Luminescent Properties and Charge Compensator Effects of SrMo0.5W0.5O4:Eu3+ for White Light LEDs

The high-temperature solid-phase approach was used to synthesize Eu3+-doped SrMo0.5W0.5O4 phosphors, whose morphological structure and luminescence properties were then characterized by XRD, SEM, FT-IR, excitation spectra, emission spectra, and fluorescence decay curves. The results reveal that the best phosphor synthesis temperature was 900 °C and that the doping of Eu3+ and charge compensators (K+, Li+, Na+, NH4+) had no effect on the crystal phase change. SrMo0.5W0.5O4:Eu3+ has major excitation peaks at 273 nm, 397 nm, and 464 nm, and a main emission peak at 615 nm, making it a potential red fluorescent material to be used as a down converter in UV LEDs (273 nm and 397 nm) and blue light LEDs (464 nm) to achieve Red emission. The emission spectra of Sr1−yMo0.5W0.5O4:yEu3+(y = 0.005, 0.01, 0.02, 0.05, 0.07) excited at 273 were depicted, with the Eu3+ concentration increasing the luminescence intensity first increases and then decreases, the emission peak intensity of SrMo0.5W0.5O4:Eu3+ achieves its maximum when the doping concentration of Eu3+ is 1%, and the critical transfer distance is calculated as 25.57 Å. When various charge compensators such as K+, Li+, Na+, and NH4+ are added to SrMo0.5W0.5O4:Eu3+, the NH4+ shows the best effect with the optimal doping concentration of 3wt%. The SrMo0.5W0.5O4:Eu3+,NH4+ color coordinate is (0.656,0.343), which is close to that of the ideal red light (0.670,0.333).


Introduction
As a new generation of the light source of solid-state lighting, white light-emitting diodes (hereafter referred to as the white light LEDs, w-LEDs, etc.) have piqued the interest of scholars both at home and abroad for their high efficiency, energy savings, and environmental protection advantages [1][2][3][4]. The white light LED used to be created by combining a GaN chip that emits blue light with yellow phosphors (YAG:Ce 3+ ) that can be effectively excited by blue light [5]. However, this approach typically generates a low color rendering index because of the lack of red light in the emission spectrum of the yellow phosphor. The solution is to add red phosphors that can be efficiently excited by blue light [6,7] or use the high-efficiency UV LED and the phosphors that can be excited by it [8,9]. As a result, it is critical to investigate red phosphors that can be successfully stimulated by blue light and UV light.
It is well known that Eu 3+ is an outstanding rare earth ion generating red light and can be effectively stimulated by blue light and UV light [10][11][12][13][14]. For instance, a new red phosphor Sr 3 NaSbO 6 :Eu 3+ doped with Eu 3+ was developed, and its emission spectra under excitation at 285 nm is located 500-700 nm, with the primary peak at 618 nm, indicating that this phosphor is a red phosphor that can be successfully stimulated by UV light [15]. Li 2.06 Nb 0. 18 Ti 0.76 O 3 :Eu 3+ phosphors by sol-gel method were prepared. When the doping proportion of Eu 3+ is x = 3 wt%, the primary excitation peak is at 396 nm, the central emission peak is at 612 nm, and its color coordinate is better than the commercial red  Figure 1a shows that the XRD patterns' peak positions and relative intensities of the XRD patterns of the sample SrMoO 4 at temperatures of 850 • C, 900 • C, 950 • C, and 1000 • C are essentially the same, which is consistent with the standard card of SrMoO 4 (JCPDS 08-0482), indicating that the synthesized samples have a tetragonal crystal system with space group I41/a, and its unit cell data are a = b = 5.3909 Å, c = 12.0118 Å and α = β = γ = 90 • . Strontium molybdate can be synthesized at these temperatures without forming an impurity phase. Furthermore, the highest peak intensity was discovered in the sample synthesized at 900 • C, indicating that the crystallinity of the sample is better at this temperature. As a result, the temperature to synthesize SrMoO 4 is set to 900 • C. Figure 1b displays that the XRD patterns' peak positions and relative intensities of the XRD patterns of the sample SrMo 0.5 W 0.5 O 4 at those temperatures are essentially consistent with the standard card of SrMoO 4 (JCPDS 08-0482), demonstrating that the synthesized samples have the structure of SrMoO 4 , and no new phase is formed. Due to the lanthanide contraction, the atomic and ionic radii of Mo and W, the second and third transition elements in the same group are very close (the atomic radii of Mo and W are both 139 pm, and the ionic radii of Mo(VI) and W(VI) are 59 pm and 60 pm, respectively), and their properties are quite similar. Besides, the structures of MoO 4 2− and WO 4 2− are the same. As a result, WO 4 2− can easily replace MoO 4 2− to form a solid solution. The peak intensity of the XRD pattern of SrMo 0.5 W 0.5 O 4 at 900 • C is higher, indicating that the sample's crystallinity is better at this temperature. As a result, 900 • C is the optimal synthesis temperature for SrMo 0.5 W 0.5 O 4 . Figure 1c shows that the XRD patterns of SrWO 4 synthesized at temperatures of 850 • C, 900 • C, 950 • C, and 1000 • C are consistent with the standard card of SrWO 4 (JCPDS 08-0490), indicating that the synthesized samples have a tetragonal crystal structure with the space group is I41/a (88), and that can synthesize pure phase strontium tungstate at these temperatures. Because the XRD peak of SrWO 4 synthesized at 900 • C is the strongest, 900 • C is the best SrWO 4 synthesis temperature.
