1. Introduction

Rare-earth (RE) silicates are good hosts for luminescence materials due to their excellent chemical and thermal stability, and high luminescence efficiency. And double salts are commonly more stable than simple salts. In our previous work [1], a new silicate scintillator material with relative low melting point was developed using silicate double salt NaLuSi₃O₉ as host, but the site surrounding of the luminescence activators need to be confirmed.

Luminescence of Eu³⁺ can be used to probe the local structure of luminescent centers in a host lattice. The numbers of observed ⁵D₀–⁷F_J (J = 0, 1, 2) emission lines are useful to determine the site symmetry of Eu³⁺ in the host [2–5]. The electric dipole transitions of Eu³⁺ are forbidden when Eu³⁺ locates in a site with inversion symmetry and only weak magnetic dipole transitions can occur (ΔJ = 0, ± 1, but J = 0 to J = 0 is forbidden). However, when Eu³⁺ occupies a site without inversion symmetry, the electric dipole transitions no longer become strictly forbidden and corresponding emission is called forced electric dipole transition. Some transitions (ΔJ = 0, ± 2) are very sensitive to surrounding and become dominated in luminescence spectra even when Eu³⁺ slightly deviates from inversion symmetry. Therefore, it can be easy to judge the point group symmetry of Eu³⁺ according to the number of ⁵D₀→⁷F_J transitions. The ⁵D₀-⁷F₁ magnetic dipole transitions (about 590 nm) dominate when Eu³⁺ locates in a site with inversion symmetry. ⁵D₀-⁷F₁ transition will show three emission peaks if Eu³⁺ occupies a site with C₁, C_2h and D_2h point group symmetry because degeneracy of ⁷F₁ energy level is totally removed. And two emission peaks occur for Eu³⁺ with C_4h, D_4h, D_3d, S₆, C_6h and D_6h point group symmetry. For high symmetry of T_h and O_h point group symmetry, only one ⁵D₀-⁷F₁ emission peak can be observed.

Meanwhile, researches have been carried out since the broad strong absorption band in excitation spectra of RE³⁺ had been assigned to charge transfer (CT) transitions [6–8]. In the past decades, growing interests has been focused on the charge transfer bands of Eu³⁺ doped compounds due to the indispensability of the material’s property for applications [9]. P. Dorenbos made a systematic study about the behavior of charge transfer bands RE³⁺ – O^2- based on the data of a large number of compounds. It was found that the CT energies of other Ln³⁺ can be predicted once the CT energy of Eu³⁺ is confirmed [4]. However, the CT energies are different for Eu³⁺ locating at the site with different symmetry. Therefore, the spectroscopic properties of Eu³⁺ in different host lattices are important not only for possible application but also for basic research. In this work, Na₃LuSi₃O₉:Eu³⁺ phosphors were prepared using a high temperature solid-state reaction method, and the site occupancy of Eu³⁺ doped in Na₃LuSi₃O₉ were analyzed.

2. Experimental

Na₃Lu_1-xEu_xSi₃O₉ samples were prepared with a high temperature solid-state reaction method in air. The raw materials included Na₂CO₃ (analytical reagent, A.R.), SiO₂ (4N), Lu₂O₃ (4N), Eu₂O₃ (4N). The starting materials according to the composition Na₃Lu_1-xEu_xSi₃O₉ (x = 0.001, 0.01, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.65, 0.70, 0.75, 0.80) were thoroughly ground and calcined at 1150 °C for 8 h in air. Finally, all the as-synthesized samples were ground into powders and collected for further measurements.

The phase purity of the samples was examined by X-ray diffraction (XRD) using a BRUKER D8 ADVANCE powder diffractometer with Cu K-alpha radiation (λ = 0.15418 nm) operating at 40 kV and 40 mA. The UV-visible excitation and emission spectra were recorded on an Edinburgh FLS920 spectrometer equipped with a CTI-cryogenics temperature control system and a 450 W xenon lamp was used as the excitation source. The decay curves were measured with a 60 W μF900 microsecond flash lamp with a pulse width of 1.5 μs and pulse repetition rate of 0.1−100 kHz.

3. Results

3.1 XRD analysis

Figure 1 shows the XRD patterns of Na₃Lu_0.70Eu_0.30Si₃O₉ and Na₃Lu_0.20Eu_0.80Si₃O₉ as the representatives. It can be deduced that the patterns of all samples are consistent with the standard card of Na₃YSi₃O₉, which means good phase purity. And it was found that the sample Na₃EuSi₃O₉ can’t be obtained because the samples will melt if the doping content of Eu³⁺ in Na₃LuSi₃O₉ exceeds 80 at%.

