Effects of Composition Variations on the Crystalline Phases and Photoluminescence Properties of Ca2+x MgSi2Eu0.025O7+x Phosphors.

A solid-state reaction method was used to synthesize Ca2+x MgSi2Eu0.025O7+x (x = 0-1.0) powders in the air atmosphere and in a reduction atmosphere (95% N2 + 5% H2) at 1350 °C and 4 h, and the reduction atmosphere was removed at 800 °C. Only the Ca2MgSi2O7 phase was found in the XRD pattern of the synthesized Ca2MgSi2Eu0.025O7 powder. The first important discovery was that when the x value of Ca2+x MgSi2Eu0.025O7+x powders was increased from 0.2 to 0.8, both Ca2MgSi2O7 and Ca3MgSi2O8 phases coexisted in the synthesized Ca2+x MgSi2Eu0.025O7+x powders, and the diffraction intensity of the Ca2MgSi2O7 (Ca3MgSi2O8) phase decreased (increased) with the x value. The second important discovery was that the Ca2MgSi2Eu0.025O7 phosphor exhibited stronger photoluminescence excitation (PLE), photoluminescence (PL), and decay curve properties than the Ca2.2MgSi2Eu0.025O7.2 phosphor, and the Ca3MgSi2Eu0.025O8 phosphor exhibited stronger PLE, PL, and decay curve properties than the Ca2+x MgSi2Eu0.025O7+x phosphors for x = 0.4, 0.6, and 0.8. For x = 0.2-0.8, the PL spectra of the Ca2+x MgSi2Eu0.025O7+x phosphors were a combination of the PL spectra of Ca2MgSi2Eu0.025O7 and Ca3MgSi2Eu0.025O8 phosphors. The third important discovery was that as the x value was increased, the maximum emission peak wavelengths of the Ca2+x MgSi2Eu0.025O7+x phosphors shifted to a lower value, the maximum emission intensity of the PL spectra increased, and the emission light changed from green and cyan to blue.

Introduction

The Ca–Mg–Si–O system exists in many different compositions, including CaMgSiO4, CaMgSi2O6, Ca2MgSi2O7, and Ca3MgSi2O8.[1,2] Ca2MgSi2O7-based and Ca3MgSi2O8-based phosphors are well known as the efficient materials of phosphors because they have good chemical and thermal stabilities. In the past, an Eu2O3-doped Ca2MgSi2O7 phosphor was synthesized in a reduction atmosphere, which emitted green light.[3,4] Sahu had found that a 327 nm-excited Ce3+-doped Ca2MgSi2O7 phosphor emitted two emission bands with the central wavelengths of 373 and 393 nm to form a broadband peak of 385 nm.[5] Sahu et al. used Eu2+ and Dy3+ as the codoped ions, and the synthesized Ca2MgSi2O7 phosphor emitted green light with a central wavelength of 535 nm.[6] Cao et al. used Eu2+ and Tm3+ as the codoped ions of the Ca2MgSi2O7 phosphor, and found that when the synthesis temperature was increased from 1250 to 1350 °C, the central emission wavelengths changed from 475 to 521 nm.[7] However, the Eu2+-doped Ca2MgSi2O7 phosphor and most of the Eu2+-based multi ion-doped Ca2MgSi2O7 phosphors emitted green light. Ca3MgSi2O8-based phosphors have also attracted intensive and long-lasting interest of the researchers because they have the characteristics of high quantum efficiencies (η)[8] and thermal stability.[9] Dewangan et al. investigated the luminescence properties of a 351 nm-excited Dy3+-doped Ca3MgSi2O8 phosphor, whose emission spectrum had three peaks located at 482, 493, and 574 nm of blue light and yellow light.[10] However, mostly Ca3MgSi2O8 was only doped with Eu2+ ions[9] or codoped with Eu2+ ions with other ions,[11] which exhibited an emission peak at about 480 nm of a blue or greenish blue light.

