Co-Doping Effect on the Optical Properties of Eu^(2+/3+) Doped in BaSiO₃

Abstract

To investigate the effect of co-doping on the optical properties of Eu^(2+/3+) doped in Ba₀.₉₈SiO₃:0.02Eu, the series of Ba₀.₉₆SiO₃:0.02Eu, 0.02R^+/3+ (R⁺ = Li⁺, K⁺ or Na⁺, R³⁺ = La³⁺ or Y³⁺) phosphors were synthesized using a solid-state reaction method. The excitation efficiency due to the charge transfer band (CTB) was enhanced via co-doping of R⁺ and the emission intensity due to Eu³⁺ was thus increased by 3.7 times compared with that of the single-doped Ba₀.₉₈SiO₃:0.02Eu³⁺. However, the co-doping of R⁺ does not increase the emission intensity of Eu³⁺ via the direct ⁷F₀→⁵L₆ excitation of Eu³⁺, but rather decreases it. On the other hand, the emission intensities due to Eu²⁺ were decreased via the co-doping of R⁺ but increased via the co-doping of La³⁺. The present work reveals that the optical properties of Eu³⁺ or Eu²⁺ doped in BaSiO₃ depend not on the charge state (+ or 3+) of the co-doped ions, but on the co-doped element itself.

1. Introduction

The photoluminescent properties of rare-earth (RE) ions have attracted considerable interest because of their potential applications as phosphors, white light-emitting diodes (WLEDs), and lasers, etc. When doped in inorganic host materials, RE ions energetically prefer the trivalent state in hosts. Some RE ions, such as Eu, Sm, and Yb, where Yb²⁺ is more stable than Yb³⁺, are doped with divalent atoms in some hosts. Specifically, Eu²⁺-doped phosphors exhibit a broad emission band that varies from ultra-violet to red, depending on the host lattice, and some of Eu³⁺-doped phosphors are used as a typical red phosphor. Thus, europium-based luminescence materials are extensively used in solid-state lighting, solar cells, red phosphors, and WLEDs.

Silicates are widely used as phosphor hosts due to their merits, such as their high thermal stability, the high energy efficiency of the silicate phosphors, and the inexpensive nature of the raw materials. Most often, the europium ions in a host are substituted into alkaline earth metals such as Ba²⁺, Sr²⁺, or Ca²⁺. When Eu³⁺ ions are doped in silicate hosts such as BaSiO₃, Ca₂SiO₄, and Sr₂SiO₄ through heat-treatment in air, there should be a charge balance mechanism involved in replacing A²⁺(A²⁺ = Ba²⁺, Ca²⁺, and Sr²⁺) with Eu³⁺. In the case of BaSiO₃, in which Eu³⁺ replaces the Ba²⁺ site based on the ionic radius, negatively charged barium ion vacancies (V_ba) are created for charge compensation. It is known that the creation of such defects leads to a deterioration in the optical performance of the synthesized phosphors. Jiang et al. have investigated the influence of Li⁺ co-doping on the photoluminescence of Eu³⁺ doped in an SrWO₄ host [1]. Li et al. also studied the effect of Li⁺, Na⁺, and K⁺ on the PL enhancement of Tb³⁺ doped in CaMoO₄ [2]. In many cases, it has been reported that the co-doping of alkali-metal ions or Y³⁺ or La³⁺ enhances the emission intensity of RE ions doped in host materials [3,4,5,6,7]. However, it has also been reported that the emission intensity decreased in ZnTa₂O₆:Pr³⁺ co-doped with alkali-metal ions [8]. The Eu³⁺ ions doped in a BaSiO₃ host will be located along with the Ba²⁺ site because the ionic radius of Eu³⁺ is similar to that of Ba²⁺. Meanwhile, the BaSiO₃:Eu²⁺ phosphors are mostly prepared via the heat-treatment of BaSiO₃:Eu³⁺ in a reducing atmosphere at high temperature. In this reduction process, the V_ba defect created for charge compensation in BaSiO₃:Eu³⁺ will lose its charge compensation function. To date, to the best of the authors’ knowledge, there have been no reports on the effect of V_ba on the optical properties of Eu²⁺ doped in BaSiO₃.

