Investigations of Photoluminescence Properties of Ca_xMg_2-xSi₂O₆:yEu²⁺ (x = 0.5–1.25, y = 0.015–0.035) Phosphors

Abstract

Previously, there were almost no relevant studies on developing the optimal Ca_xMg_2-xSi₂O₆:yEu²⁺ phosphor composition for its finest optical properties. This study employs two steps to determine the optimal composition for Ca_xMg_2-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, Ca_xMg_2-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 Ca_xMg_2-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 Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors were performed to identify the factors responsible for this outcome.

1. 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 (SiO₂) 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 CaMgSi₂O₆ have a monoclinic structure and have attracted considerable attention because they have unique structural and optical features. For example, when CaMgSi₂O₆-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 CaMgSi₂O₆-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 CaMgSi₂O₆-based powders as activators to produce phosphors of different colours and to investigate their physical and photoluminescence properties. For instance, Chandrakar et al. used Eu²⁺ as an activator and investigated the particle size, crystal structure, photoluminescence and thermoluminescence of an Eu²⁺-doped CaMgSi₂O₆ phosphor [3]. The Eu²⁺-doped CaMgSi₂O₆ phosphor emitted a blue colour, with an emission peak of approximately 449 nm [3,4]. Subsequently, Chandrakar et al. used Ce³⁺ ions as an activator, and the excitation and emission peaks of the synthesised CaMgSi₂O₆:Ce³⁺ phosphors were recorded as 295 and 327 nm, respectively [5].

Eu₂O₃ is a commonly used activator because it can produce light of different colours. When Eu₂O₃-doped phosphors are synthesised in a reducing atmosphere, the Eu³⁺ ions are reduced to Eu²⁺ ions, and most of the synthesised phosphors emit a blue or green colour. For example, when Eu₂O₃ is added to a Sr₃MgSi₂O₈-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 Eu₂O₃ is added to a Ca₂MgSi₂O₇-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 Eu₂O₃-doped phosphors are synthesised in an air atmosphere, the Eu³⁺ ions dominate the emitting properties, and the synthesised phosphors emit a red colour or a near-infrared (NIR) emission. For example, when Eu₂O₃ is added to a BaY₂O₄-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 Eu₂O₃ is added to a Sr₃La(PO₄)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 (SiO₂) and alkaline earth oxides are accompanied by changes in their emission characteristics.

Zhang and Wang researched the photoluminescence of Eu²⁺-doped CaMgSi_2xO_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_2xO_6+2x:0.01Eu²⁺ phosphors exhibited no apparent change with excess Si content, an appropriate amount of excess Si enhanced the photoluminescence intensity of CaMgSi_2xO_6+2x:0.01Eu²⁺ phosphors [10]. Lee et al. found that by controlling the substituting concentration of Ca ions to Sr ions and the Eu²⁺ concentration (range = 0.015–0.045), the photoluminescence properties of (Ca_1-xSr_x)Mg₂Si₃O₉: Eu²⁺ (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-xSr_x)Mg₂Si₃O₉:Eu²⁺ phosphors exhibit no apparent change [11]. Tseng et al. used Ca_2+xMgSi₂Eu₀.₀₂₅O_7+x as the primary composition of specimens used to investigate the crystalline phases and photoluminescence properties of Ca₂MgSi₂Eu₀.₀₂₅O₇ phosphors. They found that the Ca content has a substantial effect on the crystalline phases and photoluminescence properties of synthesised Ca_2+xMgSi₂Eu₀.₀₂₅O_7+x powders: as the Ca content increases, the central wavelength of the Ca_2+xMgSi₂Eu₀.₀₂₅O_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 Eu²⁺ ions significantly affect the crystallisation and the optical properties of phosphors based on Ca₂MgSi₂O₇-Eu²⁺ (Ca_xMg_2-xSi₂O₆:yEu²⁺). Therefore, in this study, phosphor specimens with various Ca_xMg_2-xSi₂O₆ compositions (x = 0.5, 0.75, 1.0, and 1.25) were used as the host material, and Eu₂O₃ was used as the activator to prepare Ca_xMg_2-xSi₂O₆:yEu²⁺ 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% N₂ + 5% H₂ 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 Eu²⁺-doped CaMgSi₂O₆ phosphors, only a few studies have focused on the concentration quenching effect of CaMgSi₂O₆:yEu²⁺ phosphors. Consequently, the first critical contribution of this study is the finding that the Eu²⁺ ion concentration has a substantial effect on the PLE and PL properties of the Eu²⁺-doped CaMgSi₂O₆ phosphor. To this end, CaMgSi₂O₆:yEu²⁺ (y = 0.015, 0.020, 0.025, 0.030, and 0.035) was used as the primary composition to determine the optimal Eu²⁺ ion concentration. We show that the optimal Eu²⁺ 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 CaMgSi₂O₆:yEu²⁺ phosphors are presented. We prove that the concentration quenching effect is the underlying cause of the diminished photoluminescence properties of Eu²⁺-doped CaMgSi₂O₆ phosphors at y values higher than 0.025. The effect of the Ca content on all the photoluminescence properties of Eu²⁺-doped Ca_xMg_2-xSi₂O₆ phosphors is also discussed. The second major contribution of this study is its innovative approach to this aspect of the research: synthesised Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ powders were used to investigate the effect of the Ca content on the photoluminescence properties of CaMgSi₂O₆ phosphors. The underlying cause of the observed variation in the properties of Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors were also investigated.