However, no researchers have investigated the effects of the synthesis atmosphere and variations in the composition of Ca2+MgSi2O7+ powders (x = 0–1.0) on their PLE, PL, and crystal properties. First, Ca2+MgSi2O7+ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) phosphors were synthesized in the air or a reduction atmosphere of 5% H2 + 95% N2 to find the effect of the synthesis atmosphere on their optical properties. We found that the Ca2+MgSi2Eu0.025O7+ powders synthesized under different atmospheres had different emission properties. Hence, we first analyzed the crystalline phases of Ca2+MgSi2Eu0.025O7+ compositions using their XRD patterns to find out the reasons for the variations in emission properties of their PLE and PL spectra. Thermal stability of Ca2+MgSi2Eu0.025O7+ phosphors is a very important factor for their further applications in LEDs, and an increase in temperature has a conceivable effect on their emission intensity. However, only a few publications addressed the effect of thermal stability on the PL properties of the Ca2MgSi2Eu0.025O7 and Ca3MgSi2Eu0.025O8 phosphors, and no research addressed the thermal stability of the Ca2+MgSi2Eu0.025O7+ phosphors for x = 0.2–0.8.

Therefore, the second purpose of this research is to measure the PL spectra from room temperature to 200 °C to find out the relationship between the temperature and emission intensities of the Ca2+MgSi2Eu0.025O7+ phosphors. The second important discovery is that for x values of 0 and 0.2, the Ca2MgSi2Eu0.025O7 phosphor exhibited stronger PL, PLE, and decay curve properties (temperature-dependent emission intensities) than the other phosphors. For x values equal to and larger than 0.4, the Ca3MgSi2Eu0.025O8 phosphor exhibited stronger PL properties than the other phosphors. The third important discovery is that for x = 0.2, 0.4, 0.6, and 0.8, the PL spectra of the synthesized Ca2+MgSi2Eu0.025O7+ phosphors were a combination of the PL spectra of synthesized Ca2MgSi2Eu0.025O7 and Ca3MgSi2Eu0.025O8 phosphors. Therefore, the third purpose of this research is to show that the PL spectra of Ca2+MgSi2Eu0.025O7+ phosphors synthesized in a reduction atmosphere can be divided into the PL spectra of Ca2MgSi2Eu0.025O7 and Ca3MgSi2Eu0.025O8 phosphors, which could be used to prove that the Ca2MgSi2O7 and Ca3MgSi2O8 compounds coexisted in the Ca2+MgSi2Eu0.025O7+ phosphors for x = 0.2–0.8.

Results and Discussion

The X-ray diffraction (XRD) patterns of 1350 oC-synthesized Ca2+MgSi2Eu0.025O7+ powders as a function of x value are shown in Figure , and all the source materials were not observed in the synthesized Ca2+MgSi2Eu0.025O7+ powders. For an x value of 0, only the Ca2MgSi2O7 phase was observed in the synthesized Ca2MgSi2Eu0.025O7 powder, and the crystalline phase had a good agreement with the JCPDS no. 74-0990, as shown in Figure b. The main diffraction peak of the (310) plane of the Ca3MgSi2O8 phase was observed in the Ca2.2MgSi2Eu0.025O7.2 powder. The diffraction intensity of the Ca2MgSi2O7 phase decreased and that of the Ca3MgSi2O8 phase increased as the x value was increased from 0.2 to 1. In the Ca3MgSi2Eu0.025O8 composition (x = 1), only a very weak main diffraction peak of the (211) plane of the Ca2MgSi2O7 phase was observed. The diffraction peaks of the Ca3MgSi2O8 phase were found to be in good agreement with the JCPDS no. 89-2432, as shown in Figure b. These results prove that when the Ca2+MgSi2Eu0.025O7+ powders with x = 0.2–0.8 were synthesized at 1350 °C, the Ca2MgSi2O7 and Ca3MgSi2O8 phases coexisted in the Ca2+MgSi2Eu0.025O7+ powders. Therefore, the synthesized Ca2+MgSi2Eu0.025O7+ powders with x = 0.2–0.8 can be recognized as a mixture of the Ca2MgSi2Eu0.025O7 + Ca3MgSi2Eu0.025O8 phases. However, the optical properties of the Ca2+MgSi2Eu0.025O7+ phosphors will be affected by the two Ca2MgSi2O7 and Ca3MgSi2O8 compounds.

