Investigations of Photoluminescence Properties of CaxMg2-xSi2O6:yEu2+ (x = 0.5–1.25, y = 0.015–0.035) Phosphors

Previously, there were almost no relevant studies on developing the optimal CaxMg2-xSi2O6:yEu2+ phosphor composition for its finest optical properties. This study employs two steps to determine the optimal composition for CaxMg2-xSi2O6:yEu2+ phosphors. First, CaMgSi2O6:yEu2+ (y = 0.015, 0.020, 0.025, 0.030, 0.035) was used as the primary composition of specimens synthesised in a reducing atmosphere of 95% N2 + 5% H2 to investigate the effect of Eu2+ ions on the photoluminescence properties of each variant. The emission intensities of the entire photoluminescence excitation (PLE) and photoluminescence (PL) emission spectra of the CaMgSi2O6:yEu2+ phosphors initially increased as the concentration of the Eu2+ ions increased, peaking at y = 0.025. The cause of the variations across the entire PLE and PL spectra of all five CaMgSi2O6:yEu2+ phosphors was investigated. Because the CaMgSi2O6:0.025Eu2+ phosphor had the highest PLE and PL emission intensities, in the next step, CaxMg2-xSi2O6:0.025Eu2+ (x = 0.5, 0.75, 1.0, 1.25) was used as the primary composition to investigate the effect on the photoluminescence properties when the CaO content varied. We also show that the Ca content has an apparent effect on the photoluminescence properties of CaxMg2-xSi2O6:0.025Eu2+ phosphors, and the optimal phosphor composition is Ca0.75Mg1.25Si2O6:0.025Eu2+ because it has the largest PLE and PL values. X-ray diffraction (XRD) analyses of CaxMg2-xSi2O6:0.025Eu2+ phosphors were performed to identify the factors responsible for this outcome.


Introduction
Inorganic phosphors activated by rare earth ions can emit multicolour luminescence. Therefore, they have received considerable attention because of their potential broad application in biomedical imaging, backlight displays and solid-state lighting. When the various silicate (SiO 2 ) and alkaline earth oxides (SrO, CaO and MgO) compositions are used in the host materials, synthesised phosphors possess the advantages of high stability and low price. Phosphors based on CaMgSi 2 O 6 have a monoclinic structure and have attracted considerable attention because they have unique structural and optical features. For example, when CaMgSi 2 O 6 -based phosphors are packaged with resin, they have high chemical stability and considerable thermal durability when ultraviolet (UV) radiation or blue light is used as the excitation source. Synthesised CaMgSi 2 O 6 -based powders are well known as high-efficiency phosphors and retain their high stability with rising temperatures. Consequently, they have attracted the interest of many researchers investigating them as potential candidates for application in white-light LEDs (WLEDs) [1,2]. Previously, many different oxides were doped into CaMgSi 2 O 6 -based powders as activators to produce phosphors of different colours and to investigate their physical and photoluminescence properties. For instance, Chandrakar et al. used Eu 2+ as an activator and investigated the particle size, crystal structure, photoluminescence and thermoluminescence of an Eu 2+doped CaMgSi 2 O 6 phosphor [3]. The Eu 2+ -doped CaMgSi 2 O 6 phosphor emitted a blue colour, with an emission peak of approximately 449 nm [3,4]. Subsequently, Chandrakar et al. used Ce 3+ ions as an activator, and the excitation and emission peaks of the synthesised CaMgSi 2 O 6 :Ce 3+ phosphors were recorded as 295 and 327 nm, respectively [5]. Eu 2 O 3 is a commonly used activator because it can produce light of different colours. When Eu 2 O 3 -doped phosphors are synthesised in a reducing atmosphere, the Eu 3+ ions are reduced to Eu 2+ ions, and most of the synthesised phosphors emit a blue or green colour. For example, when Eu 2 O 3 is added to a Sr 3 MgSi 2 O 8 -based powder and synthesised in a reducing atmosphere, the synthesised phosphor emits blue light with a broad and asymmetric band centred around approximately 457 nm [6]. When Eu 2 O 3 is added to a Ca 2 MgSi 2 O 7 -based powder and synthesised in a reducing atmosphere, the synthesised phosphor has only one broad emission band centred around 529 nm and emits green light [7]. When Eu 2 O 3 -doped phosphors are synthesised in an air atmosphere, the Eu 3+ ions dominate the emitting properties, and the synthesised phosphors emit a red colour or a nearinfrared (NIR) emission. For example, when Eu 2 O 3 is added to a BaY 2 O 4 -based powder and synthesised in an air atmosphere, the synthesised phosphor has multiple emission peaks in the 580-630 nm range and emits red light [8]. In addition, when Eu 2 O 3 is added to a Sr 3 La(PO 4 ) 3 -based powder and synthesised in an air atmosphere, the synthesised phosphor has multiple emission peaks in the 580-720 nm range, with the phosphor emitting red light and NIR luminescence [9]. Previous studies on this subject have found that changes in the composition of phosphors based on silicate (SiO 2 ) and alkaline earth oxides are accompanied by changes in their emission characteristics.
