Large Redshifts in Emission and Excitation from Eu 2 +-Activated Sr 2 SiO 4 and Ba 2 SiO 4 Phosphors Induced by Controlling Eu 2 + Occupancy on the Basis on Crystal-Site Engineering

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).


Introduction
Over the last decade, there have been many reports on the development of high-efficiency green-and red-emitting 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:Eu 2+ , Ba 3 Si 6 O 12 N 2 :Eu 2+ , (Ca, Sr)AlSiN 3 :Eu 2+ , 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 Si 3 N 4 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, siliconbased 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 Ca 2 SiO 4 :Eu 2+ (Ca 1.2 Eu 0.8 SiO 4 ) [14] and the orange-red emitting Sr 6 Y 2 Al 4 O 15 :Ce 3+ [15] has been prepared by the basis on crystal-site engineering [16].In the case of Ca 1.2 Eu 0.8 SiO 4 , this phosphor shows an emission peak at 653 nm under excitation at 450 nm [14].The emission wavelength of Ca  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 Eu 2+ ions (x > 0.20) to the initial composition of Ca 2−x Eu x SiO 4 , Eu 2+ substitution in the small Ca(2n) sites is promoted [14].In the case of Ca 1.2 Eu 0.8 SiO 4 , the Eu 2+ occupancy in Ca(2n) sites was about 9% of the total concentration of Eu 2+ .Thus, it is believed that a certain amount of Eu 2+ substitution in the Ca(2n) sites led to the deep-red emission of Ca 1.2 Eu 0.8 SiO 4 at about 650 nm [14].The crystal structure of Ca 2−x Eu x SiO 4 is analogous to those of Sr 2−x Eu x SiO 4 and Ba 2−x Eu x SiO 4 [17].Therefore, it is expected that the addition of large amounts of Eu 2+ ions to Sr 2−x Eu x SiO 4 and Ba 2−x Eu x SiO 4 can lead to large redshift in their emission and excitation spectra.In this study, we prepared Sr 2−x Eu x SiO 4 and Ba 2−x Eu x SiO 4 with a high concentration of Eu 2+ ions and characterized their structural and photoluminescence properties.

Experimental Detail
Polycrystalline Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 samples were synthesized by a conventional solid-state reaction method, using SrCO 3 or BaCO 3 , Eu 2 O 3 and SiO 2 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 BaCl 2 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%)-H 2 (4%) gas mixture with flow rate at 400 ml/min.In order to compare the Eu 2+ occupancies and photoluminescence (PL) properties of Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 , we also prepared Sr 1.9 Eu 0.1 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 samples with a low Eu 2+ concentration by an amorphous metal complex (AMC) method using propylene glycol-modified silane [18].This is because Sr 1.9 Eu 0.1 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 samples prepared by a solid-state reaction method contained a large amount of impurities such as SrCO 3 or BaCO 3 , compared with the samples prepared by the AMC method.These samples were then heat-treated by the same procedures as used for the Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 samples.
For single micrograins in Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 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 pat-terns in these samples were measured with optimized discriminator settings to suppress the fluorescence effect [14].PL spectra of Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 polycrystalline powders were analyzed using a fluorescence spectrometer (FP-8500, JASCO) equipped with an integrating sphere (ISF-513, JASCO).were detected in the Sr 1.9 Eu 0.1 SiO 4 and Ba 1.9 Eu 0.1 SiO 4 samples, all samples were identified as orthorhombic α'-Sr 2 SiO 4 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 1.9 Ba 0.1 SiO 4 as a starting model structure [17].Based on quantitative elemental analysis by energy-dispersive X-ray spectroscopy (EDS), the occupancies of Eu 2+ in the M(1) and M(2) sites in M 2−x Eu x SiO 4 (M: Sr and Ba) were also subjected to Rietveld refinement, as shown in Table 1 and Table 2. Here, the total Eu 2+ concentrations in both Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 after heat treatment with the BaCl 2 •2H 2 O 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 Eu 2+ is exchanged by Ba 2+ ions from BaCl 2 •2H 2 O. Irrespective of the Eu 2+ total concentration, the Sr(1) sites had higher Eu 2+ occupancies than the Sr(2) sites.The percentage of the total Eu 2+ occupancy in Sr(2) sites against Eu(1) sites increased significantly from 33% to 45% upon increasing the total Eu 2+ concentration (x) from 0.1 to 0.8.2+ were observed for all the samples.As the total concentration of Eu 2+ ions increased from 0.1 to 0.8, large emission redshifts were observed from 585 to 611 nm for Sr 2−x Eu x SiO 4 and from 513 to 545 nm for Ba 2−x Eu x SiO 4 .On the other hand, the right-hand edges of the corresponding excitation spectra of both samples were shifted to longer wavelengths as the total Eu 2+ 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−x Eu x SiO 4 and about 55 nm for Sr 2−x Eu x SiO 4 .The wavelength with the maximum excitation intensity was about 370 nm for both samples.show the emission and excitation spectra of M 1.2 Eu 0.8 SiO 4 samples (M: Ca, Sr, and Ba).Upon excitation at 450 nm, the emission peaks of M 1.2 Eu 0.8 SiO 4 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  [14].

