Synthesis and Luminescence Properties of Eu2+-Doped Sr3MgSi2O8 Blue Light-Emitting Phosphor for Application in Near-Ultraviolet Excitable White Light-Emitting Diodes

In this study, [Sr0.99Eu0.01]3MgSi2O8 phosphors were sintered at 1200–1400 °C for 1–5 h by using the solid-state reaction method. The crystallinity and morphology of these phosphors were characterized through X-ray diffraction analysis and field-emission scanning electron microscopy, respectively, to determine their luminescence. The photoluminescence properties, including the excitation and emission properties, of the prepared phosphors were investigated through fluorescence spectrophotometry. The α-Sr2SiO4, Sr2MgSi2O7, and Sr3MgSi2O8 phases coexisted in the [Sr0.99Eu0.01]3MgSi2O8 phosphors, which were synthesized at low temperatures. The particles of these phosphors had many fine hairs on their surface and resembled Clavularia viridis, which is a coral species. Transmission electron microscopy and energy dispersive X-ray spectroscopy indicated that the fine hairs contained the Sr2SiO4 and Sr2MgSi2O7 phases. However, when the [Sr0.99Eu0.01]3MgSi2O8 phosphors were sintered at 1400 °C, the Sr3MgSi2O8 phase was observed, and the Eu2+-doped Sr3MgSi2O8 phase dominated the only broad emission band, which had a central wavelength of 457 nm (blue light). The emission peaks at this wavelength were attributed to the 4f65d1–4f7 transition at the Sr2+(I) site, where Sr2+ was substituted by Eu2+. The average decay time of the synthesized phosphors was calculated to be 1.197 ms. The aforementioned results indicate that [Sr0.99Eu0.01]3MgSi2O8 can be used as a blue-emitting phosphor in ultraviolet-excited white light-emitting diodes.


Introduction
White light-emitting diodes (W-LEDs) have replaced conventional incandescent and fluorescent lamps for general illumination. Historically, artificial lighting is energy-intensive, with incandescent lamps exhibiting a luminous efficiency of only 2% and quartz halogen and fluorescent lamps reaching 4% and 15%, respectively, with most of the energy input converted to waste heat. In contrast, solid-state lighting based on W-LEDs currently attains ∼32% luminous efficiency. W-LEDs are a novel high-efficiency lighting system and fourth-generation illumination source with many advantages, including a long lifetime, high rendering index, high luminosity efficiency, low energy consumption, chemical stability, thermal stability, and eco-friendliness [1][2][3]. W-LEDs have superior luminescence characteristics relative to other lighting sources [4]. W-LEDs have many applications in various domains, such as lighting [5], biomedicine [6], communication [7], liquid crystal displays (as backlight sources) [8], and architecture [9]. However, there are several important luminescence parameters that characterize and determine the quality of W-LEDs, including

Preparation of the [Sr 1−x Eu x ] 3 MgSi 2 O 8 Phosphors
In this study, [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors were synthesized using the solid-state reaction method. The raw materials used in this synthesis were SrCO 3 (Sigma-Aldrich, St. Louis, MO, USA, 99.99%), MgO (Sigma-Aldrich, USA, 99.99%), SiO 2 (Sigma-Aldrich, USA, 99.99%), and Eu 2 O 3 (Sigma-Aldrich, USA, 99.99%) powders. These powders were mixed and ground in deionized water for 1 h by using the ball-milling method. ZrO 2 balls with a diameter of 5-8 mm were used to grind the powders. The powder mixture was then dried at 120 • C for 24 h in an oven. After drying, the mixture was ground in an agate mortar for 1 h and then calcined at 850 • C for 2 h. The mixture was placed in alumina crucibles and put in the tubular furnaces. Then, a vacuum was created in the tubular furnaces by using the mechanical pump. Finally, the reducing gas (4 vol% H 2 /96 vol% N 2 ) was led into the tubular furnaces, and the mixture was sintered at 1200-1400 • C for 1-10 h in a reducing atmosphere.

Measurements
The crystalline structures of the prepared [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors were investigated using a ceramic X-ray diffraction (XRD) source that emitted CuKα radiation (λ = 1.5406 Å). The microstructures of the phosphors were analyzed through field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The PL spectra and PL excitation (PLE) spectra were obtained using a Hitachi F-7000 spectrofluorometer with a 150-W xenon lamp as the light source. The luminance and International Commission on Illumination [Commission Internationale de l'Eclairage (CIE)] coordinates were measured using the CS-100A Konica Minolta chroma meter. All the measurements were performed at room temperature.