Zhang and Wang researched the photoluminescence of Eu 2+ -doped CaMgSi 2x O 6+2x phosphors, with x in the 1.00-1.20 range, under UV and vacuum ultraviolet (VUV) excitation; they found that although the central wavelength of CaMgSi 2x O 6+2x :0.01Eu 2+ phosphors exhibited no apparent change with excess Si content, an appropriate amount of excess Si enhanced the photoluminescence intensity of CaMgSi 2x O 6+2x :0.01Eu 2+ phosphors [10]. Lee et al. found that by controlling the substituting concentration of Ca ions to Sr ions and the Eu 2+ concentration (range = 0.015-0.045), the photoluminescence properties of (Ca 1-x Sr x )Mg 2 Si 3 O 9 : Eu 2+ (0 ≤ x ≤ 0.5) phosphors can be optimised, even though the central wavelengths of the photoluminescence excitation (PLE) and photoluminescence (PL) emission spectra of (Ca 1-x Sr x )Mg 2 Si 3 O 9 :Eu 2+ phosphors exhibit no apparent change [11]. Tseng et al. used Ca 2+x MgSi 2 Eu 0 . 025 O 7+x as the primary composition of specimens used to investigate the crystalline phases and photoluminescence properties of Ca 2 MgSi 2 Eu 0 . 025 O 7 phosphors. They found that the Ca content has a substantial effect on the crystalline phases and photoluminescence properties of synthesised Ca 2+x MgSi 2 Eu 0 . 025 O 7+x powders: as the Ca content increases, the central wavelength of the Ca 2+x MgSi 2 Eu 0 . 025 O 7+x phosphors shifts from 530 nm to 475 nm, and the emitting light changes from green, cyan, and blue-cyan to blue [12].
These findings indicate that changes in the compositions and concentration of Eu 2+ ions significantly affect the crystallisation and the optical properties of phosphors based on Ca 2 MgSi 2 O 7 -Eu 2+ (Ca x Mg 2-x Si 2 O 6 :yEu 2+ ). Therefore, in this study, phosphor specimens with various Ca x Mg 2-x Si 2 O 6 compositions (x = 0.5, 0.75, 1.0, and 1.25) were used as the host material, and Eu 2 O 3 was used as the activator to prepare Ca x Mg 2-x Si 2 O 6 :yEu 2+ powders (x = 1.0 and y = 0.015, 0.020, 0.025, 0.030, 0.035; or y = 0.025 and x = 0.5, 0.75, 1.0, 1.25). The prepared compositions were synthesised in a reducing atmosphere of 95% N 2 + 5% H 2 to produce phosphors that emit blue light. The quenching effect refers to the compositions and the synthesis processes that decrease the photoluminescence emission intensities of the PLE and PL spectra of a synthesised phosphor. A concentration quenching effect occurs under conditions in which a high activator concentration is used.
Although many studies have researched the crystal and photoluminescence properties of Eu 2+ -doped CaMgSi 2 O 6 phosphors, only a few studies have focused on the concen-tration quenching effect of CaMgSi 2 O 6 :yEu 2+ phosphors. Consequently, the first critical contribution of this study is the finding that the Eu 2+ ion concentration has a substantial effect on the PLE and PL properties of the Eu 2+ -doped CaMgSi 2 O 6 phosphor. To this end, CaMgSi 2 O 6 :yEu 2+ (y = 0.015, 0.020, 0.025, 0.030, and 0.035) was used as the primary composition to determine the optimal Eu 2+ ion concentration. We show that the optimal Eu 2+ ion concentration is y = 0.025. The maximum emission intensities of the entire PLE and PL spectra and the maximum excitation and emission intensities (PLEmax and PLmax) of CaMgSi 2 O 6 :yEu 2+ phosphors are presented. We prove that the concentration quenching effect is the underlying cause of the diminished photoluminescence properties of Eu 2+ -doped CaMgSi 2 O 6 phosphors at y values higher than 0.025. The effect of the Ca content on all the photoluminescence properties of Eu 2+ -doped Ca x Mg 2-x Si 2 O 6 phosphors is also discussed. The second major contribution of this study is its innovative approach to this aspect of the research: synthesised Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ powders were used to investigate the effect of the Ca content on the photoluminescence properties of CaMgSi 2 O 6 phosphors. The underlying cause of the observed variation in the properties of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors were also investigated.  2+ and Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ powders were mixed for 2 h using the ball milling method, with absolute alcohol used as a solute. The wetted powders were then dried at 80 • C and ground. As the synthesis temperature increased from 1200 • C to 1300 • C, the emission intensities of the PLE and PL spectra increased, peaking at 1300 • C. If 1350 • C were used as the synthesis temperature, the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ powders would melt. Therefore, the CaMgSi 2 O 6 :yEu 2+ and Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ powders were synthesised at 1300 • C for 4 h in a reducing atmosphere of 95% N 2 + 5% H 2 , which was simultaneously used to deoxidise Eu 3+ ions into Eu 2+ ions. We found that the maximum PLE (PLEmax) and maximum PL (PLmax) values for the CaMgSi 2 O 6 :0.025Eu 2+ phosphor were higher than those of other CaMgSi 2 O 6 :yEu 2+ phosphors. Therefore, a 0.025Eu 2+ concentration and a Ca x Mg 2-x Si 2 O 6 composition were used to investigate the effect of Ca content on the optical properties of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors. To determine the optimal optical properties of CaMgSi 2 O 6 :yEu 2+ and Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors, the 3D scanning method was used to detect their optimum PLE wavelengths. The PLE spectra (monitored at 450 nm) and PL spectra (excited by 346 nm) of all synthesised variants of the CaMgSi 2 O 6 :yEu 2+ and Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors were measured at room temperature in wavelength ranges of 250-400 nm and 400-700 nm, respectively, using a xenon lamp in a Hitachi F-4500 fluorescence spectrophotometer. All PLE and PL spectra, except those with different emission intensities, had similar appearances and variations.