2. Materials and Methods

SiO₂ (purity: 99.99%; Nano Structured & Amorphous Materials Inc., Houston, TX, USA), MgCO₃ (purity 99.5%; US Research Nanomaterials Inc., Houston, TX, USA), CaCO₃ (purity 98.5%; Fullin Nihon Shiyaku Bicohemical Ltd., Taoyuan, Taiwan), and Eu₂O₃ (purity 99.99%; US Research Nanomaterials Inc., Houston, TX, USA) were used as raw materials, and were measured out and weighed to match the chosen compositions of CaMgSi₂O₆:yEu²⁺ (y = 0.015, 0.020, 0.025, 0.030, and 0.035) and Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ (x = 0.5, 0.75, 1.0, and 1.25) powders. The weighed CaMgSi₂O₆:yEu²⁺ and Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ powders would melt. Therefore, the CaMgSi₂O₆:yEu²⁺ and Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ powders were synthesised at 1300 °C for 4 h in a reducing atmosphere of 95% N₂ + 5% H₂, which was simultaneously used to deoxidise Eu³⁺ ions into Eu²⁺ ions. We found that the maximum PLE (PLEmax) and maximum PL (PLmax) values for the CaMgSi₂O₆:0.025Eu²⁺ phosphor were higher than those of other CaMgSi₂O₆:yEu²⁺ phosphors. Therefore, a 0.025Eu²⁺ concentration and a Ca_xMg_2-xSi₂O₆ composition were used to investigate the effect of Ca content on the optical properties of Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors. To determine the optimal optical properties of CaMgSi₂O₆:yEu²⁺ and Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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 CaMgSi₂O₆:yEu²⁺ and Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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.

3. Results and Discussion

One of the objectives of this study was to find the concentration quenching effect of Ca_xMg_2-xSi₂O₆:Eu²⁺-based phosphors. To this end, CaMgSi₂O₆ was used as the host material, CaMgSi₂O₆:yEu²⁺ was used as the composition, and the Eu²⁺ ion concentration was altered from 0.015 to 0.035 (y = 0.015–0.035). The PLE spectra of the synthesised CaMgSi₂O₆:yEu²⁺ phosphors were measured as a function of the Eu²⁺ 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 CaMgSi₂O₆:yEu²⁺ phosphors initially increased as the Eu²⁺ ion concentration increased, peaking at y = 0.025 and then decreasing as the Eu²⁺ ion concentration increased further. These results indicate that the Eu²⁺ concentration has a substantial effect on the optical properties of Eu²⁺-doped CaMgSi₂O₆ (Ca_xMg_2-xSi₂O₆) 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 CaMgSi₂O₆:yEu²⁺ phosphors. As can be seen in the results presented in Figure 1a, we found that CaMgSi₂O₆:yEu²⁺ 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 CaMgSi₂O₆:yEu²⁺ 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 CaMgSi₂O₆:yEu²⁺ phosphors in all further analyses.

