The Effect of Heat Treatment on the Emission Color of P-Doped Ca₂SiO₄ Phosphor

Abstract

In a series of (Ca_2–x/2–yEu_y□_x/2)(Si_1–xP_x)O₄ (x = 0.06, 0.02 ≤ y ≤0.5), various color-emitting phosphors were successfully synthesized by a solid-state reaction. These phosphors were characterized by photoluminescence (PL) spectroscopy, X-ray powder diffractometry, transmission electron microscopy, and X-ray absorption fine structure spectroscopy. We evaluated the effect of heat treatment on PL properties with various annealing temperatures at 1373–1773 K for 4 h before/after reduction treatment from Eu³⁺ to Eu²⁺. In the red-emitting (Ca_1.95Eu³⁺_0.02□_0.03)(Si_0.94P_0.06)O_4+δ phosphor, the highest PL intensity exhibited when it was annealed at 1773 K. On the other hand, in the green-emitting (Ca_1.95Eu²⁺_0.02□_0.03)(Si_0.94P_0.06)O₄ phosphor, the highest PL intensity was realized when it was annealed at 1473 K and consequently treated under a reductive atmosphere. With increasing annealing temperature, the emission peak wavelength steadily decreased. Furthermore, with increasing Eu²⁺ content, the emission peak wavelength increased, with the color of emitting light becoming yellowish. Thus, the PL properties of the phosphors were affected by both the structural change from β to α’_L, which occurred by heat treatment, and the amount of doped Eu ions.

1. Introduction

Rare-earth-doped dicalcium silicate (Ca₂SiO₄, C₂S) phosphors have been investigated by several researchers for application to white light-emitting diodes (LEDs) [1,2,3,4,5]. Two types of phosphors have been reported so far: Eu³⁺-doped and Eu²⁺-doped C₂S phosphors. The former phosphors are characterized by the emission of red light due to the transition of ⁵D₀-⁷F₂ for the Eu³⁺ ion [3]. Thus, the luminescence originates from the 4f-4f dipole transitions of the Eu³⁺ ion, and hence the wavelength of the emission is almost the same among the various phosphors with different host materials. For example, a group of A₂SiO₄: Eu³⁺ (A = Ca, Sr, Ba) phosphors showed similar emission spectra at around the maximum wavelength of 620 nm [4]. On the other hand, Eu²⁺-doped C₂S phosphors are generally useful for a wide range of applications, because their luminescent colors, due to the f-d transition of the Eu²⁺ ion, are tunable by the crystal structures and/or compositions of the host materials. Recently, a crystal-site engineering technique was reported, which enabled us to customize the luminescent colors of C₂S:Eu²⁺ phosphors [5].

A series of structural studies of P-doped C₂S crystals has demonstrated that the incorporation of P most effectively stabilized the high-temperature phases of C₂S [6]. Hence, the authors present the idea of utilizing the P-doped C₂S as the host material of Eu²⁺-activated phosphors. In a previous study, the relationship between the photoluminescence (PL) intensities and crystal structures of P-doped C₂S phosphors was reported [7]. The doping of P ions in the green-emitting C₂S:Eu²⁺ phosphors were found to effectively increase the PL intensities. The polymorphs of C₂S established are, in the order of increasing temperature, γ (orthorhombic), β (monoclinic), α’_L (orthorhombic), α’_H (orthorhombic), and α (trigonal) [8]. The phase transition temperatures are, during the heating process, 963 K for β to α’_L, 1433 K for α’_L to α’_H, and 1698 K for α’_H to α. The β to α’_L phase transition of the cooling of doped C₂S has been reported to be thermoelastic martensitic [9], hence the stabilized phases at ambient temperature systematically changed from β, β+α’_L to α’_L with increasing concentrations of foreign ions such as Sr²⁺, Ba²⁺, and/or P⁵⁺ in C₂S [10,11]. Thus, the change of crystal structures by heat treatment must be potentially able to improve the photoluminescence (PL) properties of the phosphors. In fact, we have succeeded in the further improvement of the PL intensities by the annealing of the green-emitting P-doped C₂S phosphor [12].