Results and Discussion
One of the objectives of this study was to find the concentration quenching effect of Ca x Mg 2-x Si 2 O 6 :Eu 2+ -based phosphors. To this end, CaMgSi 2 O 6 was used as the host material, CaMgSi 2 O 6 :yEu 2+ was used as the composition, and the Eu 2+ ion concentration was altered from 0.015 to 0.035 (y = 0.015-0.035). The PLE spectra of the synthesised CaMgSi 2 O 6 :yEu 2+ phosphors were measured as a function of the Eu 2+ concentration; the results are presented in Figure 1a. The PLE spectra were monitored at 450 nm and recorded at room temperature in the spectral region of 250-400 nm. The emission intensities of the entire PLE spectra of the CaMgSi 2 O 6 :yEu 2+ phosphors initially increased as the Eu 2+ ion concentration increased, peaking at y = 0.025 and then decreasing as the Eu 2+ ion concentration increased further. These results indicate that the Eu 2+ concentration has a substantial effect on the optical properties of Eu 2+ -doped CaMgSi 2 O 6 (Ca x Mg 2-x Si 2 O 6 ) phosphors. Subsequently, we prove later that the concentration quenching effect is the underlying cause of the degeneration in the emission intensities across the entire PLE spectra of the CaMgSi 2 O 6 :yEu 2+ phosphors. As can be seen in the results presented in Figure 1a, we found that CaMgSi 2 O 6 :yEu 2+ phosphors have broad PLE spectra with three unapparent absorption peaks, in which the wavelengths are located at 314, 346, and 365 nm. For all the CaMgSi 2 O 6 :yEu 2+ phosphors, the PLEmax value was reached at 346 nm, and the PL spectrum excited by 346 nm recorded the PLmax value. Therefore, 346 nm was used as the excitation wavelength (λ ex ) of CaMgSi 2 O 6 :yEu 2+ phosphors in all further analyses. The critical energy transfer distance (Ro) of the concentration quenching effect happening in CaMgSi2O6:yEu 2+ phosphors is calculated using the formula: Ro = 2[(3V)/(4πxcN)] 1/3 , where xc, V, and N are the critical concentration of the Eu 2+ ions, the volume of the CaMgSi2O6 unit cell, and the number of cations in the CaMgSi2O6 unit cell, respectively [15,16]. The structure of CaMgSi2O6 is monoclinic, and its space group is C2/c; the unit cell parameters of synthesised CaMgSi2O6:0.025Eu 2+ phosphor were calculated using the Rietveld refinement method. The values of the refinement factors Rwp and Rp for CaMgSi2O6:0.025Eu 2+ phosphor were found to be 9.83% and 5.82%, respectively, which indicates that the refined values are reliable. From the calculation, the refined structural parameters of CaMgSi2O6:0.025Eu 2+ phosphor were as follows: a = 9.745 Å, b = 8.933 Å, c = 5.248 Å, α = γ = 90°, β = 105.87°, and V = 439.3 Å 3 . When N = 4, xc = 0.025 and V = 439.3 Å 3 were incorporated into the Ro = 2[(3V)/(4πxcN)] 1/3 formula to calculate the Ro value of CaMgSi2O6:yEu 2+ phosphors, which was calculated to be approximately 20.3 Å. In a host material, the energy transfer between the luminescent centres occurs via both electric multipole-multipole interactions and electric exchange interactions. If the distance between the activators exceeds 5 Å, the effect of the multipole-multipole interactions dominates the emission properties, while the exchange interactions have less effectivity on the emission properties [13,14]. However, the calculated Ro value for CaMgSi2O6:yEu 2+ phosphors is approximately 20.3 Å, which is much larger than 5 Å. Therefore, the multipole-multipole interactions are the primary mechanism dominating the decay in the emission intensities of the PLE and PL spectra of CaMgSi2O6:yEu 2+ phosphors. We believe that as the Eu 2+ ion concentration exceeds 0.025, the effectivity of the multipole-multipole interactions increases, and the concentration quenching effect of CaMgSi2O6:yEu 2+ phosphors diminishes the emission intensities of their PLE and PL spectra.
