Large Redshifts in Emission and Excitation from Eu²⁺-Activated Sr₂SiO₄ and Ba₂SiO₄ Phosphors Induced by Controlling Eu²⁺ Occupancy on the Basis on Crystal-Site Engineering

The photoluminescence properties of Eu ²⁺-activated α’-Sr ₂SiO ₄ and α’-Ba ₂SiO ₄ with a high Eu ²⁺ concentration were investigated. In the case of Sr _2-xEu _xSiO ₄, emission was shifted from 585 to 611 nm with increasing the total Eu2+ concentration (x) from 0.1 to 0.8. This trend was similar to that in Ba _2-xEu _xSiO ₄, where the emission was shifted from 513 to 545 nm. The large redshifts in both the excitation and emission spectra were discussed in terms of the Eu ²⁺ occupancies on two kinds of M sites and their local structural changes (M: Sr and Ba).

• Photoluminescence
• Red-Emitting Phosphors
• Green-Emitting Phosphors
• Orthosilicates
• Crystal-Site Engineering

1. Introduction

Over the last decade, there have been many reports on the development of high-efficiency green- and red-emit- ting phosphors that respond to excitation by blue-LEDs as this is one of the greatest practical challenges associated with the realization of warm white-LED lamps [1] -[8] . Presently, silicon-based nitride and oxy-nitride phosphors, such as β-SiAlON:Eu²⁺, Ba₃Si₆O₁₂N₂:Eu²⁺, (Ca, Sr)AlSiN₃:Eu²⁺, are the most suitable ones for white-LED applications [1] [3] - [5] because they possess excellent luminescent properties, low thermal quenching, and high chemical stability because of their high covalencies. However, they are usually prepared using refractory Si₃N₄ and unstable alkaline-earth nitrides as raw materials under high temperatures (>1500˚C) and high pressures (>0.1 MPa). Thus, their production efficiencies and costs are disadvantageous with regard to their commercial applications [9] .

In contrast to nitride and oxy-nitride phosphors, the production of oxide phosphors is more convenient as it does not require refractory or unstable raw materials, and high temperatures and pressures. Therefore, silicon- based oxide phosphors that possess properties similar to those of nitride and oxy-nitride phosphors are promising alternatives as phosphors for next-generation white-LEDs.

Recently, several oxide phosphors with excellent luminescence properties have been reported [10] - [15] . Especially, the deep-red emitting Ca₂SiO₄:Eu²⁺ (Ca_1.2Eu_0.8SiO₄) [14] and the orange-red emitting Sr₆Y₂Al₄O₁₅:Ce³⁺ [15] has been prepared by the basis on crystal-site engineering [16] . In the case of Ca_1.2Eu_0.8SiO₄, this phosphor shows an emission peak at 653 nm under excitation at 450 nm [14] . The emission wavelength of Ca_1.2Eu_0.8SiO₄ is quite similar to those of typical red-emitting CaAlSiN₃:Eu²⁺ and Sr₂Si₅N₈:Eu²⁺ [4] [6] . Ca_1.2Eu_0.8SiO₄ has been identified as an α’_L-Ca₂SiO₄ structure (space group: Pna2_1, No. 33) with two types of calcium sites: viz. with coordination numbers of 10 [Ca(1n), n = 1 - 3] and 8 [Ca(2n), n = 1 - 3]. When a small amount of Eu²⁺ ions (x < 0.20) is added into Ca_2−xEu_xSiO₄, most of the Eu²⁺ ions preferentially occupy the large Ca(1n) sites, showing green or green-yellow emission under UV light excitation [14] . However, upon adding a large amount of Eu²⁺ ions (x > 0.20) to the initial composition of Ca_2−xEu_xSiO₄, Eu²⁺ substitution in the small Ca(2n) sites is promoted [14] . In the case of Ca_1.2Eu_0.8SiO₄, the Eu²⁺ occupancy in Ca(2n) sites was about 9% of the total concentration of Eu²⁺. Thus, it is believed that a certain amount of Eu²⁺ substitution in the Ca(2n) sites led to the deep-red emission of Ca_1.2Eu_0.8SiO₄ at about 650 nm [14] . The crystal structure of Ca_2−xEu_xSiO₄ is analogous to those of Sr_2−xEu_xSiO₄ and Ba_2−xEu_xSiO₄ [17] . Therefore, it is expected that the addition of large amounts of Eu²⁺ ions to Sr_2−xEu_xSiO₄ and Ba_2−xEu_xSiO₄ can lead to large redshift in their emission and excitation spectra. In this study, we prepared Sr_2−xEu_xSiO₄ and Ba_2−xEu_xSiO₄ with a high concentration of Eu²⁺ ions and characterized their structural and photoluminescence properties.