In the present study, we successfully synthesized P-doped C₂S phosphors showing various emission colors (red, green, and yellow) by a solid-state reaction. The PL intensities were compared between the phosphors with different activators of Eu³⁺ or Eu²⁺ ions that were annealed at 1373–1773 K. We also discussed the relationship between the PL intensities and crystal structures.

2. Results and Discussion

2.1. Red-Emitting P-Doped C₂S:Eu³⁺ Phosphor

In a previous study, we prepared a series of (Ca_1.98–x/2Eu²⁺_0.02□_x/2)(Si_1–xP_x)O₄ (P-doped C₂S:Eu²⁺) phosphors with 0 ≤ x ≤ 0.20 (y = 0.02), and investigated the effect of P⁵⁺-ion doping on the PL properties. Here, the box in the chemical formula represents vacancies according to a previous paper [6]. The P-doped C₂S:Eu²⁺ phosphor with x = 0.06 exhibited the highest PL intensity among those with 0 ≤ x ≤ 0.20 [12].

In the present study, we prepared the Eu³⁺-activated red-emitting phosphor with a chemical formula of (Ca_1.95Eu³⁺_0.02□_0.03)(Si_0.94P_0.06)O_4+δ, and focused on the effect of annealing on its PL property.

Figure 1 shows the X-ray diffraction (XRD) patterns of the red-emitting phosphor annealed at various temperatures from 1373 K to 1773 K for 4 h. With increasing annealing temperature, the relative amount of the α’_L phase with respect to the β phase increased. This is caused by the phase transition from β to α’_L during annealing. Figure 2 shows the emission and excitation spectra of the red-emitting phosphors annealed at 1373 K, 1573 K, and 1773 K. Red light emission of the Eu³⁺-activated phosphors were observed at an excitation wavelength of 394 nm due to the intraconfigurational ⁷F₀-⁵L₆ transition. We observed the highest PL intensity for the specimen that was annealed at 1773 K. The sharp and strong emission peaks, induced by the transitions in the Eu³⁺ ion, appeared at around 594 nm (⁵D₀-⁷F₁, magnetic-dipole), 625 nm (⁵D₀-⁷F₂, electric-dipole), and 706 nm (⁵D₀-⁷F₄, electric-dipole). We plotted the emission intensity at 594 nm (=I₅₉₄ nm) and that at 706 nm (=I₇₀₆ nm) with excitation by 394 nm in Figure 3. The intensity ratios of I₇₀₆ nm/I₅₉₄ nm (=⁵D₀-⁷F₄/⁵D₀-⁷F₁) with different annealing temperatures are also plotted in the figure. Both I₅₉₄ nm- and I_{706 nm}-values steadily increased with increasing annealing temperature, while there was no significant change in the intensity ratio of I₇₀₆ nm/I₅₉₄ nm. The electric-dipole transitions, which is the f-f transition from ⁵D₀ to ⁷F_2,4, are, according to the theory of selection rule [13], affected by the site symmetry of the crystal fields around the Eu³⁺ ion. From the present results, the Eu³⁺ ion would be in the same Ca site, although the site environment of the Eu³⁺ ion must be changed from the β phase to the α’_L phase. Interestingly, in our previous paper, bright red emission due to hypersensitive ⁵D₀-⁷F₂ (electric-dipole) transition and very small ⁵D₀-⁷F₄ and ⁵D₀-⁷F₁ transitions were observed under the ultraviolet irradiation of 541 nm or 398 nm to Li_1.11Ta_0.89Ti_0.11O₃ phosphor [14]. The eccentricity of the Eu³⁺ positions in [(Li, Eu)O₁₂] polyhedron thus seems to be closely related to the PL efficiency of the doped lithium tantalate phosphors [15].