Because the red spectral range exhibits the characteristic f–f transitions of Eu³⁺ ions and is in the region of 570–670 nm, i.e., ⁵D₀→⁷F_j (j = 1, 2, and 3), the red spectral range was not observed in all PL spectra of CaMgSi₂O₆:yEu²⁺ 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 CaMgSi₂O₆:yEu²⁺ phosphors as a function of Eu²⁺ concentration; the observed data are presented in Figure 1b. The emission intensities of the PL spectra of CaMgSi₂O₆:yEu²⁺ phosphors showed a similar trend as the PLE spectra. For the synthesised CaMgSi₂O₆:yEu²⁺ phosphors, as can be seen in Figure 1b, the emission intensities of all peaks initially increased with the Eu²⁺ concentration, peaking at y = 0.025 and then decreasing as the Eu²⁺ concentration was increased further. The emission peak at 450 nm, which is the blue colour, is caused by a transition from the excited state 4f⁶5d¹ to the ground state 4f⁷. As the Eu²⁺ 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., Eu²⁺ 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 Eu²⁺ ion concentration in Ca_xMg_2-xSi₂O₆-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 Eu²⁺ ions in CaMgSi₂O₆:yEu²⁺ 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 CaMgSi₂O₆:yEu²⁺ phosphors is calculated using the formula: R_o = 2[(3V)/(4πx_cN)]^1/3, where x_c, V, and N are the critical concentration of the Eu²⁺ 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 CaMgSi₂O₆:0.025Eu²⁺ phosphor were calculated using the Rietveld refinement method. The values of the refinement factors Rwp and Rp for CaMgSi₂O₆:0.025Eu²⁺ 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 CaMgSi₂O₆:0.025Eu²⁺ 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 ų. When N = 4, x_c = 0.025 and V = 439.3 ų were incorporated into the R_o = 2[(3V)/(4πx_cN)]^1/3 formula to calculate the R_o value of CaMgSi₂O₆:yEu²⁺ 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 R_o value for CaMgSi₂O₆:yEu²⁺ 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:yEu²⁺ phosphors. We believe that as the Eu²⁺ ion concentration exceeds 0.025, the effectivity of the multipole–multipole interactions increases, and the concentration quenching effect of CaMgSi₂O₆:yEu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors. Therefore, the XRD patterns of synthesised Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors is monoclinic, with a space group C2/c structure, which is in good agreement with the standard JCPDS No. 75-1092. For Ca₀.₅Mg₁.₅Si₂O₆:0.025Eu²⁺ and Ca₀.₇₅Mg₁.₂₅Si₂O₆:0.025Eu²⁺ phosphors, all the diffraction peaks of the CaMgSi₂O₆ phase in the two synthesised powders were revealed, and the SiO₂ phase was also observed, as can be seen in Figure 2a. Apparently, as the CaO content of Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors increased from x = 0.75 to x = 1.25, the diffraction intensity of the SiO₂ phase decreased. As the CaO content increased further, many secondary phases were observed in the synthesised CaMgSi₂O₆:0.025Eu²⁺ and Ca₀₁.₂₅Mg₀.₇₅Si₂O₆:0.025Eu²⁺ phosphors, including SiO₂, Ca₂SiO₄, Ca₂MgSi₂O₇, and Ca₃MgSi₂O₈. 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_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ 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 Ca₀.₇₅Mg₁.₂₅Si₂O₆:0.025Eu²⁺ phosphor had the smallest full width at half maximum (FWHM). The results indicate that the CaO content affects the resulting composition of Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors, consequently transmitting their photoluminescence properties.