Figure 1

(a) XRD patterns of the synthesized Ca2+MgSi2Eu0.025O7+ powders as a function of x value. (b) Standard JCPDS nos. 74-0990 and 89-2432 for Ca2MgSi2O7 and Ca3MgSi2O8 phases.

If Eu2O3 was used as the activator, and the mixed powders were synthesized in a non-reduction atmosphere, the phosphors would emit red light[12−14] rather than blue or green light.[15,16] The PLE spectra of the air atmosphere-synthesized Ca2+MgSi2Eu0.025O7+ phosphors were recorded at room temperature and monitored at 616 nm with an excitation band in the range of 200–450 nm. As shown in Figure a, for the Ca2MgSi2Eu0.025O7 phosphor, there is one broad peak centered at about 235 nm and three sharp peaks centered at 361, 380, and 392 nm. However, for other Ca2+MgSi2Eu0.025O7+ phosphors, there is one broad peak centered at about 250 nm and also three sharp peaks centered at 361, 380, and 392 nm. The maximum emission intensity of the one broad band centered at 250 nm increased slightly and the maximum emission intensities of three sharp peaks increased apparently with x value. These results suggest that the maximum emission intensity in the PLE spectra of the Ca3MgSi2O8 phosphor is larger than that of the Ca2MgSi2O7 phosphor, which causes an increase in the maximum emission intensities of the PLE spectra of Ca2+MgSi2Eu0.025O7+ phosphors with x value. These results also suggest that x values of Ca2+MgSi2Eu0.025O7+ phosphors have a large effect on their optical properties.

Figure 2

PLE spectra of the Ca2+MgSi2Eu0.025O7+ phosphors synthesized (a) in the air atmosphere and (b) in a reduction atmosphere.

Figure b shows the results of the PLE spectra of all Ca2+MgSi2Eu0.025O7+ phosphors synthesized in a reduction atmosphere, which were recorded at room-temperature while being monitored at different wavelengths. All Ca2+MgSi2Eu0.025O7+ phosphors had one broad band with two unapparent peaks, one centered at about 280 nm, and the other was shifted from 373 nm to about 350 nm as the x value was increased. As the x value of the Ca2+MgSi2Eu0.025O7+ phosphors was changed from 0 to 0.2, the monitored wavelengths shifted from 530 nm (green light) to 529 nm, which suggested that the PLE properties of the Ca2MgSi2Eu0.025O7 phosphor dominate the two PLE curves. As the x value of the Ca2+MgSi2Eu0.025O7+ phosphors was changed from 0.2 (0.4) to 0.4 (1.0), the monitored wavelengths critically shifted from 529 (482) nm (green light) to 482 (474) nm (blue light). These results suggest that for the x value equal to and more than 0.4, the Ca3MgSi2Eu0.025O8 phosphor exhibited stronger PLE property than the Ca2MgSi2Eu0.025O7 phosphor. Figure b also shows another important result that the maximum emission intensity of the broad band increased slightly as the x value was increased from x = 0 to x = 0.4, and it increased critically as the x value was increased from x = 0.4 to x = 0.6. From the XRD patterns, we believe that the increase in the diffraction intensity of the Ca3MgSi2O8 phase with x value is the reason for the increase of maximum emission intensities of the PLE spectra. These results suggest that the composition ratio of two different compounds is the most important factor that affects the optical properties of the Ca2+MgSi2Eu0.025O7+ phosphors.