Results and Discussion
The XRD patterns of the prepared [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors were obtained to verify their crystal structures. Figure 1 shows the diffraction peaks of the [Sr 0.99 Eu 0.01 ] 3 Figure 2, in the phosphors, each Si atom was surrounded by four oxygen atoms, which resulted in the formation of a four-coordination [SiO 4 ] tetrahedral structure. Moreover, each Mg atom was surrounded by six oxygen atoms, which resulted in the formation of a [MgO 6 ] octahedron. A Sr atom could occupy three available sites, which were located in different crystallographic environments.    .01]3MgSi2O8 phosphors sintered at 1300 °C for different durations. When the sintering time was 1 h, the synthesized [Sr0.99Eu0.01]3MgSi2O8 phosphors exhibited a special surface morphology. The particles of these phosphors appeared similar to Clavularia viridis, which is a coral species, and exhibited many fine hairs on their surface. The number of fine hairs on the particle surface decreased as the sintering time increased from 1 to 6 h. In addition, to understand the microstructure of the fine hair, the prepared [Sr0.99Eu0.01]3MgSi2O8 phosphors were subjected to HR-TEM and energy dispersive X-ray spectroscopy (EDS) analyses (Figure 4). At a sintering time of 1 h, the atomic percentages of Sr, Mg, Si, and O in the fine hairs were 25.1%, 27.9%, 1.8%, and 45.2%, respectively. On the basis of this information and the XRD results ( Figure 1), we infer that the Sr2SiO4 and Sr2MgSi2O7 phases were present in the fine hairs at a sintering time of 1 h. The element distribution images of the [Sr0.99Eu0.01]3MgSi2O8 phosphors are shown in Figure S1. The resulting presence of Sr, Si, and Mg can be found, and the element content was similar to the HR-TEM/EDS result (Figure 4). At a sintering time of 5 h, the fine hairs contained Sr, Mg, Si, and O, which indicates that the Sr2MgSi2O7 phase was present in the fine hairs at a sintering time of 5 h, almost the same as the detected atomic percentage and nominal compositions in quantity. The SEM images of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered for different durations, whose BET specific   Figure S1. The resulting presence of Sr, Si, and Mg can be found, and the element content was similar to the HR-TEM/EDS result (Figure 4). At a sintering time of 5 h, the fine hairs contained Sr, Mg, Si, and O, which indicates that the Sr 2 MgSi 2 O 7 phase was present in the fine hairs at a sintering time of 5 h, almost the same as the detected atomic percentage and nominal compositions in quantity. The SEM images of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors sintered for different durations, whose BET specific surface area were 18.4 m 2 /g, 13.5 m 2 /g, 9.4 m 2 /g, 7.2 m 2 /g, 5.8 m 2 /g, and 2.5 m 2 /g, respectively, as shown in Figure 3a-f. Figure 5 [26]. Figure S2 shows the PLE spectra of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors sintered for different durations. These spectra exhibit two broad bands ranging from 240 to 320 nm and from 330 to 410 nm, with peaks at 280 and 350 nm, which are assigned to the transitions between the ground state 4f 7 and the crystal-field split state 4f 6 5d 1 . As the sintering time increased, the excitation intensity increased and reached a maximum value at a sintering time of 5 h. The aforementioned results demonstrate that as the sintering duration increased from 1 to 5 h, the crystallinity (Figure 1), particle morphologies and sizes (Figure 3), and PLE intensities of the phosphors increased. surface area were 18.4 m 2 /g, 13.5 m 2 /g, 9.4 m 2 /g, 7.2 m 2 /g, 5.8 m 2 /g, and 2.5 m 2 /g, respectively, as shown in Figure 3a-f.   .01]3MgSi2O8 phosphors sintered at 1300 °C for 5 h. The Eu 2+ excitation band of the [Sr0.99Eu0.01]3MgSi2O8 phosphors can be fitted into two Gaussian components with peaks at 280 and 350 nm, which correspond to the 4f 7 ( 8 S7/2)→4f 6 5d 1 (t2g) electron transition of Eu 2+ [26]. Figure S2 shows the PLE spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered for different durations. These spectra exhibit two broad bands ranging from 240 to 320 nm and from 330 to 410 nm, with peaks at 280 and 350 nm, which are assigned to the transitions between the ground state 4f 7 and the crystal-field split state 4f 6 5d 1 . As the sintering time increased, the excitation intensity increased and reached a maximum value at a sintering time of 5 h. The aforementioned results demonstrate that as the sintering duration increased from 1 to 5 h, the crystallinity (Figure 1), particle morphologies and sizes (Figure 3), and PLE intensities of the phosphors increased. surface area were 18.4 m 2 /g, 13.5 m 2 /g, 9.4 m 2 /g, 7.2 m 2 /g, 5.8 m 2 /g, and 2.5 m 2 /g, respectively, as shown in Figure 3a-f.   .01]3MgSi2O8 phosphors sintered at 1300 °C for 5 h. The Eu 2+ excitation band of the [Sr0.99Eu0.01]3MgSi2O8 phosphors can be fitted into two Gaussian components with peaks at 280 and 350 nm, which correspond to the 4f 7 ( 8 S7/2)→4f 6 5d 1 (t2g) electron transition of Eu 2+ [26]. Figure S2 shows the PLE spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered for different durations. These spectra exhibit two broad bands ranging from 240 to 320 nm and from 330 to 410 nm, with peaks at 280 and 350 nm, which are assigned to the transitions between the ground state 4f 7 and the crystal-field split state 4f 6 5d 1 . As the sintering time increased, the excitation intensity increased and reached a maximum value at a sintering time of 5 h. The aforementioned results demonstrate that as the sintering duration increased from 1 to 5 h, the crystallinity (Figure 1), particle morphologies and sizes (Figure 3), and PLE intensities of the phosphors increased.  Figure S3 shows the PL spectra of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors sintered at 1300 • C for different durations. The emission spectra corresponding to 280 nm excitation contain a single band at around 457 nm. As displayed in Figure S2, the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors exhibited the highest emission peak intensities when the sintering duration was 5 h, the Sr 3 MgSi 2 O 8 has a space group of P21/a, and the unit cell contains three Sr sites: one 12-coordinated Sr(I) site and two 10-coordinated Sr(II, III) sites [27]. The broad band at around 457 nm is attributed to the 4f 6 5d-4f 7 transition at the Sr 2+ (I) site, where Sr 2+ is substituted by Eu 2+ [28,29]. The electronic mechanism of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors is shown in Figure 6. The 4f 6 5d-4f 7 transition belongs to the electronic dipoleallowed transition, based on the Laporte selection rule. Kim [28]. Figure S2 does not indicate an emission peak at 570 nm; thus, only Eu 2+ ions substituted Sr 2+ at the Sr 2+ (I) site. The full width at half maximum (FWHM) of the broad band of emission peaks were approximately 50, 46, 43, 41, and 40 nm as the sintered for 1 to 5 h. This result was caused by the electron on the outer 5d-orbital of the atom, while the emission peak of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors was easily influenced by the external environment.
substituted by Eu [28,29]. The electronic mechanism of the [Sr0.99Eu0.01]3MgSi2O8 phosphors is shown in Figure 6. The 4f 6 5d-4f 7 transition belongs to the electronic dipole-allowed transition, based on the Laporte selection rule. Kim et al. indicated that the 570 nm band to Eu 2+ ions at the Sr 2+ (II, III) sites occurs at high Eu 2+ doping concentrations in Sr3MgSi2O8 [28]. Figure S2 does not indicate an emission peak at 570 nm; thus, only Eu 2+ ions substituted Sr 2+ at the Sr 2+ (I) site. The full width at half maximum (FWHM) of the broad band of emission peaks were approximately 50, 46, 43, 41, and 40 nm as the sintered for 1 to 5 h. This result was caused by the electron on the outer 5d-orbital of the atom, while the emission peak of the [Sr0.99Eu0.01]3MgSi2O8 phosphors was easily influenced by the external environment.  phors is shown in Figure 6. The 4f 6 5d-4f 7 transition belongs to the electronic dipole-allowed transition, based on the Laporte selection rule. Kim et al. indicated that the 570 nm band to Eu 2+ ions at the Sr 2+ (II, III) sites occurs at high Eu 2+ doping concentrations in Sr3MgSi2O8 [28]. Figure S2 does not indicate an emission peak at 570 nm; thus, only Eu 2+ ions substituted Sr 2+ at the Sr 2+ (I) site. The full width at half maximum (FWHM) of the broad band of emission peaks were approximately 50, 46, 43, 41, and 40 nm as the sintered for 1 to 5 h. This result was caused by the electron on the outer 5d-orbital of the atom, while the emission peak of the [Sr0.99Eu0.01]3MgSi2O8 phosphors was easily influenced by the external environment.   These data fit well with a double-exponential curve. The aforementioned curves indicate the possible interactions between Eu 2+ ions and suggest that these ions occupied the cationic sites (Sr 2+ ). To calculate the luminescence lifetimes, all the fluorescent decay curves were fitted using the double-exponential equation of Sahu et al. [30], which is expressed as follows: where I is the PL intensity, A 1 and A 2 are the fitting parameters, and τ 1 and τ 2 are the decay constants of the exponential components.