In this article, we report on the co-doping effects of alkali metal ions and La³⁺ (or Y³⁺) on the optical properties of BaSiO₃:Eu^(2+/3+) fabricated via a solid-state reaction (SSR) method. The effects of Li⁺, K⁺, Na⁺, La³⁺, and Y³⁺ co-doped with Eu^(2+/3+) on the crystalline phase and the optical properties of BaSiO₃:Eu^(2+/3+) are discussed. The results showed that the doping of Li⁺, K⁺, Na⁺, La³⁺, and Y³⁺ can significantly affect the optical properties of Eu^(2+/3+) but the element used for the co-doping ion has a greater effect on the optical properties of Eu^(2+/3+) than the charge state (+ or 3+) of the co-doping ion.

2. Materials and Methods

A series of Ba₀.₉₈SiO₃:0.02Eu, Ba₀.₉₆SiO₃:0.02Eu, and 0.02R (R = Li, K, Na, La, and Y) phosphors were prepared using a conventional solid-state reaction. The BaCO₃ (99+%), SiO₂ (particle size 5~20 nm, 99.5%), Eu₂O₃ (99.99%), K₂CO₃ (99+%), Li₂CO₃ (99+%), Li₂O (97%), Na₂CO₃ (99+%), LaCl₃ (99.99%), and Y₂O₃ (99.99%), purchased from Merck, were used as a raw materials but SiO₂ was used 20% more to obtain a single-phase BaSiO₃ sample. The raw materials were thoroughly mixed in an agate mortar and subsequently fired in air at 1100 °C for 4 h to synthesize the Eu³⁺-doped BaSiO₃. The BaSiO₃:Eu²⁺ samples were prepared by sintering the BaSiO₃:Eu³⁺ samples again under a reduction atmosphere (95% N₂ + 5% H₂) at 1200 °C for 6 h. The specific formulations of the studied samples are shown in

Table 1. Studied samples.

Sample No	Eu(3+/2+) (2%)	R	Remark
1	Eu3+	0	1st heat treated in air
2	Eu3+	0.02 Li+	R = Li, Li2O used as co-doping material, 1st heat treated in air
3	Eu3+	0.02 Li+	R = Li, Li2CO3 used as co-doping material, 1st heat treated in air
4	Eu3+	0.02 K+	R = K, K2CO3 used as co-doping material, 1st heat treated in air
5	2 Eu3+	0.02 Na+	R = Na, Na2CO3 used as co-doping material, 1st heat treated in air
6	Eu3+	0.02 La3+	R = La, LaCl3 used as co-doping material, 1st heat treated in air
7	Eu3+	0.02 Y3+	R = Y, Y2O3 used as co-doping material, 1st heat treated in air
8	Eu2+	0	same as sample 1 except 2nd heat treated in a reducing atmosphere
9	0 Eu2+	0.02 Li+	same as sample 2 except 2nd heat treated in a reducing atmosphere
10	Eu2+	0.02 Li+	same as sample 3 except 2nd heat treated in a reducing atmosphere
11	Eu2+	0.02 K+	same as sample 4 except 2nd heat treated in a reducing atmosphere
12	Eu2+	0.02 Na+	same as sample 5 except 2nd heat treated in a reducing atmosphere
13	Eu2+	0.02 La3+	same as sample 6 except 2nd heat treated in a reducing atmosphere
14	Eu2+	0.02 Y3+	same as sample 7 except 2nd heat treated in a reducing atmosphere

.

The crystal phase of the obtained samples was identified using an X-ray diffractometer (X’Pert PRO MPD-3040, Malvern Panalytical, Malvern, UK, CuK_α1, λ = 1.5406 Å) operating at 40 kV and 30 mA with a scan speed of 0.02°/sec. The photoluminescence excitation (PLE) and emission (PL) spectra were measured using a fluorescent spectrofluorometer (JASCO FP-8500, JASCO, Tokyo, Japan) equipped with an integrating sphere (ISF-834, JASCO, Tokyo, Japan). All measurements were performed at room temperature.