In this study, 392 nm was chosen as the excitation wavelength of Ca2+MgSi2Eu0.025O7+ phosphors synthesized in the air atmosphere because it had a higher emission intensity than the other wavelengths. As shown in Figure a, when 395 nm was the exciting wavelength, only two strong emission peaks of 592 and 616 nm were observed in all PL spectra. When the Eu3+ ions are used as the activator of both Ca2MgSi2Eu0.025O7 and Ca3MgSi2Eu0.025O8 phosphors, the red emission will dominate the emission mechanism, and the magnetic dipole and electric dipole transitions will coexist. The magnetic dipole transition occurs at ΔJ = 0, ±1, and the selection rule of electric dipole transition is ΔJ ≤ 6, when J = 0 and ΔJ = 2, 4, and 6.[14,17] However, for all Ca2+MgSi2Eu0.025O7+ phosphors synthesized in the air atmosphere, the prevailing emission intensity of 5D0 → 7F2 electric dipole transition at 612 nm is larger than that of 5D0 → 7F1 magnetic dipole transition at 592 nm, which suggests that the Eu3+ ions are generally located orderly in the Ca2+MgSi2O7+ phosphors.[18] Ye used a light with a wavelength of 465 nm to excite the Eu3+-doped Ca2MgSi2O7 phosphor, and the excited PL spectrum had four sharp emission peaks attributed to the transitions of 5D0 → 7F0 at ∼578 nm, 5D0 → 7F1 at ∼591 nm, 5D0 → 7F2 at ∼614 nm, and 5D0 → 7F4 at ∼702 nm, and one unapparent emission peak attributed to the transition of 5D0 → 7F3 at ∼654 nm. For the synthesized Ca2+MgSi2Eu0.025O7+ phosphors, the main emission mechanisms were the 5D0 → 7F1 (592 nm) and 5D0 → 7F2 (616 nm) transitions of Eu3+ ions, and the transitions of 5D0 → 7F for j = 0, 3, and 4 were not observed. These results prove that the air atmosphere-synthesized Ca2MgSi2Eu0.025O7 and Ca3MgSi2Eu0.025O8 phosphors emit orange-red light rather than blue or green light. Therefore, all air atmosphere-synthesized Ca2+MgSi2Eu0.025O7+ phosphors emit orange-red light rather than blue or green light.

Figure 3

Photoluminescence emission spectra of Ca2+MgSi2Eu0.025O7+ phosphors synthesized (a) in the air atmosphere and (b) in a reduction atmosphere.

The PL spectra of the reduction atmosphere-synthesized Ca2+MgSi2Eu0.025O7+ phosphors are shown in Figure b, we chose 327–374 nm as the excitation wavelength for further applications in LEDs, and the wavelength range of 400–600 nm was used to record the room-temperature PL spectra. However, as shown in Figure b, as the x value was increased from 0 to 0.2, the PLmax value decreased and the emission spectra broadened, and both emission spectra had an emission peak located at 530 nm. As the x value was increased from 0.2 to 0.4, the PLmax value had no apparent change but the emission spectrum showed a broad band. However, the PL spectrum with two emission bands located at about 530 and 475 nm was observed, which behaved in a mixing emission of both bands in the green and blue regions. As the x value was increased from 0.4 to 0.6, the PLmax critically increased and the wavelength of emission peak shifted from 530 to 480 nm. These results prove again that the emission peak located at 530 nm corresponds to the Ca2MgSi2Eu0.025O7 phosphor, and the emission peak located at 475–482 nm corresponds to the Ca3MgSi2Eu0.025O8 phosphor. The PL spectra in Figure b show clearly that the emission light changed from green to blue as the x value was increased from 0 to 1.0. The absolute photoluminescence quantum yield (PLQY) can be used to define the emission efficiency of the synthesized phosphors.[19] The standard deviation of the experimentally averaged PLQY values of eight different measurements was used to calculate the errors in PLQY values. The PLQY values of Ca2+MgSi2Eu0.025O7+ phosphors for x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 were 0.628 ± 0.036, 0.588 ± 0.033, 0.531 ± 0.029, 0.684 ± 0.028, 0.758 ± 0.031, and 0.823 ± 0.027, respectively. In the measured PL spectra of both the reduction atmosphere-synthesized Ca2MgSi2Eu0.025O7 and Ca3MgSi2Eu0.025O8 phosphors, only one emission band located at 530 and 475 nm was observed. The emission bands centered at wavelengths of 530 and 475 nm are caused by the transition of 4f7 → 4f65d1, and the difference in the wavelength is caused by the composition variations of the used host material.[9,14] Therefore, all the reduction atmosphere-synthesized Ca2+MgSi2Eu0.025O7+ phosphors have the same emission mechanism of the transition of 4f7 → 4f65d1, and they will emit the blue and green lights rather than the orange-red light.