 

Fig. 1 XRD patterns of Na₃Lu_0.70Eu_0.30Si₃O₉ (b), Na₃Lu_0.20Eu_0.80Si₃O₉ (c) and standard card of Na₃YSi₃O₉ (a).

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3.2 Photoluminescence properties of Na₃Lu_1-xEu_xSi₃O₉

In order to investigate the influence of Eu³⁺ concentration on excitation spectra of Na₃Lu_1-xEu_xSi₃O₉, the excitation spectra of Na₃Lu_1-xEu_xSi₃O₉ monitored at 611 nm are given in Fig. 2. It can be known that the excitation peaks between 360 and 480 nm are attributed to the f-f transitions of Eu³⁺ and the strongest intensity at 393 nm come from ⁷F₀→⁵L₆ transition. It can also be observed that the excitation intensity increased with increasing Eu³⁺ concentration and maximized at highest Eu³⁺ content (80 at%).

 

Fig. 2 Excitation spectra of Na₃Lu_1-xEu_xSi₃O₉ at room temperature (λ_em = 611 nm).

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To further investigate the influence of Eu³⁺ content on luminescence properties of Na₃Lu_1-xEu_xSi₃O₉, the emission spectra of Na₃Lu_1-xEu_xSi₃O₉ excited at 393 nm are given in Fig. 3. It can be observed that there are two emission peaks at 577.5 nm579 nm from ⁵D₀→⁷F₀ transition of Eu³⁺, four emission peaks at 587 nm, 590 nm, 593 nm, 596 nm from ⁵D₀→⁷F₁ transitions and six peaks at 608 nm, 611 nm, 615 nm, 618 nm, 621 nm, 625 nm from ⁵D₀→⁷F₂ transitions, respectively. It is generally known that the ⁵D₀→⁷F₁ transition of Eu³⁺ at one site can be split into three peaks at most and five for ⁵D₀→⁷F₂ transition [10]. The above results imply that Eu³⁺ occupy more than one site in Na₃LuSi₃O₉. From the inset of Fig. 3, it can be known that the emission intensity of Eu³⁺ rise up with the increasing of Eu³⁺ content and reach maximum till 80 at% Eu³⁺ content, which means that there is no concentration quenching in this host for Eu³⁺ emission, which may be attributed that the LuO₆ octahedra are isolated by SiO₄ tetrahedra in Na₃LuSi₃O₉ host which prevented energy cross relaxation among Eu³⁺ ions [1,11].

 

Fig. 3 Emission spectra of Na₃Lu_1-xEu_xSi₃O₉ at room temperature (λ_ex = 393 nm). The inset shows concentration dependence on the luminescence intensity of Na₃Lu_1-xEu_xSi₃O₉.

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3.3 Luminescence decay properties of Na₃LuSi₃O₉:Eu³⁺

As shown in Fig. 4, the decay curves of Na₃Lu_1-xEu_xSi₃O₉ overlapped very well when Eu³⁺ concentration changed, which implied that the luminescence of Eu³⁺ did not quench with increasing concentration, and keep consistent with results showed in the inset in Fig. 3. The decay curves in Fig. 4(b) cannot be fitted with a single exponential equation because Eu³⁺ ions at different sites were excited at the same time, and it fits well with a two exponential equation [12]:

 

Fig. 4 Decay curves of (A) Na₃Lu_1-xEu_xSi₃O₉ (x = 0.10, 0.30, 0.50, 0.80), (B) Na₃Lu_0.90Eu_0.10Si₃O₉ fitted with second exponential equation.

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4. Discussion

4.1 Analysis of the site occupancy of Eu³⁺ in Na₃LuSi₃O₉

The numbers of observed ⁵D₀–⁷F_J (J = 0, 1, 2) emission lines can be used to distinguish Eu³⁺ in different symmetry sites. The ⁵D₀→⁷F₁ magnetic dipole transition dominates when Eu³⁺ locates in a site with inversion symmetry. However, ⁵D₀→⁷F₂ electric dipole transition dominates when Eu³⁺ occupies a site without inversion symmetry. The ⁵D₀→⁷F₀ transition is usually very weak because it is forbidden and can only be observed in hosts with low symmetry (C_s, C_n and C_nv). And ⁵D₀→⁷F₀ transition cannot be split by crystal field, which indicates that every ⁵D₀→⁷F₀ peak corresponds to a special site. So the number of sites with C_s, C_n and C_nv symmetry in a host can be deduced by the number of ⁵D₀→⁷F₀ emission peaks of Eu³⁺.