X-ray diffraction (XRD) patterns can be used to analyse the crystalline structure of CaxMg2-xSi2O6:0.025Eu 2+ phosphors. Therefore, the XRD patterns of synthesised CaxMg2-xSi2O6:0.025Eu 2+ phosphors were measured as a function of Ca content and are presented Because the red spectral range exhibits the characteristic f-f transitions of Eu 3+ ions and is in the region of 570-670 nm, i.e., 5 D 0 → 7 F j (j = 1, 2, and 3), the red spectral range was not observed in all PL spectra of CaMgSi 2 O 6 :yEu 2+ phosphors, and only one emission peak in the 410-540 nm range was observed. Therefore, the wavelength range of 400-550 nm was used to record the room-temperature PL spectra and the PL spectra of synthesised CaMgSi 2 O 6 :yEu 2+ phosphors as a function of Eu 2+ concentration; the observed data are presented in Figure 1b. The emission intensities of the PL spectra of CaMgSi 2 O 6 :yEu 2+ phosphors showed a similar trend as the PLE spectra. For the synthesised CaMgSi 2 O 6 :yEu 2+ phosphors, as can be seen in Figure 1b, the emission intensities of all peaks initially increased with the Eu 2+ concentration, peaking at y = 0.025 and then decreasing as the Eu 2+ concentration was increased further. The emission peak at 450 nm, which is the blue colour, is caused by a transition from the excited state 4f 6 5d 1 to the ground state 4f 7 . As the Eu 2+ ion concentration increases from 0.15 to 0.25, the intensities of the entire PLE and PL spectra increase. The underlying cause of the increase is that the volume of the luminescent centres increases with an increase in activator volume (i.e., Eu 2+ ions). The concentration quenching effect is characterised by a decrease in the volume of the fluorescence quantum, which decreases as the concentration of the emission centres for the fluorophore emission increases. When the Eu 2+ ion concentration in Ca x Mg 2-x Si 2 O 6 -based phosphors exceeds a critical level, nonradiative relaxation occurs, which diminishes their PLE and PL emission intensities. To investigate the mechanism of the concentration quenching effect of Eu 2+ ions in CaMgSi 2 O 6 :yEu 2+ phosphors, relevant parameters, including the critical concentration of the ions used, the volume of a unit cell of the host material used, and the number of cations in a unit cell of the host material, must be known to calculate the critical energy transfer distance [13,14].
The critical energy transfer distance (R o ) of the concentration quenching effect happening in CaMgSi 2 O 6 :yEu 2+ phosphors is calculated using the formula: R o = 2[(3V)/(4πx c N)] 1/3 , where x c , V, and N are the critical concentration of the Eu 2+ ions, the volume of the CaMgSi 2 O 6 unit cell, and the number of cations in the CaMgSi 2 O 6 unit cell, respectively [15,16]. The structure of CaMgSi 2 O 6 is monoclinic, and its space group is C2/c; the unit cell parameters of synthesised CaMgSi 2 O 6 :0.025Eu 2+ phosphor were calculated using the Rietveld refinement method. The values of the refinement factors Rwp and Rp for CaMgSi 2 O 6 :0.025Eu 2+ phosphor were found to be 9.83% and 5.82%, respectively, which indicates that the refined values are reliable. From the calculation, the refined structural parameters of CaMgSi 2 O 6 :0.025Eu 2+ phosphor were as follows: a = 9. In a host material, the energy transfer between the luminescent centres occurs via both electric multipole-multipole interactions and electric exchange interactions. If the distance between the activators exceeds 5 Å, the effect of the multipole-multipole interactions dominates the emission properties, while the exchange interactions have less effectivity on the emission properties [13,14]. However, the calculated R o value for CaMgSi 2 O 6 :yEu 2+ phosphors is approximately 20.3 Å, which is much larger than 5 Å. Therefore, the multipolemultipole interactions are the primary mechanism dominating the decay in the emission intensities of the PLE and PL spectra of CaMgSi 2 O 6 :yEu 2+ phosphors. We believe that as the Eu 2+ ion concentration exceeds 0.025, the effectivity of the multipole-multipole interactions increases, and the concentration quenching effect of CaMgSi 2 O 6 :yEu 2+ phosphors diminishes the emission intensities of their PLE and PL spectra.