be synthesized at these temperatures without forming an impurity phase. Furthermore, the highest peak intensity was discovered in the sample synthesized at 900 ℃, indicating that the crystallinity of the sample is better at this temperature. As a result, the temperature to synthesize SrMoO4 is set to 900 ℃. Figure 1b displays that the XRD patterns' peak positions and relative intensities of the XRD patterns of the sample SrMo0.5W0.5O4 at those temperatures are essentially consistent with the standard card of SrMoO4 (JCPDS 08-0482), demonstrating that the synthesized samples have the structure of SrMoO4, and no new phase is formed. Due to the lanthanide contraction, the atomic and ionic radii of Mo and W, the second and third transition elements in the same group are very close (the atomic radii of Mo and W are both 139 pm, and the ionic radii of Mo (Ⅵ) and W(Ⅵ) are 59 pm and 60 pm, respectively), and their properties are quite similar. Besides, the structures of MoO4 2− and WO4 2− are the same. As a result, WO4 2− can easily replace MoO4 2− to form a solid solution. The peak intensity of the XRD pattern of SrMo0.5W0.5O4 at 900 ℃ is higher, indicating that the sample's crystallinity is better at this temperature. As a result, 900 ℃ is the optimal synthesis temperature for SrMo0.5W0.5O4. Figure 1c shows that the XRD patterns of SrWO4 synthesized at temperatures of 850 ℃, 900 ℃, 950 ℃, and 1000 ℃ are consistent with the standard card of SrWO4 (JCPDS 08-0490), indicating that the synthesized samples have a tetragonal crystal structure with the space group is I41/a (88), and that can synthesize pure phase strontium tungstate at these temperatures. Because the XRD peak of SrWO4 synthesized at 900 °C is the strongest, 900 °C is the best SrWO4 synthesis temperature.  Figure 2a shows that the diffraction peaks of the Sr1−xMo0.5W0.5O4:xEu 3+ XRD pattern are in line with the standard card #JCPDS 08-0482 (SrMoO4), indicating that the doping of Eu 3+ in the SrMo0.5W0.5O4 system did not cause phase change and no new phase was created. Rare earth metal Eu and alkaline-earth metal Sr have similar atomic and ionic radii  radii of Eu 3+ and Sr 2+ are 112 pm and 94.7 pm, respectively). When Eu 3+ is doped into the SrMo 0.5 W 0.5 O 4 system, it takes the position of Sr 2+ and creates a continuous solid solution. It has been reported that the O 2− is created in the system due to the imbalance in electrovalence as a result of the unequal substitution of Sr 2+ with Eu 3+ [23]. According to Figure 2b, the diffraction peaks of the XRD pattern of the phosphor Sr 0.99 MoO 4 :0.01Eu 3+ are compatible with the standard card #JCPDS 08-0482 (SrMoO 4 ), indicating that the sample forms pure phase SrMoO 4 , and no additional phases are created. That is to say, 1% Eu 3+ can be added to SrMoO 4 without generating a phase shift. As seen in Figure 2c, the diffraction peaks of the XRD pattern of the phosphor Sr 0.99 WO 4 :0.01Eu 3+ are consistent with the standard card #JCPDS 08-0490 (SrWO 4 ), indicating that the pure phase can still be obtained formed by doping 1% Eu 3+ in SrWO 4 , and no additional substances form. It has been reported that the O 2− is created in the system due to the imbalance in electrovalence as a result of the unequal substitution of Sr 2+ with Eu 3+ [23] . According to Figure 2b, the diffraction peaks of the XRD pattern of the phosphor Sr0.99MoO4:0.01Eu 3+ are compatible with the standard card #JCPDS 08-0482 (SrMoO4), indicating that the sample forms pure phase SrMoO4, and no additional phases are created. That is to say, 1% Eu 3+ can be added to SrMoO4 without generating a phase shift. As seen in Figure 2c, the diffraction peaks of the XRD pattern of the phosphor Sr0.99WO4:0.01Eu 3+ are consistent with