2.2. Green-Emitting P-Doped C₂S:Eu²⁺ Phosphor

The phase compositions of green-emitting (Ca_1.95Eu²⁺_0.02□_0.03)(Si_0.94P_0.06)O₄ phosphors were, as shown in Figure 4, different among those with different annealing temperatures. When comparing the phase compositions between the two types of phosphors, (Ca_1.95Eu²⁺_0.02□_0.03)(Si_0.94P_0.06)O₄ and (Ca_1.95Eu³⁺_0.02□_0.03)(Si_0.94P_0.06)O_4+δ, at the same annealing temperatures, they were almost equal to each other. Figure 5 shows the relationship between the PL intensity and annealing temperature in green-emitting (Ca_1.95Eu²⁺_0.02□_0.03)(Si_0.94P_0.06)O₄ phosphors. The broad excitation spectra were composed of the four bands at 310 nm, 340 nm, 360 nm, and 400 nm, each of which could be attributed to the location of an Eu²⁺ ion in the crystal structures of the β- and α’_L-phases [16,17]. The differences in peak shapes of the excitation spectra are caused by the differences in crystal structures of the host materials. The emission or excitation wavelengths were measured by monitoring them at the maximum wavelengths, as shown in

Table 1. Emission and excitation wavelengths of (Ca_1.95Eu²⁺_0.02□_0.03)(Si_0.94P_0.06)O₄ phosphors.

Annealing Temperature (K)	λex (nm)	λem (nm)
1373	323	512
1473	324	512
1573	330	509
1673	322	497
1773	319	490

. The emission peak wavelength decreased with increasing annealing temperature. This is caused by the expansion of Eu–O bond lengths by phase transition [17], because the 4f-5d transition of the Eu²⁺ ion must be closely related to the crystal field. We observed the highest PL intensity for the specimen annealed at 1473 K, of which the phase composition was both β and α’_L.

We investigated the profile intensity data of the phosphor sample annealed at 1473 K by the Rietveld method, and determined the precise phase composition. The structural parameters were taken from those reported by Jost et al. [18] for β-C₂S, Udagawa et al. [19] for α’_L-C₂S, and Dickens and Brown [20] for Ca₅P₂SiO₁₂. The sample was found to be composed of both the β- and α’_L-phases with a small amount of Ca₅P₂SiO₁₂ as the impurity phase. The phase composition was determined to be 75.3 mol % β, 23.5 mol % α’_L, and 1.2 mol % Ca₅P₂SiO₁₂ (Figure 6), under the condition of each effective particle radii being 5 μm. Because the highest PL intensity for the present sample among the phosphors annealed at 1373–1673 K, the co-existence of the β- and α’_L-phases would be essentially important for the enhancement of the PL intensity of green-emitting P-doped C₂S:Eu²⁺ phosphors. This is because the coherent interphase boundaries between α’_L and β would store the strain energy, which would distort the crystal lattices and provide a favorable environment for the efficient PL emission of the Eu²⁺ ion.

Figure 7 shows a transmission electron microscope (TEM) image of (Ca_1.95Eu²⁺_0.02□_0.03)(Si_0.94P_0.06)O₄ phosphor annealed at 1473 K taken from the ($\overline{1}$00), and the corresponding selected area electron diffraction (SAED) pattern. The relevant crystal grain was made up of the two regions of β and α’_L. The β-phase region showed a pseudo-merohedral (polysynthetic) twin structure, with the splitting of reflections for the SAED pattern. The reflection indices are based on the β-phase lattice. The coherent grain boundary (depicted by G. B. in Figure 7) was parallel to the ($01\overline{1}$) plane of the β phase (a = 5.513, b = 6.758, c = 10.460 nm, β = 117.27 degrees) and the (001) plane of the α’_L phase (a = 20.470, b = 9. 390, c = 5.437 nm). We examined the XRD profile intensity data by the Rietveld method to find that the geometrical difference in the boundaries between ($01\overline{1}$) and (001) was as small as 0.023 nm. The coherent interphase boundaries between α’_L and β would store the strain energy, which would distort the crystal lattices and, consequently, provide a favorable environment for the efficient PL emission of the Eu²⁺ ion in the green-emitting phosphor.