X-ray diffraction (XRD) patterns can be used to analyse the crystalline structure of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors. Therefore, the XRD patterns of synthesised Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors were measured as a function of Ca content and are presented in Figure 2. The diffraction peaks for standard JCPDS No. 75-1092 are also indexed in Figure 2a, and all synthesised Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors matched with these diffraction peaks, although there are some deviations in the diffraction intensities of the diffraction peaks. These results indicate that the mainly crystal structure of all the synthesised Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors is monoclinic, with a space group C2/c structure, which is in good agreement with the standard JCPDS No. 75-1092. For Ca 0 . 5 Mg 1 . 5 Si 2 O 6 :0.025Eu 2+ and Ca 0 . 75 Mg 1 . 25 Si 2 O 6 :0.025Eu 2+ phosphors, all the diffraction peaks of the CaMgSi 2 O 6 phase in the two synthesised powders were revealed, and the SiO 2 phase was also observed, as can be seen in Figure 2a. Apparently, as the CaO content of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors increased from x = 0.75 to x = 1.25, the diffraction intensity of the SiO 2 phase decreased. As the CaO content increased further, many secondary phases were observed in the synthesised CaMgSi 2 O 6 :0.025Eu 2+ and Ca 01 . 25 Figure 2a also shows that, as the value of x increased from 0.75 to 1.25, the diffraction intensities of all secondary phases increased with an increase in the CaO content. The results presented in Figure 2b show that as the CaO content of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors increased from x = 0.5 to x = 1.25, there was no apparent change in the 2θ value of the main (221) peak, and the primarily crystalline peak of the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors changed from a (221) peak to a (−311) peak. The results presented in Figure 2a also indicate that an increase in the CaO content does not alter the primary crystalline phase (monoclinic), but there is an apparent increase in the number of secondary phases. Figure 2b also shows that Ca 0 . 75 Mg 1 . 25 Si 2 O 6 :0.025Eu 2+ phosphor had the smallest full width at half maximum (FWHM). The results indicate that the CaO content affects the resulting composition of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors, consequently transmitting their photoluminescence properties. xSi2O6:0.025Eu 2+ phosphors changed from a (221) peak to a (−311) peak. The results presented in Figure 2a also indicate that an increase in the CaO content does not alter the primary crystalline phase (monoclinic), but there is an apparent increase in the number of secondary phases. Figure 2b also shows that Ca0.75Mg1.25Si2O6:0.025Eu 2+ phosphor had the smallest full width at half maximum (FWHM). The results indicate that the CaO content affects the resulting composition of CaxMg2-xSi2O6:0.025Eu 2+ phosphors, consequently transmitting their photoluminescence properties. However, from the PLE spectra, we found that all CaxMg2-xSi2O6:0.025Eu 2+ phosphors also have three unapparent absorption peaks with wavelengths at 314, 346, and 365 nm. Because the 346 nm peak also had the highest intensity, it was used as the excitation wavelength of the CaxMg2-xSi2O6:0.025Eu 2+ phosphors. The thermal stabilisation problem of synthesised phosphors is a crucial factor for their practical application in package LEDs. This is because when the excitation light source of LEDs emits heat, the thermal quenching effect occurs, reducing the emission properties and the efficiencies of the fabricated LEDs. The PLE and PL spectra of the synthesised CaxMg2-xSi2O6:0.025Eu 2+ phosphors were measured at different temperatures. However, when the temperature increased, the photoluminescence properties of all the synthesised CaxMg2-xSi2O6:0.025Eu 2+ phosphors exhibited a similar variation trend. Therefore, only the measured values of the Ca0.75Mg1.25Si2O6:0.025Eu 2+ and CaMgSi2O6:0.025Eu 2+ phosphors were used as representative values. The measured values for the PLE and PL spectra are presented in Figures 3  and 4, respectively. The PL properties of CaxMg2-xSi2O6:0.025Eu 2+ phosphors were also measured from 30 to 210 °C, with a 30 °C step below the excitation wavelength of 346 nm to find the effect of temperature on the variations in the PLmax values. By heating CaxMg2- However, from the PLE spectra, we found that all Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors also have three unapparent absorption peaks with wavelengths at 314, 346, and 365 nm. Because the 346 nm peak also had the highest intensity, it was used as the excitation wavelength of the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors. The thermal stabilisation problem of synthesised phosphors is a crucial factor for their practical application in package LEDs. This is because when the excitation light source of LEDs emits heat, the thermal quenching effect occurs, reducing the emission properties and the efficiencies of the fabricated LEDs. The PLE and PL spectra of the synthesised Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors were measured at different temperatures. However, when the temperature increased, the photoluminescence properties of all the synthesised Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors exhibited a similar variation trend. Therefore, only the measured values of the Ca 0 . 75 Mg 1 . 25 Si 2 O 6 :0.025Eu 2+ and CaMgSi 2 O 6 :0.025Eu 2+ phosphors were used as representative values. The measured values for the PLE and PL spectra are presented in Figures 3 and 4, respectively. The PL properties of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors were also measured from 30 to 210 • C, with a 30 • C step below the excitation wavelength of 346 nm to find the effect of temperature on the variations in the PLmax values. By heating Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors from~30 to 210 • C, a continuous decrease in the emission intensities of the entire PLE and PL spectra were readily observed. Shifts in the emission wavelengths of the entire PLE and PL spectra were not found for all the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors (Figures 3 and 4). Both Figures 3 and 4 also show that the relative emission wavelengths matching the PLEmax values do not change as the measured temperature increases. These results indicate that although the CaO content may change, all Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors have the same excitation and emission mechanisms. These results are presented in Figures 3 and 4, and show that the colour of the emitted by Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors is very stable and does not change with temperature.