However, from the PLE spectra, we found that all Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors were measured at different temperatures. However, when the temperature increased, the photoluminescence properties of all the synthesised Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors exhibited a similar variation trend. Therefore, only the measured values of the Ca₀.₇₅Mg₁.₂₅Si₂O₆:0.025Eu²⁺ and CaMgSi₂O₆:0.025Eu²⁺ phosphors were used as representative values. The measured values for the PLE and PL spectra are presented in Figure 3 and Figure 4, respectively. The PL properties of Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors (Figure 3 and Figure 4). Both Figure 3 and Figure 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_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors have the same excitation and emission mechanisms. These results are presented in Figure 3 and Figure 4, and show that the colour of the emitted by Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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 Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors. Therefore, the entire PLE and PL spectra of Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ 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 has a crucial effect on the photoluminescence properties of Eu²⁺-doped CaMgSi₂O₆-based phosphors. Hence, the entire PL spectra and the PLmax values of Eu²⁺-doped CaMgSi₂O₆-based phosphors can be enhanced by adjusting the CaO content. The reason behind the Ca₀.₇₅Mg₁.₂₅Si₂O₆:0.025Eu²⁺ phosphor having the highest PLmax values is not well known. However, from the XRD patterns of the Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors, we found that the Ca₀.₇₅Mg₁.₂₅Si₂O₆:0.025Eu²⁺ 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 (λ_em = 450 nm) spectrum of the Ca₀.₇₅Mg₁.₂₅Si₂O₆:0.025Eu²⁺ phosphor is presented in Figure 6, with the energy (and not the wavelength) plotted on the x-axis. The f–d excitation spectrum of Eu²⁺ ions is very complex because it is influenced by several factors. First, the 5d electrons in the 4f⁶5d excited state of the Eu²⁺ ions undergo crystal field splitting [17]. Second, the lowest states of the 4f⁶ core electrons’ configuration in each 4f⁶5d excited state of the Eu²⁺ ions are further split into seven ⁷F_J multiplets (where J = 0–6) by the spin–orbit coupling effect, which broadens each 4f⁶5d 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 4f⁶ core electrons of the Eu²⁺ ions also act to broaden the PLE spectrum [15,16]. Therefore, the transition of 4f⁷–4f⁶5d¹ produces numerous overlapping bands in the excitation spectrum of the Eu²⁺ ions in CaMgSi₂O₆-based phosphors. When the crystal field splitting for the 5d state of the Eu²⁺ 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 4f⁶5d¹ energy can be measured or analysed using other methods, except for the estimated 3.12 eV mentioned earlier. The lowest 4f⁶5d¹ 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 ⁷F₆ multiplet by subtracting the ⁷F_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 4f⁶5d¹ 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:yEu²⁺ phosphors rather than undetected impurity phases [17]. This is because there are different interactions occurring between the remaining 4f⁶ core electrons of the Eu²⁺ ions and the 5d electron. Therefore, the 4f⁷–4f⁶5d¹ excitation spectrum of the Eu²⁺ 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 4f⁶5d¹ to the ground state 4f⁷ [18,19]. The narrow emission band of the Ca₀.₇₅Mg₁.₂₅Si₂O₆:0.025Eu²⁺ 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 Ca₀.₇₅Mg₁.₂₅Si₂O₆:0.025Eu²⁺ 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 Eu²⁺ ions in the CaMgSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ 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. Figure 7 also shows that the thermal quenching effects of the PLEmax values for all the Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors have similar trends. For all Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors, their PLEmax values decreased from an initial 100% to approximately 70–78% at 120 °C and to about 35–42% at 210 °C. The commercial phosphor showed this effect as the temperature increased from 25 to 80 °C [20]. However, the synthesised Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors have higher stability than the commercial phosphor, and the wavelengths matching the PLmax values shift with the changing temperature. Figure 7 also shows that when the same measured temperature was used, the variation in the PLEmax values increased slightly with the CaO content. These results (Figure 7) also show that the thermal quenching effect is not apparent in all Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors.

The decay time of all Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:yEu²⁺ 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_xMg_2-xSi₂O₆:yEu²⁺ phosphors were at those wavelengths. The measured decay time curves of Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors presented in Figure 8 have one exponential decay stage and similar decay changes. This result further indicates that all Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors have a similar emission mechanism. Although the Ca content has an apparent effect on the PLE and PL properties of all Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors, it has no apparent effect on the crystalline structure of all the Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors, and the decay time curves were fitted using one exponential component, which is expressed as the following equation:

I₀(t) = I₁ exp[−(t/τ₁)] + I₂ exp[−(t/τ₂)] + I₃ exp[−(t/τ₃)]

(1)

where I₀(t) is the PL intensity at a defined wavelength; t is time; I₁, I₂, and I₃ are constants; and τ₁, τ₂, and τ₃ are the time constants of the exponential components. The decay time curves of all the Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors shown in Figure 8 were successfully fitted using Equation (1), and the relative τ₁, τ₂, and τ₃ parameters of all the fitting curves are presented in

Table 1. Fitting results of the decay time curves for Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors.

x Value	τ1	τ2	τ3
0.5	0.37049	0.39149	0.38108
0.75	0.36668	0.38130	0.38792
1.0	0.37125	0.38613	0.38751
1.25	0.37428	0.38828	0.38687

for x = 0.5, 0.75, 1.0, and 1.25.

4. Conclusions

The effects of Eu²⁺ concentration and CaO content on the photoluminescence properties of Ca_xMg_2-xSi₂O₆:yEu²⁺ phosphors were well investigated. The emission intensities of the entire PLE (PLEmax) spectra of the CaMgSi₂O₆:yEu²⁺ phosphors reached their maxima at y = 0.025. However, because the Eu²⁺ 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_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors reached their maxima at x = 0.75. This is because the Ca₀.₇₅Mg₁.₂₅Si₂O₆:0.025Eu²⁺ 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 CaMgSi₂O₆:0.025Eu²⁺ 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 ų. When N = 4, x_c = 0.025 and V = 439.3ų were incorporated into the R_o = 2[(3V)/(4πx_cN)]^1/3 equation to calculate the R_o value of the CaMgSi₂O₆:0.025Eu²⁺ 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 Eu²⁺ ions in the CaMgSi₂O₆:0.025Eu²⁺ phosphor was calculated to be ∼0.36 eV. For all the Ca_xMg_2-xSi₂O₆:0.025Eu²⁺ phosphors, their PLEmax values decreased from an initial 100% to approximately 70–78% at 120 °C and 35–42% at 210 °C.