.

Conclusions
Three Dy 3+ -doped scheelite structural Li 2 Ca(WO 4 ) 2 : 0.08Dy 3+ , Li 2 Ca 2 (WO 4 )(SiO 4 ):0.08Dy 3+ , and LiCa 2 (WO 4 ) (PO 4 ):0.08Dy 3+ phosphors were successfully prepared through high-temperature solid-phase process. The prepared phosphors had a wide excitation band in 320-430 nm, which could be matched with the commercially available LED chips. These phosphors mainly emit yellow light peaks at 576 nm, and the optimal doping concentration was determined to be 0.08 mol%. When [SiO 4 ] 4or [PO 4 ] 3replaced [WO 4 ] 2-, the luminescent properties, thermal stability, and lifetime decay curves of the phosphors have been significantly improved. Finally, all the three phosphors can be used to fabricate white light LED devices, in which the LiCa 2 (WO 4 )(PO 4 ):0.08Dy 3+ phosphor has the best performance. All these results reveal that the as-prepared yellow phosphors might be promising candidates for white LED luminescent materials.