3. Results and Discussion

Figure 1 shows the X-ray powder diffraction (XRD) patterns of the studied samples. The XRD patterns of all samples were relatively well matched with pure BaSiO₃ (JCPDS-702112) within the accuracy of our XRD equipment, indicating that the studied samples were single-phased BaSiO₃, which crystallizes in an orthorhombic structure and has a space group of P2₁2₁2₁ with lattice constants of a = 0.4580 nm, b = 0.5611 nm, and c = 1.2431 nm, α = β = γ = 90° [7]. This also means that the activator Eu^(2+/3+) ions and co-doping ions R^+/3+ which were small-doped in the samples, did not cause any significant changes in the crystal phase of BaSiO₃. Miscellaneous minor peaks were marked with a plus (+) and an asterisk (*). The XRD patterns of samples 8–14, prepared under a reducing atmosphere, were identical to those of samples 1–7 because there was no change in crystallinity even if the secondary heat treatment was performed under a reducing atmosphere. The XRD patterns of the samples prepared under a reducing atmosphere are thus not presented in Figure 1.

Based on the data provided by the Cambridge Crystallographic Data Center, Si⁴⁺ ions are located at the center of a tetrahedron coordinated with four O₂⁻ ions. The [SiO₄]²⁻ tetrahedrons form a chain running parallel to the c-axis by sharing corners and the Ba²⁺ ions are accommodated in the tunnels between chains [9,10].

In our previous work [11], the crystal structure of BaSiO₃ was found to be highly dependent on the particle size of the SiO₂ used as a raw material. Even though this result was not presented in this paper, it was not possible to fabricate samples not having the minor second phase with SiO₂, the particle size of which was greater than 500 nm. This means that the characteristics of raw materials having the same molecular formula can highly affect the crystal structures of the samples prepared under the same preparation conditions. Specially, the optical properties of BaSiO₃:Eu²⁺ are highly sensitive to the presence or absence of the minor second phase in the crystal structure, even though this is not true of other materials. The main reason that previously reported results are slightly different may be due to the difficulties involved in achieving single-phase BaSiO₃:Eu²⁺ fabrication without having some minor peaks [10,12,13].

The crystal structure of BaSiO₃ has been detailed in other works [10,14], where a Ba site and six oxygen atoms bond together in the BaSiO₃ unit cell. Since Ba²⁺ ions (0.135 nm) are closer to Eu³⁺ ions (0.0947 nm) than Si⁴⁺ ions (0.0400 nm) in terms of their ionic radius, Ba²⁺ is expected to be replaced by Eu³⁺ [7]. Assuming that Ba²⁺ is expected to be replaced by Eu³⁺, an imbalance of charges is generated in the sample, resulting in the creation of a negatively charged defect, V_Ba. To solve this problem, charge compensators such as Li⁺, K⁺, or Na⁺ were co-doped with Eu³⁺ and the effect of charge compensation on the emission of Eu³⁺ has been reported [7]. Yang et al. reported that doping with Na⁺ and K⁺ ions has a drastic effect on the improvement of the excitation and emission of Eu³⁺ [7]. However, through this work, we found that doping with Y³⁺ ions also has the same effect as that obtained with Na⁺ and K⁺ ions. This result means that the improvement of the excitation and emission of Eu³⁺ could be determined by the species of co-doping ions, rather than the charge state of the co-doping ions, and also shows that this process may not be explained with charge compensation alone, as described so far.

Figure 2 illustrates the excitation and emission spectra of Ba₀.₉₆SiO₃:0.02Eu³⁺, 0.02R^+/3+ samples. The PLE, measured by monitoring the strongest Eu³⁺ emission at 612 nm, is shown in Figure 2a. When Eu₂O₃, which is an optical active material, was mixed with other host materials and then heat-treated in air, the Eu ions were doped into Eu³⁺ in the host material due to its chemical stability. The PLE spectra exhibit a wide band known as a charge transfer band (CTB), where the electrons come from the entirely filled 2p orbitals of O^2- ions and are transferred to the partially filled 4f orbitals of Eu³⁺. The CTB ranged from 200 nm to 290 nm, with the highest peak centered around 228 nm. Lingxiang Yang et al. reported that the CTB peaked around 246 nm [14]. This 18 nm difference in the CTB peak position may be attributed to the crystallinity of the studied samples. The sample studied by Lingxiang Yang et al. exhibited relatively many miscellaneous peaks, which were ascribed to the BaSi₂O₅ phase (JCPDS#26-0176), compared with our samples.