Comparing the measured values in Figures 3 and 4, we found that the emission intensities of the entire PLE and PL spectra and the PLEmax and PLmax of all Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors decreased as the value of x value increased from 0.75 to 1.0. These results indicate that the Ca content affects the emission intensities of the entire PLE and PL spectra and the PLEmax and PLmax values of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors. Therefore, the entire PLE and PL spectra of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors for x = 0.50 to 1.25 were measured, and the variations in the PLmax values are compared in Figure 5. As can be seen in Figure 5, the PLmax values of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors first increased as the CaO content increased, peaking at a maximum at x = 0.75, and then decreasing as the CaO content increased further. These results indicate an important finding: that the CaO content  (Figures 3 and 4). Both Figures 3 and 4 also show that the relative emission wavelengths matching the PLEmax values do not change as the measured temperature increases. These results indicate that although the CaO content may change, all CaxMg2-xSi2O6:0.025Eu 2+ phosphors have the same excitation and emission mechanisms. These results are presented in Figures 3 and 4, and show that the colour of the emitted by CaxMg2-xSi2O6:0.025Eu 2+ phosphors is very stable and does not change with temperature.  Comparing the measured values in Figure 3 and Figure 4, we found that the emission intensities of the entire PLE and PL spectra and the PLEmax and PLmax of all CaxMg2-xSi2O6:0.025Eu 2+ phosphors decreased as the value of x value increased from 0.75 to 1.0. These results indicate that the Ca content affects the emission intensities of the entire PLE and PL spectra and the PLEmax and PLmax values of CaxMg2-xSi2O6:0.025Eu 2+ phosphors. Therefore, the entire PLE and PL spectra of CaxMg2-xSi2O6:0.025Eu 2+ phosphors for x = 0.50 to 1.25 were measured, and the variations in the PLmax values are compared in Figure 5. As can be seen in Figure 5, the PLmax values of CaxMg2-xSi2O6:0.025Eu 2+ phosphors first   (Figures 3 and 4). Both Figures 3 and 4 also show that the relative emission wavelengths matching the PLEmax values do not change as the measured temperature increases. These results indicate that although the CaO content may change, all CaxMg2-xSi2O6:0.025Eu 2+ phosphors have the same excitation and emission mechanisms. These results are presented in Figures 3 and 4, and show that the colour of the emitted by CaxMg2-xSi2O6:0.025Eu 2+ phosphors is very stable and does not change with temperature.  Comparing the measured values in Figure 3 and Figure 4, we found that the emission intensities of the entire PLE and PL spectra and the PLEmax and PLmax of all CaxMg2-xSi2O6:0.025Eu 2+ phosphors decreased as the value of x value increased from 0.75 to 1.0. These results indicate that the Ca content affects the emission intensities of the entire PLE and PL spectra and the PLEmax and PLmax values of CaxMg2-xSi2O6:0.025Eu 2+ phosphors. Therefore, the entire PLE and PL spectra of CaxMg2-xSi2O6:0.025Eu 2+ phosphors for x = 0.50 to 1.25 were measured, and the variations in the PLmax values are compared in Figure 5. As can be seen in Figure 5, the PLmax values of CaxMg2-xSi2O6:0.025Eu 2+ phosphors first The excitation (λ em = 450 nm) spectrum of the Ca 0 . 75 Mg 1 . 25 Si 2 O 6 :0.025Eu 2+ phosphor is presented in Figure 6, with the energy (and not the wavelength) plotted on the x-axis. The f-d excitation spectrum of Eu 2+ ions is very complex because it is influenced by several factors. First, the 5d electrons in the 4f 6 5d excited state of the Eu 2+ ions undergo crystal field splitting [17]. Second, the lowest states of the 4f 6 core electrons' configuration in each 4f 6 5d excited state of the Eu 2+ ions are further split into seven 7 F J multiplets (where J = 0-6) by the spin-orbit coupling effect, which broadens each 4f 6 5d excited band energy into~0.62 eV [17]. In addition, the effect of the interactions between the 5d electrons and the effect of the remaining 4f 6 core electrons of the Eu 2+ ions also act to broaden the PLE spectrum [15,16]. Therefore, the transition of 4f 7 -4f 6 5d 1 produces numerous overlapping Materials 2023, 16, 2032 8 of 12 bands in the excitation spectrum of the Eu 2+ ions in CaMgSi 2 O 6 -based phosphors. When the crystal field splitting for the 5d state of the Eu 2+ ions is roughly estimated, the merged excitation profile in the ∼4.96-3.10 eV (250-400 nm) range is fitted to the sum of five Gaussian functions, which are approximately 3.12, 3.40, 3.57, 3.99, and 4.25 eV, respectively. Apparently, the intensity of the band at 3.40 eV was too small in the fitting result.