the standard card #JCPDS 08-0490 (SrWO4), indicating that the pure phase can still be obtained formed by doping 1% Eu 3+ in SrWO4, and no additional substances form. The electrovalent imbalance induced by the unequal substitution of Sr 2+ with Eu 3+ in the Sr0.99Mo0.5W0.5O4:0.01Eu 3+ system can be rectified by adding charge compensators [31]. Figure 3 depicts the XRD patterns of SrMo0.5W0.5O4:Eu 3+ after doping with various charge compensators. Figure 3 shows that the XRD diffraction peaks of SrMo0.5W0.5O4: Eu 3+ after adding charge compensator K2CO3, Li2CO3, Na2CO3, NH4Cl are essentially consistent with the standard card of SrMoO4 (JCPDS 08-0482), that is, there is no charge in the lattice of SrMo0.5W0.5O4:Eu 3+ , and the phase is still SrMo0.5W0.5O4. The electrovalent imbalance induced by the unequal substitution of Sr 2+ with Eu 3+ in the Sr 0.99 Mo 0.5 W 0.5 O 4 :0.01Eu 3+ system can be rectified by adding charge compensators [31]. Figure 3 depicts the XRD patterns of SrMo 0.5 W 0.5 O 4 :Eu 3+ after doping with various charge compensators. Figure 3 shows that the XRD diffraction peaks of SrMo 0.5 W 0.5 O 4 : Eu 3+ after adding charge compensator K 2 CO 3 , Li 2 CO 3 , Na 2 CO 3 , NH 4 Cl are essentially consistent with the standard card of SrMoO 4 (JCPDS 08-0482), that is, there is no charge in the lattice of SrMo 0.5 W 0.5 O 4 :Eu 3+ , and the phase is still SrMo 0.5 W 0.5 O 4 . SrMo 0.5 W 0.5 O 4 has a tetragonal crystal system with a scheelite structure, and each of its units contains one Sr site, one Mo/W site, and four O sites. According to Figure 4, there is only one type of cationic site, Sr, in the lattice, and each, on average, has eight coordinated oxygen ions which include four MoO 4   SrMo0.5W0.5O4 has a tetragonal crystal system with a scheeli its units contains one Sr site, one Mo/W site, and four O sites. Acc is only one type of cationic site, Sr, in the lattice, and each, on av nated oxygen ions which include four MoO4 2− /WO4 2− that belong have no inversion center. Each central W/Mo site is coordinated forming a MoO4 2− /WO4 2− tetrahedron. As the MoO4 2− /WO4 2− tetra quite stable, SrMo0.5W0.5O4 retains its lattice structure when Sr 2+ is The FT-IR spectrum of the sample SrMo0.5W0.5O4 was obtai  SrMo0.5W0.5O4 has a tetragonal crystal system with a its units contains one Sr site, one Mo/W site, and four O si is only one type of cationic site, Sr, in the lattice, and eac nated oxygen ions which include four MoO4 2− /WO4 2− that have no inversion center. Each central W/Mo site is coo forming a MoO4 2− /WO4 2− tetrahedron. As the MoO4 2− /W quite stable, SrMo0.5W0.5O4 retains its lattice structure whe The FT-IR spectrum of the sample SrMo0.5W0.5O4 w disc method. As shown in Figure 5, the FT-IR spectra of sorption peaks at 818 cm −1 , 1630 cm −1 , and 3420 cm −1 , w cm −1 corresponds to the stretching vibration of O-W/M WO4 2− and MoO4 2− groups in the prepared samples. The ab 3420 cm −1 are respectively attributed to the bending an causing the water vapor on the surface of the SrMo0.5W0.5 The FT-IR spectrum of the sample SrMo 0.5 W 0.5 O 4 was obtained by the KBr pressed disc method. As shown in Figure 5 Figure 6 shows the SEM photos of the phosphor Sr0 using a high-temperature solid phase technique Sr0.99Mo0.5W0.5O4:0.01Eu 3+ has sharp edges and corners, an size of around 2 μm, with agglomeration produced by preparation. Figure 6. SEM of Sr0.99Mo0.5W0.5O4:Eu 3+ .   Figure 6 shows the SEM photos of the phosphor Sr0.99Mo0.5W0.5O4:Eu 3+ synthe using a high-temperature solid phase technique at 900 °C. The pho Sr0.99Mo0.5W0.5O4:0.01Eu 3+ has sharp edges and corners, an irregular form, and a pa size of around 2 μm, with agglomeration produced by high-temperature solidpreparation.