Based on our previous study, we knew that the optical properties of Eu^(2+/3+) doped in BaSiO₃ were more highly dependent upon the crystallinity of the host materials, compared to other host materials. Several narrow bands from 300 nm to 550 nm, due to the intra-4f transitions of Eu³⁺ ions, were also observed. Since the transitions of Eu³⁺ were very stable, and are almost certainly not affected by the lattice structure or neighboring anion ligands of Eu³⁺, the excitation peaks were almost same for all samples. However, in the case of the CTB, things were quite different. It has been reported in the literature that the CTB depends greatly on the lattice structure of the matrix in which Eu³⁺ was introduced [15]. For example, the CTB peak position for Sr₂SiO₄:Eu³⁺ is 293 nm and it was 250 nm for the Gd₂O₃ [16]. It is thus reasonable to assume that the 18 nm difference in the CTB peak position may be attributed to the crystallinity of the studied samples.

One interesting aspect of our results is that the excitation efficiency due to the CTB in sample 5, co-doped with Na⁺ ions, was enhanced by 3.7 times compared with that of the single-doped sample 1. The CTB excitation efficiency of sample 7, co-doped with Y³⁺, was two times higher than that of sample 6, co-doped with La³⁺. Furthermore, The CTB excitation efficiency of samples co-doped with Li⁺ or K⁺ was twice as high as that of the single-doped sample 1. These results mean that the co-doped R^+/3+ ions act as a sensitizer and can thus contribute to the enhancement of the emission intensity due to Eu³⁺. It is thus relatively reasonable to conclude that the CTB excitation efficiency does not depend greatly on the charge state of the co-doping element, but rather on the species of co-doping ions.

Upon the excitation of CTB’s 230 nm region, as shown in Figure 2a, the ⁵D₀→⁷F₂ maximum transition intensity of Eu³⁺ for sample 5, co-doped with Na⁺, was enhanced by a factor of 3.7 times compared with sample 1. In the case of samples 2 and 3, which were co-doped with the same Li⁺, the ⁵D₀→⁷F₂ transition intensity of sample 2 was stronger than that of sample 3. We have thus shown through this study, for the first time, that the emission efficiency of Eu³⁺ is dependent upon the raw material of the co-doping ions, rather than the charge state of the co-doping ions. Figure 2c shows the emission spectra obtained at an excitation wavelength of 393 nm, which represents a ⁷F₀→⁵L₆ excitation of Eu³⁺. The emission intensities of samples 2, 3, and 6, co-doped with Li⁺ or La³⁺ ions, were stronger than that of sample 1 but that of sample 4, co-doped with K⁺, Na⁺ and K⁺, was much weaker than that of sample 1.

Figure 3 shows the strongest Eu³⁺ emission intensity that was obtained at 612 nm when excited by 230 nm and 393 nm, and it also shows the difference in the excitation efficiency through the CTB and the direct ⁷F₀→⁵L₆ excitation of Eu³⁺. When excited by CTB’s 230 nm region, the results clearly showed that the excitation efficiencies of sample 5, co-doped with Na⁺, and sample 4, co-doped with K⁺, were much better than those of other samples. However, the excitation efficiency of sample 4, excited by 393 nm, which represents ⁷F₀→⁵L₆ excitation of Eu³⁺, was the lowest among the studied samples.