Na₃LuSi₃O₉ belongs to orthorhombic structure and P2₁2₁2₁ space group. As shown in Table 1, orthorhombic structure consists of C_2v, D_2h and D₂ point groups. Considering the space group, Na₃LuSi₃O₉ belongs to D₂ point group [13]. It can be known from the structure of Na₃LuSi₃O₉ that all Lu sites belong to C₁ symmetry. ⁵D₀→⁷F₀ transition will show only one emission peak if Eu³⁺ occupy a site with C₁ symmetry (3 and 5 peaks for ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions, respectively). It can be observed that there are two ⁵D₀→⁷F₀ emission peaks located at 577.5 nm and 579 nm in Fig. 5 which implies that Eu³⁺ ions occupy two different sites in Na₃LuSi₃O₉.

Table 1. Lu³⁺ sites and Lu–O bond lengths in Na₃LuSi₃O₉

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Fig. 5 Emission spectra of Na₃Lu_1-xEu_xSi₃O₉ (x = 0.001, 0.10, 0.20, 0.80) at room temperature (λ_ex = 393 nm).

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As shown in Table 1, the average length of Lu-O bond at Lu4 site is longest and then Lu1 site [1]. And the lengths of Lu-O bonds at Lu2 and Lu3 sites are very close. Hence, Eu³⁺ would prefer to enter Lu4 site because of larger ionic radius of Eu³⁺ than Lu³⁺.

Figure 5 displays the emission spectra of Na₃Lu_1-xEu_xSi₃O₉ (x = 0.001, 0.10, 0.20, 0.80) under 393 nm excitation. Two ⁵D₀-⁷F₀ emission peaks at 577.5 nm and 579 nm can be obviously observed with different Eu³⁺ content. The intensity of emission peak at 579 nm was relatively strong at low Eu³⁺ concentration, then weakened until almost disappear with increasing Eu³⁺ content, as the red dot line shown in Fig. 5. In contrast, the intensity of emission peak at 577.5 nm was very weak at low Eu³⁺ concentration and became stronger and stronger with increasing Eu³⁺ content. There are four Lu sites in Na₃LuSi₃O₉ host. The emission at 579 nm was considered to be caused by ⁵D₀-⁷F₀ transition of Eu³⁺ at Lu4 site and emission at 577.5 nm was due to Eu³⁺ at the other Lu sites, because Eu³⁺ preferred to occupy Lu4 site at low concentration and then other sites with increasing content. However, only two obvious ⁵D₀-⁷F₀ emissions were observed. It may be due to that the site symmetry of Lu sites (other than Lu4 site) was similar and their ⁵D₀-⁷F₀ emissions overlapped. It can also be known from Fig. 5 that a new ⁵D₀-⁷F₁ emission peak at 585 nm and two new ⁵D₀-⁷F₂ emission peaks at 608.5 and 613.5 nm appeared with increasing Eu³⁺ concentration, which may be caused by that Eu³⁺ mainly occupied Lu4 site at low concentration and then started to enter other sites with increasing content.

4.2 Charge transfer bands (CTB) of Eu³⁺ in Na₃LuSi₃O₉

Crystal structure and properties of active ions have great influence on charge transfer band [14,15]. Figure 6(a) shows normalized charge transfer bands of Na₃Lu_0.99Eu_0.01Si₃O₉ monitored at 577 nm and 579 nm. The charge transfer band centered at 270 nm (λ_em = 579 nm) is attributed to Eu³⁺ at Lu4 site, and the peaks at 298 nm, 319 nm and 321 nm were due to transitions from ⁷F₀ energy level to higher 4f levels (⁵H_J, ⁵F_J) of Eu³⁺ [16]. The charge transfer band centered at 287 nm (λ_em = 577 nm) comes from Eu³⁺ at other Lu sites except Lu4.

 

Fig. 6 Normalized CTB excitation spectra of (A) Na₃Lu_0.99Eu_0.01Si₃O₉ (λ_em = 577 nm, 579 nm), (B) Na₃Lu_1-xEu_xSi₃O₉ (λ_em = 579 nm), (C) Na₃Lu_1-xEu_xSi₃O₉ (λ_em = 577 nm), (D) Na₃Lu_0.99Eu_0.01Si₃O₉ (λ_em = 608 nm, 611 nm, 615 nm, 618 nm, 621 nm, 625 nm).