Results and Discussion
Finally, we discuss the origin of the large redshifts observed in the emission and excitation spectra of Sr 2−x Eu x SiO 4 and Ba 2−x Eu x SiO 4 on changing the total Eu 2+ concentration (x) from 0.1 to 0.8 in terms of the occupancies of Eu 2+ 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'-Sr 2 SiO 4 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 x Ba 2−x SiO 4 :Eu 2+ (Eu 2+ = 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−x Eu x SiO 4 were smaller and more distorted than those of the Sr(1) sites, regardless of Eu 2+ concentration.If Eu 2+ ions occupy the Sr(2) sites, the coordination environment of the Eu 2+ ions can lead to strong crystal field splitting of the 5d orbitals of Eu 2+ ions.Therefore, the red emission band (λ em = 612 nm) observed in Sr 1.2 Eu 0.8 SiO 4 is mainly produced from the Eu 2+ ions in Sr(2) sites, since a certain amount of Eu 2+ ions is present in the Sr(2) sites.It is conceivable that the addition of a large amount of Eu 2+ based on crystal-site engineering is mainly responsible for the large redshifts in the emission and excitation spectra of Sr 2−x Eu x SiO 4 .The relationship between PL properties and Eu 2+ occupancy is quite similar to that in the case of Ca 2−x Eu x SiO 4 [14].In contrast, the yellow emission band (λ em = 585 nm) observed in Sr 1.9 Eu 0.1 SiO 4 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 Eu 2+ ions occupied Sr(1) sites in Sr 1.9 Eu 0.1 SiO 4 , as shown in Table 1.
The polyhedral volumes and distortion indices of Ba

Conclusion
We observed large redshifts in both the emission and excitation spectra of M

Figure 1 (
Figure 1(a) and Figure 1(b) show the X-ray diffraction (XRD) patterns of Sr 2−x Eu x SiO 4 and Ba 2−x Eu x SiO 4 samples with total Eu 2+ concentrations (x) of 0.1 and 0.8.Although trace amounts of SrCO 3 and BaCO 3 impuritieswere detected in the Sr 1.9 Eu 0.1 SiO 4 and Ba 1.9 Eu 0.1 SiO 4 samples, all samples were identified as orthorhombic α'-Sr 2 SiO 4 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 1.9 Ba 0.1 SiO 4 as a starting model structure[17].Based on quantitative elemental analysis by energy-dispersive X-ray spectroscopy (EDS), the occupancies of Eu 2+ in the M(1) and M(2) sites in M 2−x Eu x SiO 4 (M: Sr and Ba) were also subjected to Rietveld refinement, as shown in Table1and Table2.Here, the total Eu 2+ concentrations in both Sr 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 after heat treatment with the BaCl 2 •2H 2 O 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 Eu 2+ is exchanged by Ba 2+ ions from BaCl 2 •2H 2 O. Irrespective of the Eu 2+ total concentration, the Sr(1) sites had higher Eu 2+ occupancies than the Sr(2) sites.The percentage of the total Eu 2+ occupancy in Sr(2) sites against Eu(1) sites increased significantly from 33% to 45% upon increasing the total Eu 2+ concentration (x) from 0.1 to 0.8.