The thermal stability is a very important factor for the practical applications of the synthesized phosphors because the exciting light will emit heat, and the heat will have a large effect on their emission properties reducing their efficiencies. The PL spectra of Ca2+MgSi2Eu0.025O7+ phosphors synthesized in a reduction atmosphere were excited by the wavelengths shown in Figure b and measured at different temperatures, and the obtained results are shown in Figure . By heating the Ca2+MgSi2Eu0.025O7+ phosphors from room temperature (∼25 °C) to 200 °C, a continuous decrease of the emission intensities and a slight shift of the emission wavelengths at around 530 (x = 0), 480 (x = 0.6), and 475 (x = 1.0) nm were really observed for all Ca2+MgSi2Eu0.025O7+ phosphors, as shown in Figure . Upon increasing the temperature from 25 to 200 °C, the wavelengths of emission peaks of Ca2MgSi2Eu0.025O7, Ca2.4MgSi2Eu0.025O7.4, and Ca3MgSi2Eu0.025O8 phosphors slightly shifted from 530 to 528, 480 to 478, and 475 to 474 nm, respectively. However, the wavelength of the emission peak of the Ca2.2MgSi2Eu0.025O7.2 phosphor shifted from 529 to ∼476 nm, and the reason for this result will be explained below.

Figure 4

Temperature-dependent PL spectra of Ca2+MgSi2Eu0.025O7+ phosphors for (a) x = 0, (b) x = 0.2, (c) x = 0.6, and (d) x = 1.

The PLmax values of all Ca2+MgSi2Eu0.025O7+ phosphors measured at 25 °C were used as the standard values to normalize the PLmax values measured at the temperatures from 25 to 200 °C, and the variations in the relative PLmax values are shown in Figure . For the Ca2+MgSi2Eu0.025O7+ phosphors with x = 0 and 0.2, the PLmax value decreased from the initial 100% to about 60.0% at 100 °C and to lower than 40.0% at 150 °C, respectively. For another group Ca2+MgSi2Eu0.025O7+ phosphors with x = 0.4, 0.6, 0.8, and 1.0, the PLmax value gradually decreased from the initial 100 to 84.8% (x = 0.4) or higher 92% (x = 0.6, 0.8, and 1.0) at 100 °C and to 70.0% (x = 0.4) or higher 76.9% (x = 0.6, 0.8, and 1.0) at 200 °C. These results in Figure show that the thermal quenching effect of the Ca2MgSi2Eu0.025O7 green phosphor is more obvious than that of the Ca3MgSi2Eu0.025O8 blue phosphor. Figure also shows that the thermal quenching effect of the Ca2.2MgSi2Eu0.025O7.2 phosphor is similar to that of the Ca2MgSi2Eu0.025O7 phosphor, and the thermal quenching effects of Ca2+MgSi2Eu0.025O7+ phosphors with x = 0.4, 0.6, and 0.8 are similar to that of the Ca3MgSi2Eu0.025O8 phosphor. These results prove that for an x value of 0.2 of the Ca2+MgSi2Eu0.025O7+ phosphors, their PL property is stronger than that of the Ca2MgSi2Eu0.025O7 phosphor. For x values of 0.4, 0.6, and 0.8 of the Ca2+MgSi2Eu0.025O7+ phosphors, their PL property is stronger than that of the Ca3MgSi2Eu0.025O8 phosphor.

Figure 5

Temperature-dependent emission intensities of Ca2+MgSi2Eu0.025O7+ phosphors.