2. Experimental Detail

Polycrystalline Sr_1.2Eu_0.8SiO₄ and Ba_1.2Eu_0.8SiO₄ samples were synthesized by a conventional solid-state reaction method, using SrCO₃ or BaCO₃, Eu₂O₃ and SiO₂ powders as raw materials. These raw powders were mixed in stoichiometric proportions in ethanol by using an agate mortar. The resulting mixtures were calcined at 1000˚C in air for 12 h. Subsequently, the calcined powders were reground and mixed with a small amount of BaCl₂ as flux reagents. Finally, the powders were heat-treated in a tube furnace at 1200˚C for 4 h under a flow of an Ar (96%)-H₂ (4%) gas mixture with flow rate at 400 ml/min. In order to compare the Eu²⁺ occupancies and photoluminescence (PL) properties of Sr_1.2Eu_0.8SiO₄ and Ba_1.2Eu_0.8SiO₄, we also prepared Sr_1.9Eu_0.1SiO₄and Ba_1.2Eu_0.8SiO₄ samples with a low Eu²⁺ concentration by an amorphous metal complex (AMC) method using propylene glycol-modified silane [18] . This is because Sr_1.9Eu_0.1SiO₄ and Ba_1.2Eu_0.8SiO₄ samples prepared by a solid-state reaction method contained a large amount of impurities such as SrCO₃ or BaCO₃, compared with the samples prepared by the AMC method. These samples were then heat-treated by the same procedures as used for the Sr_1.2Eu_0.8SiO₄ and Ba_1.2Eu_0.8SiO₄ samples.

For single micrograins in Sr_1.2Eu_0.8SiO₄ and Ba_1.2Eu_0.8SiO₄ polycrystalline powders, quantitative analysis of metal elements such as Sr, Ba and Eu was firstly carried out by scanning electron microscopy (SEM, SU1510, Hitachi) together with energy-dispersive X-ray spectroscopy (EDS, X-act, Horiba). The applied voltage used for EDS analysis was 20 kV. Then, these final products were characterized by X-ray diffraction (XRD, D2 PHASER, BrukerAXS) using Cu-Kα radiation. XRD patterns were collected using the continuous scan mode with a step interval of 0.02˚. In the case of XRD measurements of samples containing higher concentrations of Eu, a significant increase in background signal was observed due to sample fluorescence. Therefore, XRD patterns in these samples were measured with optimized discriminator settings to suppress the fluorescence effect [14] . PL spectra of Sr_1.2Eu_0.8SiO₄ and Ba_1.2Eu_0.8SiO₄ polycrystalline powders were analyzed using a fluorescence spectrometer (FP-8500, JASCO) equipped with an integrating sphere (ISF-513, JASCO).