Analysis of Luminescence Performance
Molecules 2023, 28, x FOR PEER REVIEW implying that Eu 3+ is located in the non-inversion symmetry center lattice site of ho tices of SrWO4:Eu 3+ , SrMo0.5W0.5O4: Eu 3+ , and SrMoO4:Eu 3+ . So SrMoO4:Eu 3+ can be u a down converter in UV LEDs and blue light LEDs to achieve red emission.    Figure 9a shows that all samples' peak forms and po remain constant. However, with the Eu 3+ concentration increasing, the luminescen tensity first increases and then decreases. The emission peak intensi Sr0.99Mo0.5W0.5O4:0.01Eu 3+ achieves its maximum when the doping concentration of 1%, and if the concentration of Eu 3+ continues to increase, the phenomenon of conc tion quenching appears. This is because although the transition of emitted light inc with the increase of the Eu 3+ concentration, which can effectively improve the inten the emitted light, the continuous increase of the doping amount of Eu 3+ will narro distance between Eu 3+ , resulting in a decrease in emission intensity due to nonrad energy transfer between Eu 3+ . To look into the energy transfer of Eu 3+ ions in SrMo0.5W the critical distance of Eu 3+ ions is first estimated using the formula below.
The critical distance Rc can be computed using the Blass theory formula [34]: In this equation, V denotes the unit cell volume, Xc is the critical concentration in SrMo0.5W0.5O4(the optimal doping concentration), and N denotes the number of c per unit cell of SrMo0.5W0.5O4 crystal. Figure 9a shows the critical threshold concen of Eu 3+ is 0.01 in SrMo0.5W0.5O4 crystal, N = 4, V = 349.78 Å 3 . According to the Blass fo Rc = 25.57 Å . In general, non-radiative energy transfer modes are broadly classif electron exchange interaction and electric multipole interaction. When the critical di Rc is around 5 Å , the non-radiative energy transfer mode is electron exchange inter When Rc reaches 25.57 Å , much more than 5 Å , the energy transfer between E SrMo0.5W0.5O4: Eu 3+ is electric multipolar interaction.
The energy transfer formula for the electric multipole interaction can be deriv ing Van Uitert's theory [35]: In this formula, I is the integrated emission intensity, X is the activator concen above the critical concentration, and K and β are constants for a given matrix. Ana the constant θ confirms the energy transfer mode of the electric multipole interactio the number of cations in the unit cell of SrMo0.5W0.5O4 crystal can be deduced. θ = 6, 10 correspond to dipole-dipole (d-d), dipole-quaternary (d-q), and quaternary-quat Intensity/a.u.