of a rare-earth ion can be calculated using the following equation [31]: The average luminescence lifetimes of the [Sr0.99Eu0.01]3MgSi2O8 phosphors were calculated to be 3.406, 3.191, and 1.143 ms for the sintering durations of 1, 2, and 5 h, respectively. The parameter τ* decreased with sintering time. This phenomenon might be attributed to the energy transfer between the Eu 2+ ions located at the Sr 2+ sites [32].  On the basis of the aforementioned equation, the average luminescence lifetimes (τ*) of a rare-earth ion can be calculated using the following equation [31]: The average luminescence lifetimes of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors were calculated to be 3.406, 3.191, and 1.143 ms for the sintering durations of 1, 2, and 5 h, respectively. The parameter τ* decreased with sintering time. This phenomenon might be attributed to the energy transfer between the Eu 2+ ions located at the Sr 2+ sites [32]. Sintering temperature affects the PL properties and structure of phosphors. Therefore, we attempted to determine the optimal sintering temperature for preparing [ Figure 9. Sintering temperature affects the PL properties and structure of phosphors. Therefore, we attempted to determine the optimal sintering temperature for preparing [Sr0.99Eu0.01]3MgSi2O8 phosphors. XRD patterns of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered using the solid-state method at temperatures of 1200, 1250, 1300, 1350 and 1400 °C for 5 h are depicted in Figure 9. Figure  The aforementioned results indicate that the row material of SrCO3 decomposed into SrO and CO2, then SrO reacted with SiO2 to form Sr2SiO4, and finally SrO and MgO reacted with SiO2 to form the Sr2MgSi2O7 and Sr3MgSi2O8 phases. When the sintering temperature was lower than 1000 °C, the following reaction occurred: When the sintering temperature was between 1000 and 1200 °C, the following reaction occurred [33,34]: When the sintering temperature was between 1200 and 1300 °C, the following reaction occurred [35]: At 1450 °C, the [Sr0.99Eu0.01]3MgSi2O8 phosphor melted. Consequently, the crystalline structures and PL properties of the [Sr0.99Eu0.01]3MgSi2O8 phosphors were not examined at sintering temperatures higher than 1450 °C.  The findings for the crystal structure of the [Sr0.99Eu0.01]3MgSi2O8 phosphor sintered at 1400 °C was fitted using the following parameters: a = 5.341 Å, b = 9.700 Å, and c = 7.184 Å (Sr3MgSi2O8 phosphors). Subsequently, Rietveld refinement was conducted on the XRD data of this phosphor ( Figure 10). The final refinement convergence was achieved when χ 2 = 5.42, which is marginally higher than the optimal value χ 2 value of <2. This result was due to the coexistence of the Sr2MgSi2O7 (2θ = 29.7° and 30.2°) and α-Sr2SiO4 (2θ = 35  The aforementioned results indicate that the row material of SrCO 3 decomposed into SrO and CO 2 , then SrO reacted with SiO 2 to form Sr 2 SiO 4 , and finally SrO and MgO reacted with SiO 2 to form the Sr 2 MgSi 2 O 7 and Sr 3 MgSi 2 O 8 phases. When the sintering temperature was lower than 1000 • C, the following reaction occurred: When the sintering temperature was between 1000 and 1200 • C, the following reaction occurred [33,34]: When the sintering temperature was between 1200 and 1300 • C, the following reaction occurred [35] The findings for the crystal structure of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphor sintered at 1400 • C was fitted using the following parameters: a = 5.341 Å, b = 9.700 Å, and c = 7.184 Å (Sr 3 MgSi 2 O 8 phosphors). Subsequently, Rietveld refinement was conducted on the XRD data of this phosphor ( Figure 10). The final refinement convergence was achieved when χ 2 = 5.42, which is marginally higher than the optimal value χ 2 value of <2. This result was due to the coexistence of the  The findings for the crystal structure of the [Sr0.99Eu0.01]3MgSi2O8 