Figure 4a–d shows the PLE and PL spectra of the studied BaSiO₃:Eu²⁺ phosphors (samples 8–14). The PLE spectra, measured by monitoring the 577 nm emission intensity, are shown in Figure 4a. The PLE spectra cover a very broad region, from 200 to 450 nm, with two peaks of 280 nm and 340 nm. The broad excitation covered the UV/vis range widely, which matches well with commercial LED chip emissions, so this phosphor is suitable for solid-state lighting based on an n-UV LED chip. The broad excitation band is commonly ascribed to the crystal-field split 4f⁶5d¹ configuration of Eu²⁺ ions. It is well known that the 4f⁶5d¹ electron state is not shielded from anion ligands in the compounds. It has thus been suggested that the 4f⁶5d¹ electron state has a strong interaction with neighboring anion ligands in the BaSiO₃ host, which results in a broad excitation band, as shown in Figure 4a. Since samples 8–14 were actually the same as samples 1–7, which were heat-treated in air, except for the fact that samples 8–14 were heat-treated in a reducing atmosphere, and the ionic radius of Eu³⁺ (0.0947 nm) was smaller than that of Ba (0.135 nm), it was expected that Eu²⁺ ions in sample 8–14 would replace the Ba sites of BaSiO₃. The excitation spectrum shown in Figure 4a has two peaks at 280 nm and 340 nm, which corresponds to the crystal-field splitting of the 5d levels in the excited 4f⁶5d¹ configuration of the Eu²⁺. The excitation spectra for samples 8–14 show a similar pattern, even though the relative excitation intensity over the whole excitation band is slightly different.

Figure 4b shows the room-temperature PL spectra of samples 8–14 excited at 340 nm, in which the PL features of Eu²⁺ showed slight changes in peak position depending on the co-doping ions. Since only one Ba site exists crystallographically in BaSiO₃, a single broad emission band from 450 nm to 750 nm with the peak location at 577 nm was expected, which was attributed to the typical 4f⁶5d¹→4f⁷ transition. This single and symmetrical broad emission band indicated that the Eu²⁺ ions were replaced into the only Ba²⁺ site in BaSiO₃, as expected. The full width at half maximum (FWHM) of the emission spectrum was about 131 nm, which is broader than the common values of FWHM (50–100 nm) of Eu²⁺ in most phosphors [3,12,17] and is also broader than that (119 nm) of the well-known yellow-emitting Y₃Al₅O₁₂:Ce³⁺ phosphor [18]. This broad emission may indicate that the interaction of Eu²⁺ with the ligands of the BaSiO₃ host was strong and thus it may also be suitable as a warm white light source using an n-UV Led chip.

We also found the emission due to Eu³⁺ ions when samples 8–14 were excited at 280 nm was efficient in exciting Eu³⁺ ions. This implies that not the all Eu³⁺ ions present in samples 1–7 were completely reduced into Eu²⁺ via heat treatment under the mixed nitrogen (90%) and hydrogen (10%) atmosphere. Co-doping ions such as Li⁺, K⁺, and Na⁺ were effective in enhancing the emission of Eu³⁺ via the excitation of the CTB (refer to Figure 2b) but they decreased the emission intensity of Eu²⁺ ions, as shown in Figure 4b. Based on the radii of the compound elements, all Eu³⁺ ions were expected to replace the Ba²⁺ sites of BaSiO₃ due to the occurrence of the charge imbalance. To solve this charge imbalance effect, charge compensators such as Li⁺, K⁺, and Na⁺ ions were co-doped with Eu³⁺ ions. When Eu³⁺ and R⁺ (R⁺ = Li⁺, K⁺ and Na⁺) were co-doped, two kinds of defects were created, since one Eu³⁺ and one R⁺ occupied two Ba²⁺ sites in BaSiO₃:Eu³⁺, R⁺ (samples 2–5). Considering that the ionic sizes of cations varied with their coordination numbers (CN), we expected that Eu³⁺ and R⁺ ions would prefer to replace the Ba²⁺ (CN = 8, r = 1.42Å) in BaSiO₃ because the ionic radius of Eu³⁺ (CN = 6, r = 1.07~1.087 Å) was closer to that of Ba²⁺ and the Si⁴⁺ sites with an ionic radius of 0.42 Å were too small for Eu³⁺ to occupy [7,9,19]. The ionic radii of Li⁺, Na⁺, and K⁺ ions are also known to be 0.9, 1.16, and 1.52 Å, respectively [19,20].

