1. Introduction

Trivalence cerium ion, Ce³⁺ has been widely applied in the phosphor materials for phosphor-converted white light-emitting diodes (pc-white LEDs) as an activator ion, because the Ce³⁺-activated phosphors show the strong broad excitation and emission band due to the spin allowed energy transitions between the 4f ground state and 5d excited state of Ce³⁺ [1]. The 5d electrons are significantly affected by the ligand or the crystal field because the 5d electrons are located in the outermost orbitals unshielded by outer oribital electrons, and the position of excitation and emission bands strongly depend on the crystal field splitting of the 5d excited level [2]. In particular, Ce³⁺-doped Y₃Al₅O₁₂ phosphor (YAG:Ce) has been used in pc-white LEDs as a commercial yellow emission phosphor. The YAG:Ce phosphor shows strong optical absorption in the blue light regions, which correspond to the emission wavelength of InGaN-based blue LED chips, and exhibits strong broad yellow emission under blue light excitation [2,3]. White LEDs combined with YAG:Ce and InGaN-based blue LED chips are used widely in LED lamps and backlight in liquid crystal displays, because of their high luminescence efficiency, designable feature, and low cost. However, since the yellow-emitting YAG:Ce phosphor has weak emission efficiency in red light region, these white LEDs have low reproducibility and low color rendering index [4–8]. To solve these problems, it is necessary to develop novel phosphor materials of high emission efficiencies in red light region under blue light excitation.

In this study, therefore, we focused on Sr₆Y₂Al₄O₁₅ as a host material for the phosphor to develop a novel Ce³⁺-doped phosphor having high emission intensity in red emission region. Figure 1 shows the crystal structure of the Sr₆Y₂Al₄O₁₅, which is illustrated using the VESTA program [9]. The Sr₆Y₂Al₄O₁₅ crystal structure is built out of corner-connected YO₆ octahedra and AlO₄ tetrahedra, and Sr²⁺ ions coordinated 8 or 9 oxide anions in the crystal structure. In the Sr₆Y₂Al₄O₁₅ structure, Ce³⁺ can be possible to dope into the three different sites with two Sr²⁺ site of SrO₈ and SrO₉ polyhedrons, and one Y³⁺ site of YO₆ octahedra. In addition, ionic radii of Sr²⁺ for 8- and 9-fold coordinations are 0.1260 and 0.1310 nm, respectively, and ionic radius of Y³⁺ for 6-fold coordination is 0.0900 nm [10]. These ionic radii of Sr²⁺ and Y³⁺ are similar to that of Ce³⁺ (0.1010, 0.1143, and 0.1196 nm for 6, 8, and 9-fold coordinations) [10]. However, the bond length between Y³⁺ and the nearest O²⁻ in the YO₆ octahedra is 0.2143 nm, which is shorter than that of the Sr²⁺−O²⁻ in the SrO₈ (0.2413 nm) and SrO₉ (0.2242 nm) polyhedrons [11]. The bond length of Y³⁺−O²⁻ in the Sr₆Y₂Al₄O₁₅ lattice is also shorter than that of the Y³⁺−O²⁻ in the Y₃Al₅O₁₂ (YAG; 0.2397 nm) lattice. Since the crystal field splitting of the 5d excited level remarkably depends on the bond length between an activator ion and the nearest anion, it is expected to obtain the excitation and emission bands in longer wavelength side when Ce³⁺ doped selectively into the Y³⁺ site in the Sr₆Y₂Al₄O₁₅ lattice. S. Tezuka et al. have reported that the emission peak wavelength of the Eu²⁺-activated CaSrSiO₄ phosphors, which is well known as green emitting phosphor excitable with near-UV, shifted to longer wavelength regions (red spectral region; 600~700 nm) as the Eu²⁺ concentration was increased beyond 5 mol% [12]. They are explained with respect to the red shift of emission wavelength that the CaSrSiO₄ structure has two different Ca/Sr sites coordinated 10 (1n site) or 8 (2n site) oxide anions possible to substitute with Eu²⁺, and Eu²⁺ begin to occupy in Ca/Sr (2n site) site with increasing the Eu²⁺ concentration beyond 5 mol%; consequently, the red emission with peak at greater than 600 nm is observed in the samples with Eu²⁺ doping beyond 5 mol%. Considering this, we adjusted the Ce³⁺ concentration between 0.5 and 20 mol% in the novel Sr₆(Y_1-xCe_x)₂Al₄O₁₅ phosphors prepared in this study, as a result, these phosphors exhibit orange-red emission with a peak at 600 nm under blue light excitation, which is the first report of a Ce³⁺-doped oxide phosphor having strong emission in the red spectral region.