However, we tried to find out the reason for the variations in the wavelengths in the emission peaks of all Ca2+MgSi2Eu0.025O7+ phosphors shown in Figures b and 4. Therefore, we used the emission spectra of the Ca2MgSi2Eu0.025O7 and Ca3MgSi2Eu0.025O8 phosphors as reference curves 1 and 2. The emission spectra of Ca2.2MgSi2Eu0.025O7.2, Ca2.4MgSi2Eu0.025O7.4, Ca2.6MgSi2Eu0.025O7.6, and Ca2.8MgSi2Eu0.025O7.8 were successfully divided into a combination of intensities of 0.79 curve 1 (Ca2MgSi2Eu0.025O7, shown in Figure b) + 0.05 curve 2 (Ca3MgSi2Eu0.025O8, Figure a), 0.64 curve 1 + 0.155 curve 2 (Figure b), 0.40 curve 1 + 0.75 curve 2 (Figure c), and 0.20 curve 1 + 0.89 curve 2 (Figure d). These results also prove that the compositions of the Ca2MgSi2Eu0.025O7 phosphors have an emission peak centered at 530 nm and the Ca3MgSi2Eu0.025O8 phosphor has an emission peak centered at 475 nm. When the x value increases, the measured XRD patterns show that the composition of the Ca2MgSi2O7 phase decreases and that of the Ca3MgSi2O8 phase increases, therefore the wavelength of PLmax changes from 530 to 475 nm. Also, the Ca3MgSi2O8-based phosphor has less dependence on the temperature than the Ca2MgSi2O7-based phosphor. When the measured temperature increases, the emission light generated by the Ca3MgSi2Eu0.025O8 phosphor has a less change as compared with that generated by the Ca2MgSi2Eu0.025O7 phosphor, therefore the wavelength of PLmax shifts from 530 to 475 nm, as shown by the results in Figure a–d.

Figure 6

Decomposition of the PL spectra of Ca2+MgSi2Eu0.025O7+ phosphors, for (a) x = 0.2 (excitation wavelength was 373 nm), (b) x = 0.4 (346 nm), (c) x = 0.6 (327 nm), and (d) x = 0.8 (340 nm).

The large shift in the wavelength of PLmax of the Ca2.2MgSi2Eu0.025O7.2 phosphor is clearly explained below. When the measured temperature increases, the intensity of emission curve 1 has a larger reduction rate but that of the emission curve 2 has a smaller reduction rate. When the higher measured temperature is used, the PLmax and the emission intensity of the whole PL spectrum are reduced, as shown in Figure . As shown in Figure a, the emission curve of the Ca2.2MgSi2Eu0.025O7.2 phosphor can be divided into a combination of intensities of 0.79 curve 1 + 0.05 curve 2, therefore, the emission curve of the 100 °C (200 °C)-measured Ca2.2MgSi2Eu0.025O7.2 phosphor can also be divided into a combination of intensities of 0.79 100 °C (200 °C)-measured curve 1 + 0.05 100 °C (200 °C)-measured curve 2, as shown in Figure a (Figure b). As compared with the spectrum measured at 25 °C, the proportions of 100 and 200 oC measured curve 1 become lower and those of 100 and 200 oC measured curve 2 in the PL spectrum become higher. Therefore, for the Ca2+MgSi2Eu0.025O7+ phosphors with x = 02, 0.4, and 0.6, their wavelengths to reveal the PLmax value are shifted to lower values as the measured temperature increases.

Figure 7

Decomposition of the PL spectra of Ca2+MgSi2Eu0.025O7+ phosphors, for x = 0.2 and measured at (a) 100 and (b) 200 °C.

As the x value was increased from 0 to 1, the maximum emission intensities of PLE and PL increased and the emitting light changed from green (Ca2MgSi2O7 and Ca2.2MgSi2O7.2), cyan (Ca2.4MgSi2O7.4), and blue-cyan (Ca2.6MgSi2O7.6 and Ca2.8MgSi2O7.8) to blue (Ca3MgSi2O8), as shown in Figure . The color coordinates of Ca2+MgSi2Eu0.025O7+ phosphors were located at (0.2993, 0.4245) green, (0.2988, 0.4238) green, (0.2655, 0.3762) cyan, (0.2218, 0.3292) cyan-blue, (0.1992, 0.3122) blue, and (0.1980, 0.3103) blue corresponding to the Ca2+MgSi2O7+ phosphors with x = 0, 02, 04, 06, 08, and 1.0, respectively. The results of CIE chromaticity diagrams prove that as the x value increases from 0 to 1.0, the color of the emitting light of the Ca2+MgSi2O7+ phosphors changes from green to blue.