Figure 2 .
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 .
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 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).

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.

Table 2 .
[14]ice 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.Hence, the trend in Sr 2−x Eu x SiO 4 is similar to that in Ca 2−x Eu x SiO 4[14].On the other hand, the trend of variation in Eu 2+ occupancies between the Ba(1) and Ba(2) sites in Ba 2−x Eu x SiO 4 is opposite that in Ca 2−x Eu x SiO 4[14]and Sr 2−x Eu x SiO 4 , as shown in Table2.Eu 2+ ions predominantly occupy Ba(2) sites in Ba 1.9 Eu 0.1 SiO 4 .However, as the total Eu 2+ concentration is increased, Eu 2+ 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−x Eu x SiO 4 and Ba 2−x Eu x SiO 4 samples with total Eu 2+ concentrations (x) of 0.1 and 0.8.Broad emission peaks assigned to the 4f 6 5d 1 → 4f 7 transition of Eu 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 were approximately 1.6 times higher than that of Ca 1.2 Eu 0.8 SiO 4 .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 1.2 Eu 0.8 SiO 4 are comparable to that of red light.On the other hand, the x (0.39) and y (0.58) coordinates of Ba 1.2 Eu 0.8 SiO 4 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 1.2 Eu 0.8 SiO 4 and 47% and 53% for Ba 1.2 Eu 0.8 SiO 4 , respectively, which are similar to those for Ca 1.2 Eu 0.8 SiO 4 (external QE: 44% and internal QE: 50%)

Table 4 .
2−x Eu x SiO 4 samples with total Eu 2+ concentrations (x) of 0.1 and 0.8 are listed in The polyhedral volume of the Ba(2) sites in Ba 1.2 Eu 0.8 SiO 4 is smaller than that

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).

Relative intensity / a. u.
[25].1 SiO 4 .In addition, the distortion index of the Ba(2) sites in Ba 1.2 Eu 0.8 SiO 4 is larger than that in Ba 1.9 Eu 0.1 SiO 4 , while the distortion index of the Ba(1) sites in Ba 1.2 Eu 0.8 SiO 4 and Ba 1.9 Eu 0.1 SiO 4 is almost the same.Therefore, the large redshifts in both the excitation and emission spectra of Ba 2−x Eu x SiO 4 could be attributed to the decrease in polyhedral volume and the increase in distortion of the Ba(2) site.This trend for Ba 2−x Eu x SiO 4 matches with that for intermediate compositions of the solid-solution Ba 2−x Sr x SiO 4 :Eu 2+[25].
(1) Eu x SiO 4 (M: Sr and Ba) at high concentration of Eu 2+ ions.In the case of Sr 2−x Eu x SiO 4 , the emission peak was shifted from 585 nm for Sr 1.9 Eu 0.1 SiO 4 to 611 nm for Sr 1.2 Eu 0.8 SiO 4 .On the other hand, in the case of Ba 2−x Eu x SiO 4 , the emission peak was shifted from 513 nm for Ba 1.9 Eu 0.1 SiO 4 to 545 nm for Ba 1.2 Eu 0.8 SiO 4 .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 Eu 2+ 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 1.2 Eu 0.8 SiO 4 and Ba 1.2 Eu 0.8 SiO 4 are suitable as red-and green-emitting phosphors for next-generation white-LED applications.