3. Results and Discussion

Figure 1(a) and Figure 1(b) show the X-ray diffraction (XRD) patterns of Sr_2−xEu_xSiO₄ and Ba_2−xEu_xSiO₄ samples with total Eu²⁺ concentrations (x) of 0.1 and 0.8. Although trace amounts of SrCO₃ and BaCO₃ impurities were detected in the Sr_1.9Eu_0.1SiO₄ and Ba_1.9Eu_0.1SiO₄ samples, all samples were identified as orthorhombic α'-Sr₂SiO₄ structures with the Pnma (No. 62) space group [17] [19] . Subsequently, the XRD patterns were subjected to Rietveld refinement (RIETAN-FP program [20] ) using the crystal structure of Sr_1.9Ba_0.1SiO₄ as a starting model structure [17] . Based on quantitative elemental analysis by energy-dispersive X-ray spectroscopy (EDS), the occupancies of Eu²⁺ in the M(1) and M(2) sites in M_2−xEu_xSiO₄ (M: Sr and Ba) were also subjected to Rietveld refinement, as shown in Table 1 and Table 2. Here, the total Eu²⁺ concentrations in both Sr_1.2Eu_0.8SiO₄ and Ba_1.2Eu_0.8SiO₄ after heat treatment with the BaCl₂∙2H₂O flux were refined as 0.70, which was lower than that in the samples with initial compositions. This indicates that, during heat treatment, some amount of Eu²⁺ is exchanged by Ba²⁺ ions from BaCl₂∙2H₂O. Irrespective of the Eu²⁺ total concentration, the Sr(1) sites had higher Eu²⁺ occupancies than the Sr(2) sites. The percentage of the total Eu²⁺ occupancy in Sr(2) sites against Eu(1) sites increased significantly from 33% to 45% upon increasing the total Eu²⁺ concentration (x) from 0.1 to 0.8.

Figure 1

X-ray diffraction patterns of (a) Sr 2-x Eu x SiO 4 (x = 0.1 and 0.8) and (b) Ba 2-x Eu x SiO 4 (x = 0.1 and 0.8) polycrystalline samples

Table 1

Lattice constants and Eu 2+ occupancies at each Sr site of Sr 2-x Eu x SiO 4 samples, estimated by Rietveld refinements of their X-ray diffraction patterns

Total concentration of Eu2+ (x)	Lattice constants (Å)			Occupancy of Eu2+
	a	b	c	Total (Refinement)	Sr(1) site	Sr(2) site
0.10	7.1017	5.6724	9.7458	0.09	0.06	0.03
0.80	7.1090	5.6706	9.7502	0.70	0.38	0.32

Table 2

Lattice constants and Eu 2+ occupancies at each Ba site of Ba 2-x Eu x SiO 4 samples, estimated by Rietveld refinements of their X-ray diffraction patterns

Total concentration of Eu2+ (x)	Lattice constants (Å)			Occupancy of Eu2+
	a	b	c	Total (Refinement)	Ba(1) site	Ba(2) site
0.10	7.4884	5.8014	10.1787	0.10	0.01	0.09
0.80	7.3691	5.7423	9.9205	0.70	0.15	0.55

Hence, the trend in Sr_2−xEu_xSiO₄ is similar to that in Ca_2−xEu_xSiO₄ [14] . On the other hand, the trend of variation in Eu²⁺ occupancies between the Ba(1) and Ba(2) sites in Ba_2−xEu_xSiO₄ is opposite that in Ca_2−xEu_xSiO₄ [14] and Sr_2−xEu_xSiO₄, as shown in Table 2. Eu²⁺ ions predominantly occupy Ba(2) sites in Ba_1.9Eu_0.1SiO₄. However, as the total Eu²⁺ concentration is increased, Eu²⁺ ions occupy not only Ba(2) sites, but also Ba(1) sites.

Figure 2(a) and Figure 2(b) show the emission and excitation spectra of Sr_2−xEu_xSiO₄ and Ba_2−xEu_xSiO₄ samples with total Eu²⁺ concentrations (x) of 0.1 and 0.8. Broad emission peaks assigned to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ were observed for all the samples. As the total concentration of Eu²⁺ ions increased from 0.1 to 0.8, large emission redshifts were observed from 585 to 611 nm for Sr_2−xEu_xSiO₄ and from 513 to 545 nm for Ba_2−xEu_xSiO₄. On the other hand, the right-hand edges of the corresponding excitation spectra of both samples were shifted to longer wavelengths as the total Eu²⁺ concentration increased from 0.1 to 0.8. The increase in the widths of the right-hand edges in the excitation spectra were about 40 nm for Ba_2−xEu_xSiO₄ and about 55 nm for Sr_2−xEu_xSiO₄. The wavelength with the maximum excitation intensity was about 370 nm for both samples.

