Optics and Photonics Journal
Vol.05 No.11(2015), Article ID:61445,8 pages

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

Yasushi Sato1*, Hiroki Kuwahara2, Hideki Kato2, Makoto Kobayashi2, Takaki Masaki3, Masato Kakihana2*

1Department of Chemistry, Faculty of Science, Okayama University of Science, Okayama, Japan

2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan

3School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Republic of Korea

Copyright © 2015 by authors and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).


Received 17 October 2015; accepted 26 October 2015; published 25 November 2015


The photoluminescence properties of Eu2+-activated α’-Sr2SiO4 and α’-Ba2SiO4 with a high Eu2+ concentration were investigated. In the case of Sr2−xEuxSiO4, 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 Ba2−xEuxSiO4, 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 Eu2+ 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+ (Ca1.2Eu0.8SiO4) [14] and the orange-red emitting Sr6Y2Al4O15:Ce3+ [15] has been prepared by the basis on crystal-site engineering [16] . In the case of Ca1.2Eu0.8SiO4, this phosphor shows an emission peak at 653 nm under excitation at 450 nm [14] . The emission wavelength of Ca1.2Eu0.8SiO4 is quite similar to those of typical red-emitting CaAlSiN3:Eu2+ and Sr2Si5N8:Eu2+ [4] [6] . Ca1.2Eu0.8SiO4 has been identified as an α’L-Ca2SiO4 structure (space group: Pna21, 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 Ca2−xEuxSiO4, 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 Ca2−xEuxSiO4, Eu2+ substitution in the small Ca(2n) sites is promoted [14] . In the case of Ca1.2Eu0.8SiO4, 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 Ca1.2Eu0.8SiO4 at about 650 nm [14] . The crystal structure of Ca2−xEuxSiO4 is analogous to those of Sr2−xEuxSiO4 and Ba2−xEuxSiO4 [17] . Therefore, it is expected that the addition of large amounts of Eu2+ ions to Sr2−xEuxSiO4 and Ba2−xEuxSiO4 can lead to large redshift in their emission and excitation spectra. In this study, we prepared Sr2−xEuxSiO4 and Ba2−xEuxSiO4 with a high concentration of Eu2+ ions and characterized their structural and photoluminescence properties.

2. Experimental Detail

Polycrystalline Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4 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 Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4, we also prepared Sr1.9Eu0.1SiO4and Ba1.2Eu0.8SiO4 samples with a low Eu2+ concentration by an amorphous metal complex (AMC) method using propylene glycol-modified silane [18] . This is because Sr1.9Eu0.1SiO4 and Ba1.2Eu0.8SiO4 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 Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4 samples.

For single micrograins in Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4 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 Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4 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 Sr2−xEuxSiO4 and Ba2−xEuxSiO4 samples with total Eu2+ concentrations (x) of 0.1 and 0.8. Although trace amounts of SrCO3 and BaCO3 impurities were detected in the Sr1.9Eu0.1SiO4 and Ba1.9Eu0.1SiO4 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 Sr1.9Ba0.1SiO4 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 M2−xEuxSiO4 (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 Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4 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) Sr2-xEuxSiO4 (x = 0.1 and 0.8) and (b) Ba2-xEuxSiO4 (x = 0.1 and 0.8) polycrystalline samples.

Table 1. Lattice constants and Eu2+ occupancies at each Sr site of Sr2-xEuxSiO4 samples, estimated by Rietveld refinements of their X-ray diffraction patterns.

Table 2. Lattice constants and Eu2+ occupancies at each Ba site of Ba2-xEuxSiO4 samples, estimated by Rietveld refinements of their X-ray diffraction patterns.