 

Fig. 1 Crystal structure of monoclinic Sr₆Y₂Al₄O₁₅.

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2. Experimental

SrCO₃, Y₂O₃, Al₂O₃, and CeO₂ were mixed in a stoichiometric ratio using a mortar with acetone, in which the Ce³⁺ content was varied from 0.5 to 20 mol%, and the homogeneous mixture was calcined at 1500 °C for 12 h in a flow of 5%H₂-95%N₂ gas. After calcination, the samples were reground in a mortar. The crystal structure of the samples was identified by X-ray powder diffraction (XRD; Mac Science Ltd. MX-Labo) analysis. Photoluminescence (PL) excitation and emission spectra were measured at room temperature with a spectrofluorometer (Jasco Corp. FP-6500/6600).

3. Results and discussion

The XRD patterns of the Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.005 ≤ x ≤ 0.20) phosphors are shown in Fig. 2. The standard XRD pattern of Sr₆Y₂Al₄O₁₅ from the inorganic crystal structure database (ICSD #262993) is also shown in Fig. 2 as a reference. The XRD patterns of all samples were in good agreement with a single phase of monoclinic Sr₆Y₂Al₄O₁₅ with high crystalline, and no impurity phase was observed in the patterns.

 

Fig. 2 XRD patterns of the Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.005 ≤ x ≤ 0.20) phosphors.

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Figure 3 shows the excitation and emission spectra of Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.005 ≤ x ≤ 0.15) phosphors. The excitation and emission spectra were recorded at maximum peak wavelength for each samples. All the excitation spectra consist of three broad bands from 260 to 380 nm centered on about 290, 322, and 354 nm, these excitation bands are due to the absorption of the incident radiation by Ce³⁺ ions which leads to the excitation of electrons from the 4f¹ ground state (²F_5/2 and ²F_7/2) to the excited 5d¹ level (²D) [13]. The emission spectra exhibit two broad blue emission band peaked at 396 and 450 nm, which corresponds to the 5d → 4f transition of Ce³⁺. The emission peak intensity increases with the amount of Ce³⁺ and reaches a maximum at x = 0.01 in Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.005 ≤ x ≤ 0.20), and then decreased probably due to concentration quenching. However, the emission peak intensity tends to increase with increasing Ce³⁺ concentration beyond the x = 0.05 and the maximum peak wavelength changed from 450 nm to 396 nm. The maximum peak intensity at 396 nm was obtained for Sr₆(Y_0.85Ce_0.15)₂Al₄O₁₅. In addition, the body color of the obtained phosphors was changed from white to yellow with increasing in the Ce³⁺ concentration beyond x = 0.10.

 

Fig. 3 Excitation and emission spectra of the Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.005 ≤ x ≤ 0.15) phosphors. The excitation and emission spectra were recorded at maximum peak wavelength for each samples.