Figure 3(a) and Figure 3(b) show the emission and excitation spectra of M_1.2Eu_0.8SiO₄ samples (M: Ca, Sr, and Ba). Upon excitation at 450 nm, the emission peaks of M_1.2Eu_0.8SiO₄ were located at 653, 613, and 541 nm for Ca, Sr, and Ba, respectively. The wavelength with the maximum emission intensity was systematically shifted towards shorter wavelengths on moving towards alkaline earth elements with larger ionic radii (Ca → Sr → Ba) [21] . The intensities of Sr_1.2Eu_0.8SiO₄ and Ba_1.2Eu_0.8SiO₄ were approximately 1.6 times higher than that of Ca_1.2Eu_0.8SiO₄. The CIE chromaticity coordinates of the samples upon excitation at 450 are shown in Figure 4. The x (0.58) and y (0.42) coordinates of Sr_1.2Eu_0.8SiO₄ are comparable to that of red light. On the other hand, the x (0.39) and y (0.58) coordinates of Ba_1.2Eu_0.8SiO₄ are comparable to that of the green light region. The external and internal QE values for excitation at 450 nm were 46% and 58% for Sr_1.2Eu_0.8SiO₄ and 47% and 53% for Ba_1.2Eu_0.8SiO₄, respectively, which are similar to those for Ca_1.2Eu_0.8SiO₄ (external QE: 44% and internal QE: 50%) [14] .

Finally, we discuss the origin of the large redshifts observed in the emission and excitation spectra of Sr_2−xEu_xSiO₄ and Ba_2−xEu_xSiO₄ on changing the total Eu²⁺ concentration (x) from 0.1 to 0.8 in terms of the occupancies of Eu²⁺ ions on the M(1) and M(2) sites and the local structural changes at both M sites (M: Sr and Ba). In the α’-Sr₂SiO₄ structure, the polyhedral volume of the M(1) sites is greater than that of the M(2) sites and the coordination numbers are 10 and 9, respectively [17] Based on the crystal parameters obtained from Rietveld refinement, the polyhedral volumes and distortion indices of the M(1) and M(2) sites in both samples were estimated by a VESTA program [22] using the methods suggested by Swanson and Peterson [23] and Baur [24] . The similar estimations for Sr_xBa_2−xSiO₄:Eu₂₊ (Eu²⁺ = 0.1) have been also reported by Denault et al. [25] .

As is evident from Table 3, the polyhedra of the Sr(2) sites in Sr_2−xEu_xSiO₄ were smaller and more distorted than those of the Sr(1) sites, regardless of Eu²⁺ concentration. If Eu²⁺ ions occupy the Sr(2) sites, the coordination environment of the Eu²⁺ ions can lead to strong crystal field splitting of the 5d orbitals of Eu²⁺ ions. Therefore, the red emission band (λ_em = 612 nm) observed in Sr_1.2Eu_0.8SiO₄ is mainly produced from the Eu²⁺ ions in Sr(2) sites, since a certain amount of Eu²⁺ ions is present in the Sr(2) sites. It is conceivable that the addition of a large amount of Eu²⁺ based on crystal-site engineering is mainly responsible for the large redshifts in the emission and excitation spectra of Sr_2−xEu_xSiO₄. The relationship between PL properties and Eu²⁺ occupancy is quite similar to that in the case of Ca_2−xEu_xSiO₄ [14] . In contrast, the yellow emission band (λ_em = 585 nm) observed in Sr_1.9Eu_0.1SiO₄ was composed of at least two emission peaks: one originating from the Sr(1) sites and the other from the Sr(2) sites. It is plausible that the main contribution to the yellow emission band was from emissions originating from the Sr(1) sites, since most of the Eu²⁺ ions occupied Sr(1) sites in Sr_1.9Eu_0.1SiO₄, as shown in Table 1.