Figure 8

Emission photographs of Ca2+MgSi2O7+ phosphors synthesized in a reduction atmosphere.

In this study, the optimum wavelengths to excite the Ca2+MgSi2Eu0.025O7+ phosphors were 374, 373, 346, 327, 340, and 357 nm for x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0. The wavelengths to measure the intensity decay were 530 (x = 0), 529 (x = 0.2), 482 (x = 0.4), 480 (x = 0.6), 476 (x = 0.8), and 475 nm (x = 1.0) because the maximum emission intensities of Ca2+MgSi2Eu0.025O7+ phosphors were at those wavelengths. The measured decay curves of the Ca2+MgSi2Eu0.025O7+ phosphors are shown in Figure a, which were 0.81, 0.77, 0.38, 0.34, 0.36, and 0.38 ms for the x values of 0, 0.2, 0.4, 0.6, 0.8, and 1.0. The decay curves of all Ca2+MgSi2Eu0.025O7+ phosphors shown in Figure a can be divided into two stages with different decay slopes, the first stage has a larger decay slope as the decay time is less than about 0.9 ms and the second stage has a smaller decay slope as the decay time is more than about 0.9 ms. As shown in Figure a, the decay times of all Ca2+MgSi2Eu0.025O7+ phosphors can also be divided into two groups. For x = 0 and 0.2, the Ca2MgSi2Eu0.025O7 phase shows strong PL properties, and the two phosphors had a shorter decay time; for x = 0.4, 0.6 0.8, and 1.0, the Ca3MgSi2Eu0.025O8 phase shows strong PL properties, and they had a longer decay time. These results prove again that as the x value is changed from 0.2 to 0.4, the dominating phase of the compound changing from Ca2MgSi2Eu0.025O7 to Ca3MgSi2Eu0.025O8 is the reason for the variations in the decay times. However, the variations in decay curves of all Ca2+MgSi2Eu0.025O7+ phosphors match the results shown in Figures and 5. The decay processes for the PL intensities of all Ca2+MgSi2Eu0.025O7+ phosphors can be simulated by a curve-fitting technology, and the curves fitted by the sum of two exponential components can be expressed in an equation as shown below[14]where I(t) is the PL intensity at a specified wavelength, I1 and I2 are constants, t is the time, and τ1 and τ2 are the time constants of the two exponential components. Figure a shows that the variations in the emission intensities of all Ca2+MgSi2Eu0.025O7+ phosphors had two stages, and the decay curves could be divided into two different groups.

Figure 9

Decay curves of Ca2+MgSi2O7+ phosphors. (a) Measured results and (b) comparisons of the measured and simulated results.

The decay time curves of all Ca2+MgSi2Eu0.025O7+ phosphors can be constructed using a curve-fitting method, the decay curves shown in Figure b have been successfully fitted using eq , and Table compares the relative parameters of all the fitting curves. Clearly, as the x value is increased from 0.2 to 0.4, τ1 decreased sharply from 7.52645 to 0.33776 ms, but τ2 did not go through a dramatic change and instead fluctuated between 0.41193 and 0.33776 ms. Generally, the green- and blue-emitting Ca2+MgSi2Eu0.025O7+ phosphors are suitable for UV-LEDs. However, the reason for the change of the decay time is that there are different host materials (crystalline phases) formed in the Ca2+MgSi2Eu0.025O7+ phosphors, and the traps generated in the Ca2MgSi2Eu0.025O7 phosphor are deeper than those generated in the Ca3MgSi2Eu0.025O8 phosphor. Therefore, the characteristic emissions of Ca2+MgSi2Eu0.025O7+ phosphors for x = 0 and 0.2 have a longer afterglow (or decay time).[14] The investigated results have proven that when Ca2+MgSi2Eu0.025O7+ phosphors are synthesized in a reduction atmosphere, the properties of the Ca2MgSi2Eu0.025O7 phosphor will be stronger than the optical properties of the Ca2.2MgSi2Eu0.025O7.2 phosphor, and the Ca3MgSi2Eu0.025O8 phosphor will exhibit properties stronger than the optical properties of Ca2+MgSi2Eu0.025O7+ phosphors for x = 0.4, 0.6, and 0.8, respectively.