Hence, the trend in Sr2−xEuxSiO4 is similar to that in Ca2−xEuxSiO4 [14] . On the other hand, the trend of variation in Eu2+ occupancies between the Ba(1) and Ba(2) sites in Ba2−xEuxSiO4 is opposite that in Ca2−xEuxSiO4 [14] and Sr2−xEuxSiO4, as shown in Table 2. Eu2+ ions predominantly occupy Ba(2) sites in Ba1.9Eu0.1SiO4. 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 Sr2−xEuxSiO4 and Ba2−xEuxSiO4 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 Sr2−xEuxSiO4 and from 513 to 545 nm for Ba2−xEuxSiO4. 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 Ba2−xEuxSiO4 and about 55 nm for Sr2−xEuxSiO4. 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 M1.2Eu0.8SiO4 samples (M: Ca, Sr, and Ba). Upon excitation at 450 nm, the emission peaks of M1.2Eu0.8SiO4 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 Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4 were approximately 1.6 times higher than that of Ca1.2Eu0.8SiO4. 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 Sr1.2Eu0.8SiO4 are comparable to that of red light. On the other hand, the x (0.39) and y (0.58) coordinates of Ba1.2Eu0.8SiO4 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 Sr1.2Eu0.8SiO4 and 47% and 53% for Ba1.2Eu0.8SiO4, respectively, which are similar to those for Ca1.2Eu0.8SiO4 (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 Sr2−xEuxSiO4 and Ba2−xEuxSiO4 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 SrxBa2−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 Sr2−xEuxSiO4 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 (λem = 612 nm) observed in Sr1.2Eu0.8SiO4 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 Sr2−xEuxSiO4. The relationship between PL properties and Eu2+ occupancy is quite similar to that in the case of Ca2−xEuxSiO4 [14] . In contrast, the yellow emission band (λem = 585 nm) observed in Sr1.9Eu0.1SiO4 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 Sr1.9Eu0.1SiO4, as shown in Table 1.

The polyhedral volumes and distortion indices of Ba2−xEuxSiO4 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 Ba1.2Eu0.8SiO4 is smaller than that

Figure 2. Emission (λex = 365 nm) and excitation spectra of (a) Sr2-xEuxSiO4 and (b) Ba2-xEuxSiO4 with total Eu2+ 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 (λex = 450 nm) and (b) excitation spectra of Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4. For comparison, the emission and corresponding excitation spectra of Ca1.2Eu0.8SiO4 are also shown in both the figures. The photographs of Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4 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 Sr2-xEuxSiO4 samples (total Eu2+ concentration (x) equal to 0.1 and 0.8).

Figure 4. The CIE chromaticity coordinates of Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4 phosphors (open circles). The coordinates of three other orthosilicate phosphors (Ca1.2Eu0.8SiO4, Sr1.9Eu0.1SiO4 and Ba1.9Eu0.1SiO4: solid circles), commercial YAG: Ce3+ (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 Ba2-xEuxSiO4 samples (total Eu2+ concentration (x) equal to 0.1 and 0.8).

in Ba1.9Eu0.1SiO4. In addition, the distortion index of the Ba(2) sites in Ba1.2Eu0.8SiO4 is larger than that in Ba1.9Eu0.1SiO4, while the distortion index of the Ba(1) sites in Ba1.2Eu0.8SiO4 and Ba1.9Eu0.1SiO4 is almost the same. Therefore, the large redshifts in both the excitation and emission spectra of Ba2−xEuxSiO4 could be attributed to the decrease in polyhedral volume and the increase in distortion of the Ba(2) site. This trend for Ba2−xEuxSiO4 matches with that for intermediate compositions of the solid-solution Ba2−xSrxSiO4:Eu2+ [25] .

4. Conclusion

We observed large redshifts in both the emission and excitation spectra of M2−xEuxSiO4 (M: Sr and Ba) at high concentration of Eu2+ ions. In the case of Sr2−xEuxSiO4, the emission peak was shifted from 585 nm for Sr1.9Eu0.1SiO4 to 611 nm for Sr1.2Eu0.8SiO4. On the other hand, in the case of Ba2−xEuxSiO4, the emission peak was shifted from 513 nm for Ba1.9Eu0.1SiO4 to 545 nm for Ba1.2Eu0.8SiO4. 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 Sr1.2Eu0.8SiO4 and Ba1.2Eu0.8SiO4 are suitable as red- and green-emitting phosphors for next-generation white-LED applications.


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”.

Cite this paper

YasushiSato,HirokiKuwahara,HidekiKato,MakotoKobayashi,TakakiMasaki,MasatoKakihana, (2015) 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. Optics and Photonics Journal,05,326-333. doi: 10.4236/opj.2015.511031


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