phosphor sintered at 1400 °C was fitted using the following parameters: a = 5.341 Å, b = 9.700 Å, and c = 7.184 Å (Sr3MgSi2O8 phosphors). Subsequently, Rietveld refinement was conducted on the XRD data of this phosphor ( Figure 10). The final refinement convergence was achieved when χ 2 = 5.42, which is marginally higher than the optimal value χ 2 value of <2. This result was due to the coexistence of the Sr2MgSi2O7 (2θ = 29.7° and 30.2°) and α-Sr2SiO4 (2θ = 35.4°, 43.9°, 45.1°, and 60.7°) phases in the aforementioned phosphor. The remaining diffraction peak of 2θ values, in addition to those mentioned above, were assigned to the [Sr0.99Eu0.01]3MgSi2O8 phase. It demonstrated that the Sr 2+ ions were substituted by Eu 2+ ions in the [Sr0.99Eu0.01]3MgSi2O8 phosphors. As the sintering temperature increased, the PLE intensity also increased, and the maximum PLE intensity was achieved when the sintering temperature was 1400 • C ( Figure 11). As depicted in Figure 12 [Sr0.99Eu0.01]3MgSi2O8 phosphors increased with sintering temperature. The [Sr0.99Eu0.01]3MgSi2O8 phosphor sintered at 1400 °C exhibited the highest PL intensity, and the broad and asymmetric band with an FWHM value of 38 nm was observed at around 457 nm. The FWHM of the broad band of emission peaks were approximately 38, 40, 43 and 45 nm as the sintered temperature decreased from 1400 °C to 1200 °C. The blue emission band of the [Sr0.99Eu0.01]3MgSi2O8 phosphors at 457 nm was attributed to the 5d-4f electron transition of Eu 2+ .   Figure 13 shows the Eu 3d XPS spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at different temperatures. The results shows that there is no Eu 2+ -related peaks at the sintered temperature of 900 °C (Figure 13a), and the Eu 2+ peak appeared at the sintering temperature of 1400 °C (Figure 13b). The Eu 3d XPS spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at 1400 °C is shown in Figure 14, revealing the Eu 3d peak deconvolution of the electron binding energies of Eu 3+ 3d3/2 (1164 eV), Eu 2+ 3d3/2 (1155 eV), Eu 3+ 3d5/2 (1134 eV), and Eu    Figure 13 shows the Eu 3d XPS spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors s at different temperatures. The results shows that there is no Eu 2+ -related peaks at tered temperature of 900 °C (Figure 13a), and the Eu 2+ peak appeared at the sinteri perature of 1400 °C (Figure 13b). The Eu 3d XPS spectra of the [Sr0.99Eu0.01]3MgSi2O phors sintered at 1400 °C is shown in Figure 14, revealing the Eu 3d peak deconv of the electron binding energies of Eu 3+ 3d3/2 (1164 eV), Eu 2+ 3d3/2 (1155 eV), Eu 3+ 3d eV), and Eu 2+ 3d5/2 (1125 eV). This result demonstrated that the Eu 3+ ions are succ   (Figure 13a), and the Eu 2+ peak appeared at the sintering temperature of 1400 • C (Figure 13b). The Eu 3d XPS spectra of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors sintered at 1400 • C is shown in Figure 14, revealing the Eu 3d peak deconvolution of the electron binding energies of Eu 3+ 3d 3/2 (1164 eV), Eu 2+ 3d 3/2 (1155 eV), Eu 3+ 3d 5/2 (1134 eV), and Eu 2+ 3d 5/2 (1125 eV). This result demonstrated that the Eu 3+ ions are successfully reduced to Eu 2+ ions at a 1400 • C sintering temperature. In general, Eu 3+ →Eu 2+ reduction requires a higher temperature in the reducing atmosphere. reduced to Eu 2+ ions at a 1400 °C sintering temperature. In general, Eu 3+ →Eu 2+ reduction requires a higher temperature in the reducing atmosphere.   