Table 1

Time Constants of the Fitting Decay Time Curves of Ca2+MgSi2Eu0.025O7+ Phosphors

x value	I1	τ1 (ms)	I2	τ2 (ms)	x value	I1	τ1 (ms)	I2	τ2 (ms)
0	0.22976	6.93125	0.71389	0.42261	0.6	0.53647	0.32729	0.53647	0.32730
0.2	0.21156	7.52645	0.72719	0.41193	0.8	0.56381	0.40604	0.56381	0.40602
0.4	0.53411	0.33776	0.53411	0.33776	1.0	0.55098	0.37853	0.55098	0.37854

Conclusions

All Ca2+MgSi2Eu0.025O7+ phosphors synthesized in the air atmosphere emit red color. As all Ca2+MgSi2Eu0.025O7+ phosphors were synthesized in a reduction atmosphere, the maximum emission intensities of PLE and PL increased with the x value and the emitting light changed from green (x = 0 and 0.2), cyan (0.4), cyan-blue (0.6), and blue lights (0.8 and 1.0), respectively. As the x value of reduction atmosphere-synthesized Ca2+MgSi2Eu0.025O7+ phosphors was changed from 0.2 (0.4) to 0.4 (1.0), the monitored wavelengths changed critically from 529 (482) to 482 (474) nm. The decay curves of all Ca2+MgSi2Eu0.025O7+ phosphors synthesized in reduction atmosphere had two stages, but as the x value was changed, the decay curves had two different groups. These results of this study have proven that when Ca2+MgSi2Eu0.025O7+ phosphors are synthesized in a reduction atmosphere, the Ca2MgSi2Eu0.025O7 phosphor will exhibit stronger properties than the optical properties of the Ca2.2MgSi2Eu0.025O7.2 phosphor, and the Ca3MgSi2Eu0.025O8 phosphor will exhibit stronger properties than the optical properties of Ca2.4MgSi2Eu0.025O7.4, Ca2.6MgSi2Eu0.025O7.6, and Ca2.8MgSi2Eu0.025O7.8.

Experimental Procedures

When Eu2O3 was used as a dopant and synthesized in a reduction atmosphere, the Eu3+ ions could be deoxidized into the Eu2+ ions, therefore we used it as the precursor of Eu2+ ions. Eu2O3, SiO2, MgCO3, and CaCO3 were used as the raw materials and they were wetted according to the compositions of Ca2+MgSi2Eu0.025O7+ powders with x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0. After Ca2+MgSi2Eu0.025O7+ powders were ball-milled for 2 h with absolute alcohol, dried, and ground, they were sintered at 1350 °C for 4 h in an air atmosphere or in a reduction atmosphere of 95% N2 + 5% H2. At first, XRD patterns were used to analyze the effect of composition variations on the crystalline properties of the synthesized Ca2+MgSi2Eu0.025O7+ powders. A “3D scanning method” was used to find out the optimum PLE wavelengths of the Ca2+MgSi2Eu0.025O7+ phosphors, and we found that their optimal exciting wavelengths changed with their composition. PLE and PL properties of the Ca2+MgSi2Eu0.025O7+ phosphors were measured in the wavelength ranges of 200–450 and 400–650 nm at room temperature with a Hitachi F-4500 fluorescence spectrophotometer (FL). The multiplied detecting value of the photomultiplier tube was set at 350 because as the set value exceeded 350, the maximum intensities of the PLE and PL spectra exceeded the displayable range of the FL spectrophotometer. The thermal stability was an important factor for the further applications of reduction atmosphere-synthesized Ca2+MgSi2Eu0.025O7+ phosphors, therefore the PL spectra were measured from room temperature to 200 °C to find out the effect of measured temperatures on the variations of maximum emission intensities (PLmax).