Wavelength/nm x=0 x=0.5 x=1   Figure 9a shows that all samples' peak forms and positions remain constant. However, with the Eu 3+ concentration increasing, the luminescence intensity first increases and then decreases. The emission peak intensity of Sr 0.99 Mo 0.5 W 0.5 O 4 :0.01Eu 3+ achieves its maximum when the doping concentration of Eu 3+ is 1%, and if the concentration of Eu 3+ continues to increase, the phenomenon of concentration quenching appears. This is because although the transition of emitted light increases with the increase of the Eu 3+ concentration, which can effectively improve the intensity of the emitted light, the continuous increase of the doping amount of Eu 3+ will narrow the distance between Eu 3+ , resulting in a decrease in emission intensity due to nonradiative energy transfer between Eu 3+ . To look into the energy transfer of Eu 3+ ions in SrMo 0.5 W 0.5 O 4 , the critical distance of Eu 3+ ions is first estimated using the formula below. The partial substitution of Sr 2+ by Eu 3+ in SrMo0.5W0.5O4:Eu 3+ will result in a charge imbalance, leading to excessive charge defects in the lattice and thus decreasing the phosphor luminous efficiency. However, adding the right amount of good charge compensator can increase the sample's luminous efficiency [31]. Figure 10 depicts the emission spectra of phosphors SrMo0.5W0.5O4: Eu 3+ , M (M = K + , Li + , Na + , NH4 + ) doped with various charge compensators. The addition of the charge compensator doesn't modify the position of the emission peak of SrMo0.5W0.5O4: Eu 3+ . Various charge compensators have different effects on the luminescence intensity of SrMo0.5W0.5O4:Eu 3+ , but their doping will improve the luminescence intensity, with NH4 + having the best effect. The critical distance R c can be computed using the Blass theory formula [34]: In this equation, V denotes the unit cell volume, X c is the critical concentration of Eu 3+ in SrMo 0.5 W 0.5 O 4 (the optimal doping concentration), and N denotes the number of cations per unit cell of SrMo 0.5 W 0.5 O 4 crystal. Figure 9a shows the critical threshold concentration of Eu 3+ is 0.01 in SrMo 0.5 W 0.5 O 4 crystal, N = 4, V = 349.78 Å 3 . According to the Blass formula, R c = 25.57 Å. In general, non-radiative energy transfer modes are broadly classified as electron exchange interaction and electric multipole interaction. When the critical distance R c is around 5 Å, the non-radiative energy transfer mode is electron exchange interaction. When R c reaches 25.57 Å, much more than 5 Å, the energy transfer between Eu 3+ in SrMo 0.5 W 0.5 O 4 : Eu 3+ is electric multipolar interaction.
The energy transfer formula for the electric multipole interaction can be derived using Van Uitert's theory [35]: In this formula, I is the integrated emission intensity, X is the activator concentration above the critical concentration, and K and β are constants for a given matrix. Analyzing the constant θ confirms the energy transfer mode of the electric multipole interaction, and the number of cations in the unit cell of SrMo 0.5 W 0.5 O 4 crystal can be deduced. θ = 6, 8, and 10 correspond to dipole-dipole (d-d), dipole-quaternary (d-q), and quaternary-quaternary (q-q) interactions, respectively. Figure 9b reveals the connection between log(I/X) and log(X) of SrMo 0.5 W 0.5 O 4 : Eu 3+ . If the slope -1.64 is -θ/3, then θ will be 4.92, which the value is closer to 6. As a result, the electric dipole-electric dipole (d-d) interaction causes the quenching concentration in Sr 1−x Mo 0.5 W 0.5 O 4 :xEu 3+ .