increased as the CaO content increased, peaking at a maximum at x = 0.75, and then decreasing as the CaO content increased further. These results indicate an important finding: that the CaO content has a crucial effect on the photoluminescence properties of Eu 2+doped CaMgSi2O6-based phosphors. Hence, the entire PL spectra and the PLmax values of Eu 2+ -doped CaMgSi2O6-based phosphors can be enhanced by adjusting the CaO content. The reason behind the Ca0.75Mg1.25Si2O6:0.025Eu 2+ phosphor having the highest PLmax values is not well known. However, from the XRD patterns of the CaxMg2-xSi2O6:0.025Eu 2+ phosphors, we found that the Ca0.75Mg1.25Si2O6:0.025Eu 2+ phosphor has the lowest residual SiO2 phase and a low number of secondary phases. This may be the underlying reason for the result. The excitation (λem = 450 nm) spectrum of the Ca0.75Mg1.25Si2O6:0.025Eu 2+ phosphor is presented in Figure 6, with the energy (and not the wavelength) plotted on the x-axis. The f-d excitation spectrum of Eu 2+ ions is very complex because it is influenced by several factors. First, the 5d electrons in the 4f 6 5d excited state of the Eu 2+ ions undergo crystal field splitting [17]. Second, the lowest states of the 4f 6 core electrons' configuration in each 4f 6 5d excited state of the Eu 2+ ions are further split into seven 7 FJ multiplets (where J = 0-6) by the spin-orbit coupling effect, which broadens each 4f 6 5d excited band energy into ~0.62 eV [17]. In addition, the effect of the interactions between the 5d electrons and the effect of the remaining 4f 6 core electrons of the Eu 2+ ions also act to broaden the PLE spectrum [15,16]. Therefore, the transition of 4f 7 -4f 6 5d 1 produces numerous overlapping bands in the excitation spectrum of the Eu 2+ ions in CaMgSi2O6-based phosphors. When the crystal field splitting for the 5d state of the Eu 2+ ions is roughly estimated, the merged excitation profile in the ∼4.96-3.10 eV (250-400 nm) range is fitted to the sum of five Gaussian functions, which are approximately 3.12, 3.40, 3.57, 3.99, and 4.25 eV, respectively. Apparently, the intensity of the band at 3.40 eV was too small in the fitting result. However, the lowest 4f 6 5d 1 energy can be measured or analysed using other methods, except for the estimated 3.12 eV mentioned earlier. The lowest 4f 6 5d 1 energy is evaluated as ∼3.12 eV if we consider the 3.57 eV as the approximate main peak and the transition from the lowest 5d state to the 7 F6 multiplet by subtracting the 7 FJ (J = 0, 6) energy difference of 0.45 eV from the main peak [17]. Based on the PLE spectra in Figure 6, the median value of 3.57 eV is recognised as the lowest 4f 6 5d 1 energy of four estimations: 3.12, 3.40, 3.57, and 3.99 eV. The 3.12, 3.40, and 3.99 eV estimates can be attributed to the excited states of some defects in the synthesised CaMgSi2O6:yEu 2+ phosphors rather than undetected impurity phases [17]. This is because there are different interactions occurring between the remaining 4f 6 core electrons of the Eu 2+ ions and the 5d electron. Therefore, the 4f 7 -4f 6 5d 1 excitation spectrum of the Eu 2+ ions in the CaMgSi2O6 phosphor contain numerous overlapping bands, and these results prove that the emission peak at 450 nm is caused by a transition from the excited state 4f 6 5d 1 to the ground state 4f 7 [18,19]. The narrow emission band of the Ca0.75Mg1.25Si2O6:0.025Eu 2+ phosphor is located at ∼2.76 eV (∼450 nm), and its FWHM of the PL spectrum is about 0.15 eV. When a molecule or atom absorbs However, the lowest 4f 6 5d 1 energy can be measured or analysed using other methods, except for the estimated 3.12 eV mentioned earlier. The lowest 4f 6 5d 1 energy is evaluated as ∼3.12 eV if we consider the 3.57 eV as the approximate main peak and the transition from the lowest 5d state to the 7 F 6 multiplet by subtracting the 7 F J (J = 0, 6) energy difference of 0.45 eV from the main peak [17]. Based on the PLE spectra in Figure 6, the median value of 3.57 eV is recognised as the lowest 4f 6 5d 1 energy of four estimations: 3.12, 3.40, 3.57, and 3.99 eV. The 3.12, 3.40, and 3.99 eV estimates can be attributed to the excited states of some defects in the synthesised CaMgSi 2 O 6 :yEu 2+ phosphors rather than undetected impurity phases [17]. This is because there are different interactions occurring between the remaining 4f 6 core electrons of the Eu 2+ ions and the 5d electron. Therefore, the 4f 7 -4f 6 5d 1 excitation spectrum of the Eu 2+ ions in the CaMgSi 2 O 6 phosphor contain numerous overlapping bands, and these results prove that the emission peak at 450 nm is caused by a transition from the excited state 4f 6 5d 1 to the ground state 4f 7 [18,19]. The narrow emission band of the Ca 0 . 75 Mg 1 . 25 Si 2 O 6 :0.025Eu 2+ phosphor is located at ∼2.76 eV (∼450 nm), and its FWHM of the PL spectrum is about 0.15 eV. When a molecule or atom absorbs a photon to gain its energy, it enters an excited state. However, when the absorbed photon has more energy than the emitted photon, the difference between the two energies is the Stokes shift. For the Ca 0 . 75 Mg 1 . 25 Si 2 O 6 :0.025Eu 2+ phosphor, the Stokes shift is the difference in energy between the positions of the band maxima of the emission and absorption spectra for the fluorescence of the same electronic transition. Accordingly, the Stokes shift for the Eu 2+ ions in the CaMgSi 2 O 6 :0.025Eu 2+ phosphor is calculated to be ∼0.36 eV using the energy difference between the maxima of the emission band of ∼2.76 eV and the estimated lowest 5d excitation band of ∼3.12 eV.