Figure S5 displays the fluorescent decay curves of the [Sr0.99Eu0.01]3MgSi2O8 phosphors excited at 280 nm and monitored at 457 nm. The data fit well with a double-exponential curve. The average luminescence lifetimes of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at 1200, 1300, and 1400 °C were calculated from Equation (2) to be 1.074, 1.144, and 1.197 ms, respectively. The parameter τ* decreased with sintering temperature. This result demonstrates that energy transfer occurred between the Eu 2+ ions located at the Sr 2+ sites [32]. Figure 15 shows the CIE chromaticity coordinates and photographs of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at different temperatures. The CIE chromaticity diagram was obtained for an excitation wavelength of 280 nm. When the sintering temperature was increased from 1200 to 1400 °C, the CIE chromaticity coordinates shifted reduced to Eu 2+ ions at a 1400 °C sintering temperature. In general, Eu 3+ →Eu 2+ reduction requires a higher temperature in the reducing atmosphere.   Figure S5 displays the fluorescent decay curves of the [Sr0.99Eu0.01]3MgSi2O8 phosphors excited at 280 nm and monitored at 457 nm. The data fit well with a double-exponential curve. The average luminescence lifetimes of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at 1200, 1300, and 1400 °C were calculated from Equation (2) to be 1.074, 1.144, and 1.197 ms, respectively. The parameter τ* decreased with sintering temperature. This result demonstrates that energy transfer occurred between the Eu 2+ ions located at the Sr 2+ sites [32]. Figure 15 shows the CIE chromaticity coordinates and photographs of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at different temperatures. The CIE chromaticity diagram was obtained for an excitation wavelength of 280 nm. When the sintering temperature was increased from 1200 to 1400 °C, the CIE chromaticity coordinates shifted  [32]. Figure 15 shows the CIE chromaticity coordinates and photographs of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors sintered at different temperatures. The CIE chromaticity diagram was obtained for an excitation wavelength of 280 nm. When the sintering temperature was increased from 1200 to 1400 • C, the CIE chromaticity coordinates shifted from a light blue region (x = 0.1659, y = 0.1382) to an ultramarine blue region (x = 0.1494, y = 0.0942). Therefore, the optimal sintering temperature in the production of blue phosphors is 1400 • C. Images of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors sintered at different temperatures under UV light irradiation are displayed in the inset of Figure 15 and in Figure S6. from a light blue region (x = 0.1659, y = 0.1382) to an ultramarine blue region (x = 0.1494, y = 0.0942). Therefore, the optimal sintering temperature in the production of blue phosphors is 1400 °C. Images of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at different temperatures under UV light irradiation are displayed in the inset of Figure 15 and in Figure  S6.

Conclusions
In this study, Eu 2+ -doped [Sr1−xEux]3MgSi2O8 phosphors were prepared in a reducing atmosphere by using a solid-state reaction method, and the photoluminescence properties of these phosphors were investigated. The optimal sintering temperature and duration for the preparation of the [Sr0.99Eu0.01]3MgSi2O8 phosphors was found to be 1400 °C and 5 h, respectively. The blue emission of these phosphors at 457 nm is attributed to the 5d-4f electron transition of Eu 2+ . In addition, the average decay time of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at 1400 °C for 5 h was calculated to be 1.197 ms. The CIE chromaticity coordinates of the phosphors sintered at 1400 °C were (x = 0.1494, y = 0.0942), and this point lies in an ultramarine blue region in the CIE chromaticity diagram.

Conclusions
In this study, Eu 2+ -doped [Sr 1−x Eu x ] 3 MgSi 2 O 8 phosphors were prepared in a reducing atmosphere by using a solid-state reaction method, and the photoluminescence properties of these phosphors were investigated. The optimal sintering temperature and duration for the preparation of the [Sr 0.99 Eu 0.01 ] 3 MgSi 2 O 8 phosphors was found to be 1400 • C and 5 h, respectively. The blue emission of these phosphors at 457 nm is attributed to the 5d-4f electron transition of Eu 2+