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The decrease of the peak intensity at 450 nm and the change of body color to yellow (complementary color to blue) suggests that the high concentration of Ce³⁺-doped phosphors are expected to have the optical absorption at blue light region. Figure 4 shows the excitation and emission spectra of the Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.10 ≤ x ≤ 0.20) phosphors. The excitation spectra were obtained for emission at 600 nm and the emission spectra were obtained for excitation at 460 nm. The excitation spectra consist of two broad bands from 280 to 500 nm centered on about 330 and 460 nm due to the 4f → 5d transition of Ce³⁺, which is broader than that of the reported Ce³⁺ doped phosphors and indicates that these phosphors are very suitable for a color converter using any excitation wavelength as the primary light source. The emission spectra exhibit a broad orange-red emission band centering at 600 nm corresponding to the 5d → 4f transition of Ce³⁺ under excitation at 460 nm. The emission band based on the 5d → 4f transition of Ce³⁺ shifted to the shorter wavelength side with the increase in the Ce³⁺ concentration in the Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.10 ≤ x ≤ 0.20) phosphors, while the peak splitting was not observed in the emission spectra with the Ce³⁺ concentration increasing.

 

Fig. 4 Excitation and emission spectra of the Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.10 ≤ x ≤ 0.20) phosphors. The excitation spectra were obtained for emission at 600 nm and the emission spectra were obtained for excitation at 460 nm.

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The reason for the deformation of excitation and emission spectra profile with the Ce³⁺ concentration increasing in the Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.005 ≤ x ≤ 0.20) phosphors can be explained by the change of Ce³⁺ doped site in the host Sr₆Y₂Al₄O₁₅ lattice. The Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.005 ≤ x ≤ 0.20) phosphors show the broad blue emission band having two centers at 396 and 450 nm and the orange-red emission band was observed with the increase in the Ce³⁺ concentration. These results indicate that the existence of three kind of excitation and emission centers in accordance with the three different types of crystallographic environment of Ce³⁺ in the host Sr₆Y₂Al₄O₁₅ lattice. As mentioned above, the Sr₆Y₂Al₄O₁₅ has three different doping sites having different crystallographic environments with SrO₈ and SrO₉ polyhedron and YO₆ octahedron, and these sites are possible to substitute with Ce³⁺ ion. The bond length between Sr²⁺ and the nearest O²⁻ in the SrO₈ (0.2413 nm) and SrO₉ (0.2242 nm) polyhedron is larger than that of the Y³⁺−O²⁻ in YO₆ octahedron (0.2143 nm) and the coordination numbers of the SrO₈ and SrO₉ polyhedron are larger than that of YO₆ [11]. This indicates that the crystal field strength of O²⁻ around Ce³⁺ in the Sr²⁺ site is weaker than that of Y³⁺ site. In addition, the blue emission band has two centers at 396 and 450 nm, and the orange-red emission band has one center at 600 nm. Therefore, it is possible to consider that the Ce³⁺ ions is preferentially occupy the Sr²⁺ site in the host Sr₆Y₂Al₄O₁₅ lattice and presents the blue emission. With Ce³⁺ concentration increased beyond the solid solubility limit in replacement of the Sr²⁺ site, the Ce³⁺ ion begin to occupy the Y³⁺ site in the host Sr₆Y₂Al₄O₁₅ lattice, consequently, the samples with x ≥ 0.10 exhibit the orange-red emission. To the best of our knowledge, this is the first report of the Ce³⁺ doping oxide phosphor exhibiting orange-red emission centered at 600 nm under blue light excitation. Furthermore, Ce³⁺ doped Sr₆Y₂Al₄O₁₅ phosphors allow us to tune the emission color from blue to orange-red by controlling the Ce³⁺ doping site in the Sr₆Y₂Al₄O₁₅ lattice.

4. Conclusion

Novel Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.005 ≤ x ≤ 0.20) phosphors were synthesized by a conventional solid state reaction method. These phosphors obtained in the present study adopt the monoclinic Sr₆Y₂Al₄O₁₅ structure with high crystallinity. The excitation spectra of Sr₆(Y_1-xCe_x)₂Al₄O₁₅ (0.005 ≤ x ≤ 0.20) phosphors are consisted of a broad band from 270 to 380 nm, and emission spectra showed the broad blue emission band. When the Ce³⁺ concentration increased beyond 10 mol% (x = 0.10), the strong optical absorption at blue light region appeared and these phosphors present orange-red emission centered on 600 nm.