The polyhedral volumes and distortion indices of Ba_2−xEu_xSiO₄ samples with total Eu²⁺ concentrations (x) of 0.1 and 0.8 are listed in Table 4. The polyhedral volume of the Ba(2) sites in Ba_1.2Eu_0.8SiO₄ is smaller than that

Figure 2

Emission (λ ex = 365 nm) and excitation spectra of (a) Sr 2-x Eu x SiO 4 and (b) Ba 2-x Eu x SiO 4 with total Eu 2+ concentrations (x) of 0.1 and 0.8. The intensities of the emission and excitation spectra were normalized according to the maximum intensity of the spectra

Figure 3

(a) Emission (λ ex = 450 nm) and (b) excitation spectra of Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 . For comparison, the emission and corresponding excitation spectra of Ca 1.2 Eu 0.8 SiO 4 are also shown in both the figures. The photographs of Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 phosphors upon blue-light excitation with blue LED are inserted in Figure 3 (a)

Table 3

Polyhedral volumes and distortion indices of the Sr(1) and Sr(2) sites in Sr 2-x Eu x SiO 4 samples (total Eu 2+ concentration (x) equal to 0.1 and 0.8)

Total concentration of Eu2+ (x)	Polyhedral volume (Å3)		Distortion index
	Sr(1) site	Sr(2) site	Sr(1) site	Sr(2) site
0.10	46.903	37.267	0.0388	0.0652
0.80	47.710	36.538	0.0425	0.0687

Figure 4

The CIE chromaticity coordinates of Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 phosphors (open circles). The coordinates of three other orthosilicate phosphors (Ca 1.2 Eu 0.8 SiO 4 , Sr 1.9 Eu 0.1 SiO 4 and Ba 1.9 Eu 0.1 SiO 4 : solid circles), commercial YAG: Ce 3+ (P46) phosphors (solid square), and blue LEDs (open triangle) are also shown in this figure

Table 4

Polyhedral volumes and distortion indices of the Ba(1) and Ba(2) sites in Ba 2-x Eu x SiO 4 samples (total Eu 2+ concentration (x) equal to 0.1 and 0.8)

Total concentration of Eu2+ (x)	Polyhedral volume (Å3)		Distortion index
	Ba(1) site	Ba(2) site	Ba(1) site	Ba(2) site
0.10	53.244	43.064	0.0437	0.0475
0.80	51.431	39.356	0.0418	0.0584

in Ba_1.9Eu_0.1SiO₄. In addition, the distortion index of the Ba(2) sites in Ba_1.2Eu_0.8SiO₄ is larger than that in Ba_1.9Eu_0.1SiO₄, while the distortion index of the Ba(1) sites in Ba_1.2Eu_0.8SiO₄ and Ba_1.9Eu_0.1SiO₄ is almost the same. Therefore, the large redshifts in both the excitation and emission spectra of Ba_2−xEu_xSiO₄ could be attributed to the decrease in polyhedral volume and the increase in distortion of the Ba(2) site. This trend for Ba_2−xEu_xSiO₄ matches with that for intermediate compositions of the solid-solution Ba_2−xSr_xSiO₄:Eu²⁺ [25] .

4. Conclusion

We observed large redshifts in both the emission and excitation spectra of M_2−xEu_xSiO₄ (M: Sr and Ba) at high concentration of Eu²⁺ ions. In the case of Sr_2−xEu_xSiO₄, the emission peak was shifted from 585 nm for Sr_1.9Eu_0.1SiO₄ to 611 nm for Sr_1.2Eu_0.8SiO₄. On the other hand, in the case of Ba_2−xEu_xSiO₄, the emission peak was shifted from 513 nm for Ba_1.9Eu_0.1SiO₄ to 545 nm for Ba_1.2Eu_0.8SiO₄. The right-hand edges of the excitation spectra for both the samples were significantly shifted by 40 - 55 nm to longer wavelengths, allowing for excitation by blue light. The induction of large redshifts in the emission and excitation spectra of both samples could be attributed to the occupancy of Eu²⁺ ions in the polyhedra of Sr(2) or Ba(2) sites, which is smaller and more distorted than the Sr(1) or Ba(1) sites. These results indicate that Sr_1.2Eu_0.8SiO₄ and Ba_1.2Eu_0.8SiO₄ are suitable as red- and green-emitting phosphors for next-generation white-LED applications.

Acknowledgements

This work was partially supported by a Grant-in-Aid for Scientific Research (No. 22107002) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). This work was also partially supported by the Cooperative Research Program of the “Network Joint Research Center for Materials and Devices”.

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