Ba₀.₉₆SiO₃:0.02Eu²⁺, 0.02R^+/3+ (samples 8–14) phosphors were prepared by carrying out a second heat-treatment of Ba₀.₉₆SiO₃:0.02Eu³⁺, 0.02R^+/3+ samples (samples 1–7) in a mixed nitrogen (90%) and hydrogen (10%) atmosphere to reduce Eu³⁺ into Eu²⁺. When Ba₀.₉₆SiO₃:0.02Eu³⁺, 0.02R^+/3+ phosphors were heated in the mixed nitrogen (90%) and hydrogen (10%) atmosphere, hydroxyl bonds were formed, which seemed to play an important role in the reduction of Eu³⁺ ions. We assume that these hydroxyl bonds may have reacted with the negatively charged barium ion vacancies (V_ba) which were generated due to the charge imbalance when Eu³⁺ replaced the Ba²⁺ site in Ba₀.₉₆SiO₃:0.02Eu³⁺, 0.02R^+/3+, resulting in the decreased PL intensity of Eu². However, more experimental and theoretical studies are needed to reveal the mechanism underlying the decrease in PL intensity due to Eu²⁺.

Figure 4d shows the room-temperature PL spectra of samples 8–14 excited at 254 nm, which belonged to the CTB and was also efficient to excite Eu³⁺ ions. As shown in Figure 4c, the PL intensity of sample 13, co-doped with La³⁺, was stronger than that of samples 9–12, co-doped with R⁺. Furthermore, the PL intensity of sample 13 was stronger than that of sample 14, which was co-doped with Y³⁺. Figure 2 shows that the PL intensity of the ⁵D₀→⁷F₂ transition of Eu³⁺ for sample 6, co-doped with La³⁺, was weaker than that of the ⁵D₀→⁷F_4–7 transition but the PL intensity of the ⁵D₀→⁷F₂ transition of Eu³⁺ for samples 1–7 (except for sample 6) was stronger than that of the ⁵D₀→⁷F_4–7 transition. In general, it is known that the emission intensity due to the ⁵D₀→⁷F_4–7 transition is weaker than the emission intensity caused by the ⁵D₀→⁷F₂ transition because Ω₄ is much smaller than Ω₂ according to the Judd–Ofelt theory [20]. However, depending on the symmetry of Eu³⁺, it has been reported that the luminescence intensity due to the ⁵D₀→⁷F₄ transition is stronger than that of ⁵D₀→⁷F₂ [21]. The results shown in Figure 2 and Figure 4d may indicate that the PL intensity of Eu²⁺ or Eu³⁺ doped in BaSiO₃ depends on the co-doped element or the environment around the Eu²⁺ or Eu³⁺, rather than the charge state of the co-doped elements such as R⁺, La³⁺, and Y³⁺.

4. Conclusions

In summary, Ba₀.₉₈₊₀._02RSiO₃: 0.02Eu^(2+/3+), R^+/3+(R = Li⁺, Na⁺, K⁺, La³⁺ and Y³⁺) phosphors were prepared by means of a solid-state reaction method and studies were carried out on the effect of co-doping with ions such as Li⁺, Na⁺, K⁺, La³⁺, and Y³⁺ on the optical properties of the Ba₀.₉₈SiO₃: 0.02Eu^(2+/3+) phosphors.

One notable result of this work is that the excitation efficiency due to CTB was enhanced by co-doping of R^+/3+, which acted as a kind of sensitizer, and thus increased the emission intensity due to Eu³⁺ by 3.7 times. However, the co-doped R^+/3+ ions did not increase the emission intensity of Eu³⁺ via direct ⁷F₀→⁵L₆ excitation of Eu³⁺, but rather decreased it. On the other hand, the co-doped R⁺ ions did not enhance the emission intensity of Eu²⁺ via the excitation of the CTB or through a direct excitation of Eu²⁺, but the co-doped La³⁺ did achieve this effect. The present work thus reveals for the first time that the optical properties of Eu²⁺ or Eu³⁺ doped in BaSiO₃ depend on the co-doped element itself, rather than the charge state of the co-doped elements such as R⁺, La³⁺, and Y³⁺.