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It can be observed in Fig. 5 that the ⁵D₀-⁷F₀ emission intensity of Eu³⁺ decreases at 579 nm and increases at 577.5 nm with increasing Eu³⁺ concentration. To investigate the changing rules, CTB excitation spectra of Na₃Lu_1-xEu_xSi₃O₉ were measured by monitored 577 nm and 579 nm, as shown in Fig. 6(b) and Fig. 6(c).

Figure 6(b) displays the normalized charge transfer bands excitation spectra of Na₃Lu_1-xEu_xSi₃O₉ monitored at 579 nm. The charge transfer bands overlapped with curve a when doping concentration of Eu³⁺ is below 10 at%. Then it moves to longer wavelength with increasing Eu³⁺ concentration from 10 at% to 30 at%, as shown of curve b. And charge transfer bands of Na₃Lu_1-xEu_xSi₃O₉ overlapped with curve c with Eu³⁺ content in the range from 30 at% to 65 at%. And it comes to curve d when Eu³⁺ concentration exceeded 65 at%. Curve c and d are almost the same except the intensity of excitation peaks at around 320 nm. Because Eu³⁺ prefers to occupy Lu4 site at low content (below 10 at%) which corresponds to CTB of curve a. More and more Eu³⁺ entered other Lu sites (except Lu4) with high Eu³⁺ content (above 30 at%), leading to the movement of charge transfer band to curve c and d.

Figure 6(c) shows the normalized CTB excitation spectra of Na₃Lu_1-xEu_xSi₃O₉ (x = 0.10, 0.20, 0.30, 0.50, 0.65, 0.70, 0.75, 0.80) monitored at 577 nm. The position of the charge transfer bands of Eu³⁺ kept still when Eu³⁺ concentration varied. But the CTB broadened and the intensity of excitation peaks at 298 nm, 304 nm, 319 nm and 321 nm from ⁷F₀ energy level to higher 4f levels (⁵H_n, ⁵F_n) of Eu³⁺ increased with increasing Eu³⁺ content.

To investigate the variation of charge transfer bands by monitoring at different wavelength, the normalized CTB excitation spectra of Na₃Lu_0.99Eu_0.01Si₃O₉ (λ_em = 608 nm, 611 nm, 615 nm, 618 nm, 621 nm, 625 nm) are shown in Fig. 6(d). Three kinds of charge transfer bands can be observed in Fig. 6(d). The first one centered at 270 nm (λ_em = 611 nm, 618 nm) belonging to Eu³⁺ at Lu4 site. The second kind centered at 287 nm (λ_em = 615 nm, 625 nm) which may be attributed to Eu³⁺ at the other Lu sites except Lu4. The CTB located between 270 nm and 287 nm when monitored at 621 nm which may be due to the overlap of the above two charge transfer bands. The last one (λ_em = 608) is different because it showed strong f-f transition excitation peaks and the charge transfer band was relatively weak. But the position of the charge transfer band is the same with the second kind.

5. Conclusions

A series of Na₃Lu_1-xEu_xSi₃O₉ phosphors were prepared with a high temperature solid-state reaction method. All samples were pure phase below 80 at% Eu³⁺ and the sample will melt if the content of Eu³⁺ exceeded 80 at%. From the luminescence spectra, it can be known that the emission and excitation intensity of Eu³⁺ increased with increasing Eu³⁺ content and maximized at highest Eu³⁺ content (80 at%). No concentration quenching occurred.

Eu³⁺ prefers to occupy Lu4 site at low concentration (below 10 at%) because of the longest average Lu-O bond length at Lu4 site, then starts to enter the other Lu sites with increasing Eu³⁺ concentration. The ⁵D₀-⁷F₀ emission of Eu³⁺ at Lu4 site locates at 579 nm and corresponding charge transfer band sits at 270 nm. For the other sites, the emission from ⁵D₀-⁷F₀ emission of Eu³⁺ locates at 577.5 nm. A new ⁵D₀-⁷F₁ emission peak at 585 nm and two new ⁵D₀-⁷F₂ emission peaks at 608.5 and 613.5 nm appeared with increasing Eu³⁺ concentration, probably because Eu³⁺ mainly occupies Lu4 site at low concentration and then starts to enter other sites with increasing content. The clarified site occupancy of Eu³⁺ doped in the host Na₃LuSi₃O₉ will be beneficial to develop new phosphors using double salt silicate Na₃RESi₃O₉ as hosts.