In order to determine the environment of the Eu ion, X-ray absorption fine structure (XAFS) spectra were obtained with fluorescence mode at room temperature. Figure 8 shows the radial structure function of (Ca_1.95Eu²⁺_0.02□_0.03)(Si_0.94P_0.06)O₄ phosphor annealed at 1473 K. The simulation was performed using the software Artemis [21] (Figure 8b), in which the Eu ion is substituted in the Ca(1n) site with 10 coordination of β-Ca₂SiO₄ (structural parameters by Jost et al. (1977) [18]. Figure 8c shows the calculated peaks as a sum of each peak shown in Figure 8b. With radial distance less than 2 Å, a distinct doublet peak appeared in the simulation in Figure 8c, although the corresponding peak was single for the measurement data in Figure 8a. The doping of the Eu ion in the Ca site would equalize the Ca(Eu)–O lengths, and hence we observed the single peak for the radial structure function in Figure 8a. This result agreed well with that obtained by Sato et al. (2014) [4]. For the doping of a large amount of Eu ions, the Eu ions would preferentially occupy the Ca (2n) site, leading to the change in emission color from green to yellow, and then to red, because the PL property is closely related to the crystal field of the host material.

2.3. Green-Yellow-Emitting P-Doped C₂S Phosphor with a Large Amount of Eu²⁺ Ions

In the formula of (Ca_1.97–yEu²⁺_y□_0.03)(Si_0.94P_0.06)O₄, the phosphors were synthesized with a large Eu²⁺ content (y = 0.1, 0.3, and 0.5). Figure 9 shows XRD patterns of the phosphors that were annealed at 1473 K and consequently reduced under a 97% Ar–3% H₂ reductive atmosphere. With increasing Eu content, it was obvious that the structural change occurred after the reduction process. Before the reduction treatment, the β and α’_L phases, together with the Ca₅(PO₄)₂(SiO₄) phase, coexisted. However, after the reduction, the main constituent phase was the α’_L phase. For the doping of Eu with a smaller amount, the phase compositions were almost the same between the phosphors before and after the reduction treatment, as shown in Figure 1 and Figure 4. Therefore, the phase compositions were effectively affected by the doping of a large amount of Eu²⁺, the ionic radius of which is larger than that of Eu³⁺. Figure 10 shows the PL intensities of phosphors with the Eu²⁺ content of y = 0.02, 0.1, 0.3, and 0.5. With increasing Eu content, the PL intensity steadily decreased due to the effect of concentration quenching. The top peaks shifted to the longer wavelength sides; they are 512 nm with y = 0.02, 525 nm with y = 0.1, and 533 nm with y = 0.3. With the Eu²⁺ content of y = 0.5, two peak-tops appeared at around 550 nm for one top and around 640 nm for the other. As described above, the emission peak wavelength decreased by the expansion of Eu–O bond lengths by the phase transition from β to α’_L [18]. The emission color would change in accordance with the site preference of Eu²⁺ ions between the distinct two sites of Ca(1n) and Ca(2n). Sato et al. reported that the emission color of Ca_2–xEu_xSiO₄ phosphor changed from green-yellow to deep-red with increasing Eu²⁺ content [4], which was closely related to the peculiar coordination environments of Eu²⁺ in the two Ca sites of the host C₂S [22].