The partial substitution of Sr 2+ by Eu 3+ in SrMo 0.5 W 0.5 O 4 :Eu 3+ will result in a charge imbalance, leading to excessive charge defects in the lattice and thus decreasing the phosphor luminous efficiency. However, adding the right amount of good charge compensator can increase the sample's luminous efficiency [31]. Figure 10  The partial substitution of Sr 2+ by Eu 3+ in SrMo0.5W0.5O4:Eu 3+ will result in a char imbalance, leading to excessive charge defects in the lattice and thus decreasing the pho phor luminous efficiency. However, adding the right amount of good charge compensat can increase the sample's luminous efficiency [31]. Figure 10 depicts the emission spect of phosphors SrMo0.5W0.5O4: Eu 3+ , M (M = K + , Li + , Na + , NH4 + ) doped with various char compensators. The addition of the charge compensator doesn't modify the position of t emission peak of SrMo0.5W0.5O4: Eu 3+ . Various charge compensators have different effe on the luminescence intensity of SrMo0.5W0.5O4:Eu 3+ , but their doping will improve the l minescence intensity, with NH4 + having the best effect.  Figure 11 depicts the luminescence intensity of Sr0.99Mo0.5W0.5O4:0.01Eu 3+ at vario NH4 + doping concentrations (0%, 3%, 6%, 10%, 15%). The figure shows that when the co centration of NH4 + is low, the luminescence intensity of the sample increases as the co centration of NH4 + increases. The sample's emission peak intensity reaches its maximu highest when the NH4 + doping concentration is 3%. As the concentration of NH4 + conti ues to increase, concentration quenching will occur. This is due to the fact that when t concentration of NH4 + is low, NH4 + can replace the position of Sr 2+ in the lattice, loweri the symmetry of the lattice and modifying the local crystal field environment around Eu which eventually increases the sample's luminescence performance [36,37]; At the sam  Figure 11 depicts the luminescence intensity of Sr 0.99 Mo 0.5 W 0.5 O 4 :0.01Eu 3+ at various NH 4 + doping concentrations (0%, 3%, 6%, 10%, 15%). The figure shows that when the concentration of NH 4 + is low, the luminescence intensity of the sample increases as the concentration of NH 4 + increases. The sample's emission peak intensity reaches its maximum highest when the NH 4 + doping concentration is 3%. As the concentration of NH 4 + continues to increase, concentration quenching will occur. This is due to the fact that when the concentration of NH 4 + is low, NH 4 + can replace the position of Sr 2+ in the lattice, lowering the symmetry of the lattice and modifying the local crystal field environment around Eu 3+ , which eventually increases the sample's luminescence performance [36,37]; At the same time, due to the difference in the quantities of electric charges of NH 4 + and Sr 3+ , oxygen vacancies will be formed after replacing Sr 2+ in order to maintain the electrical neutrality of NH 4 + . These oxygen vacancies can transfer charge with Eu 3+ [34], thereby increasing the sample's luminescence intensity. On the other hand, the excess NH 4 + will enter the lattice gaps and induce lattice distortions, affecting the luminescence intensity of the samples.
olecules 2023, 28, x FOR PEER REVIEW 10 of NH4 + . These oxygen vacancies can transfer charge with Eu 3+ [34], thereby increasing sample's luminescence intensity. On the other hand, the excess NH4 + will enter the la gaps and induce lattice distortions, affecting the luminescence intensity of the sampl Figure 11. Emission spectra of SrMo0.5W0.5O4:Eu 3+ with different concentrations of NH4 + . Figure 12 shows the luminescence decay curves of SrMo0.5W0.5O4:Eu 3+ phosp doped with several charge compensators (K + , Li + , Na + , NH4 + ) at an excitation wavele of 464 nm and an emission wavelength of 615 nm. As illustrated in Figure 12, the d curves of all samples' emitted light satisfy a bi-exponential equation [38]: In the formula, I(t) denotes the emission intensity at time t, I0 represents the in emission intensity, A1 and A2 are the pre-exponential factors of each decay compon and τ1 and τ2 are the decay times of each component. The average emission decay (τave) can be calculated using the below [38].
The average emission decay time τave shown in Figure 12, was calculated to be ms for Sr0.99Mo0.5W0.5O4:0.01Eu 3+ and 0.0.51, 0.0.57, 0.56, and 0.0.58 ms Sr0.99Mo0.5W0.5O4:0.01Eu 3+ , A (A = Li + , Na + , K + , NH4 + ), respectively. The emission d times of all ceramic samples were very similar and slightly lower than that of the pow sample. This suggests that, in the ceramic samples, the electronic relaxation time from split 5 D2 energy levels to the lowest transition energy level 5 D0 was reduced. When charge compensator NH4 + concentration is 3% in the Sr0.99Mo0.5W0.5O4:0.01Eu 3+ , A (A Li + , Na + , NH4 + ) system, the fluorescence lifespan of the sample achieves a maximum 0.58ms. It is also demonstrated that adding NH4 + can significantly improve the lumi cent characteristics of the samples.  Figure 12 shows the luminescence decay curves of SrMo 0.5 W 0.5 O 4 :Eu 3+ phosphors doped with several charge compensators (K + , Li + , Na + , NH 4 + ) at an excitation wavelength of 464 nm and an emission wavelength of 615 nm. As illustrated in Figure 12, the decay curves of all samples' emitted light satisfy a bi-exponential equation [38]: Figure 11. Emission spectra of SrMo0.5W0.5O4:Eu 3+ with different concentrations of NH4 + . Figure 12 shows the luminescence decay curves of SrMo0.5W0.5O4:Eu 3+ ph doped with several charge compensators (K + , Li + , Na + , NH4 + ) at an excitation wav of 464 nm and an emission wavelength of 615 nm. As illustrated in Figure 12, th curves of all samples' emitted light satisfy a bi-exponential equation [38]: In the formula, I(t) denotes the emission intensity at time t, I0 represents th emission intensity, A1 and A2 are the pre-exponential factors of each decay com and τ1 and τ2 are the decay times of each component. The average emission dec (τave) can be calculated using the below [38].