To find the variations in the relative PLEmax values, the PLEmax values of all the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors measured at 30 • C were used as standard values to normalise the PLEmax values measured at temperatures changed from 30 to 210 • C; the results are presented in Figure 7a,b, respectively.  The decay time of all CaxMg2-xSi2O6:0.025Eu 2+ phosphors is defined as when the intensity of the PLmax decreases from its maximum value to 36.8% (1/e). In this study, the optimum wavelength to excite the CaxMg2-xSi2O6:yEu 2+ phosphors was 346 nm, and the wavelength to measure the intensity decay was 450 nm because the excitation and emission peaks of all the CaxMg2-xSi2O6:yEu 2+ phosphors were at those wavelengths. The measured decay time curves of CaxMg2-xSi2O6:0.025Eu 2+ phosphors are presented in Figure 8, from which the decay time was measured to be 0.85, 0.82, 0.79, and 0.86 ms for the x values x = 0.5, 0.75, 1.0, and 1.25, respectively. The decay time curves of all the CaxMg2-xSi2O6:0.025Eu 2+ phosphors presented in Figure 8 have one exponential decay stage and similar decay changes. This result further indicates that all CaxMg2-xSi2O6:0.025Eu 2+ phosphors have a similar emission mechanism. Although the Ca content has an apparent effect on the PLE and PL properties of all CaxMg2-xSi2O6:0.025Eu 2+ phosphors, it has no apparent effect on the crystalline structure of all the CaxMg2-xSi2O6:0.025Eu 2+ phosphors. Thus, it has no apparent effect on the variations in decay time curves. A curve-fitting technology was The decay time of all Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors is defined as when the intensity of the PLmax decreases from its maximum value to 36.8% (1/e). In this study, the optimum wavelength to excite the Ca x Mg 2-x Si 2 O 6 :yEu 2+ phosphors was 346 nm, and the wavelength to measure the intensity decay was 450 nm because the excitation and emission peaks of all the Ca x Mg 2-x Si 2 O 6 :yEu 2+ phosphors were at those wavelengths. The measured decay time curves of Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors are presented in Figure 8, from which the decay time was measured to be 0.85, 0.82, 0.79, and 0.86 ms for the x values x = 0.5, 0.75, 1.0, and 1.25, respectively. The decay time curves of all the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors presented in Figure 8 have one exponential decay stage and similar decay changes. This result further indicates that all Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors have a similar emission mechanism. Although the Ca content has an apparent effect on the PLE and PL properties of all Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors, it has no apparent effect on the crystalline structure of all the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors. Thus, it has no apparent effect on the variations in decay time curves. A curve-fitting technology was used to simulate the decay processes for the PL intensities of all the synthesised Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors, and the decay time curves were fitted using one exponential component, which is expressed as the following equation: where I 0 (t) is the PL intensity at a defined wavelength; t is time; I 1 , I 2 , and I 3 are constants; and τ 1 , τ 2 , and τ 3 are the time constants of the exponential components. The decay time curves of all the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors shown in Figure 8 were successfully fitted using Equation (1), and the relative τ 1 , τ 2 , and τ 3 parameters of all the fitting curves are presented in Table 1

Conclusions
The effects of Eu 2+ concentration and CaO content on the photoluminescence properties of Ca x Mg 2-x Si 2 O 6 :yEu 2+ phosphors were well investigated. The emission intensities of the entire PLE (PLEmax) spectra of the CaMgSi 2 O 6 :yEu 2+ phosphors reached their maxima at y = 0.025. However, because the Eu 2+ concentration was higher than 0.025, the concentration quench effect caused them to diminish. The emission intensities of the entire PLE (PLEmax) spectra of the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors reached their maxima at x = 0.75. This is because the Ca 0 . 75 Mg 1 . 25 Si 2 O 6 :0.025Eu 2+ phosphor had the smallest FWHM value and the least raw materials and secondary phases. Using the Rietveld refinement method, the calculated structural parameters of the CaMgSi 2 O 6 :0.025Eu 2+ phosphor were a = 9.745 Å, b = 8.933 Å, c = 5.248 Å, α = γ = 90 • , β = 1 05.87 • , and V = 439.3 Å 3 . When N = 4, x c = 0.025 and V = 439.3Å 3 were incorporated into the R o = 2[(3V)/(4πx c N)] 1/3 equa-tion to calculate the R o value of the CaMgSi 2 O 6 :0.025Eu 2+ phosphor, which was calculated to be approximately 20.3 Å. The merged excitation profile in the ∼4.96-3.10 eV (250-400 nm) range was fitted to the sum of five Gaussian functions, which were approximately 3.12, 3.40, 3.57, 3.99, and 4.25 eV. The Stokes shift for the Eu 2+ ions in the CaMgSi 2 O 6 :0.025Eu 2+ phosphor was calculated to be ∼0.36 eV. For all the Ca x Mg 2-x Si 2 O 6 :0.025Eu 2+ phosphors, their PLEmax values decreased from an initial 100% to approximately 70-78% at 120 • C and 35-42% at 210 • C.