3. Materials and Methods

The phosphors were synthesized by a conventional solid-state reaction without flux regents because their compositions should be precisely controlled. The starting materials used were the chemicals of CaCO₃, SiO₂, CaHPO₄·2H₂O, and Eu₂O₃ (>99.9% grade) for the preparation of the Eu²⁺-doped C₂S phosphors. The chemicals were weighted in molar ratios of [Ca:Eu:Si:P] = [2−x/2−y:y:1−x:x] with 0 ≤ x ≤ 0.20 and 0.02 ≤ y ≤ 0.5, the chemical variations of which correspond to the general formula (Ca_2–x/2–yEu²⁺_y□_x/2)(Si_1–xP_x)O₄. These powder specimens were thoroughly mixed with a small amount of acetone in a planetary ball mill (Pulverisette P-6, Fritsch, Dresden, Germany. The well-mixed materials were subsequently pressed into pellets, heated at 498 K for 6 h, 973 K for 2 h, and then at 1273 K for 8 h in air. The densely sintered disc-shaped specimens thus obtained were subsequently annealed for 4 h at 1373–1773 K, which correspond to the stable temperature regions of α’_L (1373 K), α’_H (1473, 1573, and 1673 K), and α (1773 K), and then cooled to ambient temperature in the electric furnace. Finally, these samples were heated at 1473 K for 3 h under a 97% Ar–3% H₂ reductive atmosphere. Phase identification was made based on the XRD data (CuKα), which were obtained on a RINT 2500 device (Rigaku Co., Ltd., Tokyo, Japan) operated at 40 kV and 200 mA. The phase compositions were determined from the X-ray profile intensity data (CuKα₁) collected on another diffractometer in the 2θ range of 25.0°–43.0° (X’Pert PRO Alpha-1, PANalytical B.V., Almelo, The Netherlands) operated at 45 kV and 40 mA. The profile intensity data were examined by the Rietveld method [23] using a computer program RIETAN-FP [24,25]. The refinement resulted in the satisfactory reliability (R) indices of R_wp = 4.74%, S (=R_wp/R_e) = 1.72, and R_p = 3.61%. TEM images were observed using a conventional TEM (JEM-2100F, JEOL, Tokyo, Japan) equipped with an energy-dispersive spectroscopy. Eu L3-edge X-ray absorption fine structure spectroscopy (XAFS) was measured at beam line BL5S1 in Aichi Synchrotron Center with fluorescence mode at room temperature. The simulation of XAFS was performed on the software Artemis [21]. Excitation and emission spectra were obtained using a fluorescence spectrophotometer (F-7000, HITACHI, Tokyo, Japan).

4. Conclusions

A series of (Ca_2–x/2–yEu²⁺_y□_x/2)(Si_1–xP_x)O₄ (x = 0.06, 0.02 ≤ y ≤ 0.5) with various color-emitting phosphors were successfully synthesized by a solid-state reaction. We clarified the effect of heat treatment on the emission color using X-ray powder diffractometry, transmission electron microscopy, and X-ray absorption fine structure spectroscopy. The PL properties of the phosphors were closely related to the structural change from β to α’_L, which occurred by heat treatment, and the amount of doped Eu ions.

• With red-emitting (Ca_1.95Eu³⁺_0.02□_0.03)(Si_0.94P_0.06)O_4+δ phosphor, the PL intensity increased with increasing annealing temperature, with the highest PL intensity reached when annealed at 1773 K. There was no significant change in the ratio of electric-dipole transition/magnetic-dipole transition during the annealing.
• With green-emitting (Ca_1.95Eu²⁺_0.02□_0.03)(Si_0.94P_0.06)O₄ phosphor, the highest PL intensity was observed when annealed at 1473 K. Because the phase composition was both α’_L and β, there must be many α’_L/β boundaries, which would provide a favorable luminescent environment of the Eu²⁺ ion in the host material. We confirmed that the Eu²⁺ ion preferentially occupied the Ca(1n) site, based on the simulation and experimental data. With increasing annealing temperature, the emission peak wavelength decreased due to the expansion of the Eu–O bond lengths.
• With the increase of the y-value for (Ca_1.97–yEu²⁺_y□_0.03)(Si_0.94P_0.06)O₄, the emission color accordingly changed from green (y = 0.02) to yellow (y = 0.5). This color change was caused by the increase of Eu²⁺ occupancy at the Ca(2n) site with respect to the Ca(1n) site.