The average emission decay time τave shown in Figure 12, was calculated to ms for Sr0.99Mo0.5W0.5O4:0.01Eu 3+ and 0.0.51, 0.0.57, 0.56, and 0.0.58 Sr0.99Mo0.5W0.5O4:0.01Eu 3+ , A (A = Li + , Na + , K + , NH4 + ), respectively. The emissio times of all ceramic samples were very similar and slightly lower than that of the sample. This suggests that, in the ceramic samples, the electronic relaxation time f split 5 D2 energy levels to the lowest transition energy level 5 D0 was reduced. W charge compensator NH4 + concentration is 3% in the Sr0.99Mo0.5W0.5O4:0.01Eu 3+ , A Li + , Na + , NH4 + ) system, the fluorescence lifespan of the sample achieves a maxim 0.58ms. It is also demonstrated that adding NH4 + can significantly improve the lu cent characteristics of the samples. In the formula, I(t) denotes the emission intensity at time t, I 0 represents the initial emission intensity, A 1 and A 2 are the pre-exponential factors of each decay component, and τ 1 and τ 2 are the decay times of each component. The average emission decay time (τ ave ) can be calculated using the below [38].

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The average emission decay time τ ave shown in Figure 12, was calculated to be 0.57 ms for Sr 0.99 Mo 0.5 W 0.5 O 4 :0.01Eu 3+ and 0.0.51, 0.0.57, 0.56, and 0.0.58 ms for Sr 0.99 Mo 0.5 W 0.5 O 4 :0.01Eu 3+ , A (A = Li + , Na + , K + , NH 4 + ), respectively. The emission decay times of all ceramic samples were very similar and slightly lower than that of the powder sample. This suggests that, in the ceramic samples, the electronic relaxation time from the split 5 D 2 energy levels to the lowest transition energy level 5 D 0 was reduced. When the charge compensator NH 4 + concentration is 3% in the Sr 0.99 Mo 0.5 W 0.5 O 4 :0.01Eu 3+ , A (A = K + , Li + , Na + , NH 4 + ) system, the fluorescence lifespan of the sample achieves a maximum of 0.58ms. It is also demonstrated that adding NH 4 + can significantly improve the luminescent characteristics of the samples.

Sample Testing and Characterization
The structures of the samples were studied using a Bruker (Billerica, MA, USA) AXS D8 X-ray diffractometer (XRD), with Cu Kα lines as a radiation source. An operating voltage of 40 KV, An operating current of 30 mA, and a scanning range of 2θ = 15°-80°; the microscopic morphology of the samples was characterized using a JSM-6490LV scanning

Sample Preparation
All samples were synthesized in an air atmosphere using a high-temperature solid phase method. The raw materials included SrCO 3 (A.R.), MoO 3 (A.R.), WO 3 (A.R.), Eu 2 O 3 (99.99%), Na 2 CO 3 (A.R.), Li 2 CO 3 (A.R.), K 2 CO 3 (A.R.), and NH 4 Cl (A.R.). They were accurately weighed based on the stoichiometric ratio of Sr (1−y) Mo x W 1−x O 4 :yEu 3+ , transferred to an agate mortar, added a tiny amount of anhydrous ethanol, ground for 30 min, then transferred the blended powder was to a high-temperature furnace and calcined at a certain temperature for 5 h.

Sample Testing and Characterization
The structures of the samples were studied using a Bruker (Billerica, MA, USA) AXS D8 X-ray diffractometer (XRD), with Cu Kα lines as a radiation source. An operating