1. Introduction

Recently, the alkali-alkaline earth borates have captured much attention because of their potential application as new phosphor materials in solid state lighting [1–4]. For example, NaSrBO₃ crystallizes in the monoclinic space group $P{2}_1/c$, in which one Sr²⁺ ion is surrounded by nine O^2- ions [5]. By doping Ce³⁺ in Sr²⁺ sites, a blue-emitting phosphor which is used to fabricate LED successfully is synthesized [6].

KSr₄(BO₃)₃, one alkali-alkaline earth borate, synthesized by our group firstly. It crystallized with space group Ama2 [7]. The structure of this compound contains one K site and three different Sr sites. KO₈ polyhedra are connected via a bridging oxygen atom to form infinite long chains along the a axis, and share four edge and two corners with the BO₃ triangles. The Sr atoms appear in three crystallographically different environments in the compound. The Sr(1) atoms (in the 4b site) are coordinated with eight oxygen atoms, forming distorted bicapped trigonal prisms, while the Sr(2) (in the 8c site) are eight-coordinated to oxygen atoms, forming distorted trigonal dodecahedra. The Sr(3) atoms (in the 4a site) are nine-coordinated by oxygen atoms, forming distorted tricapped trigonal prisms, and they are connected via their corners along the a axis. Because of the plenty of crystallographic sites, KSr₄(BO₃)₃ is a potential host to investigate phosphor with good luminescence properties. Our former studies have confirmed that Dy³⁺, Tm³⁺, and Eu³⁺ codoped KSr₄(BO₃)₃ phosphors exhibit great potential for use as single-component phosphors for warm ultraviolet white light-emitting diodes [8,9]. In those studies, it is disclosed that rare earth ions will occupy different Sr sites when they doped into host. For example, Eu³⁺, Tm³⁺, and Dy³⁺ ions occupy Sr(2) site when they are single doped into host, while Dy³⁺ ions occupy Sr(2) and Sr(3) sites simultaneously when they codoped with Tm³⁺.

Sm³⁺ ions, a kind of rare earth ions, generate intense reddish orange emitting light because of the typical transition between the ground state and the excited state configuration. So, plenty of studies on Sm³⁺ doped phosphors are reported [10–14]. To the best of our knowledge, the luminescence properties of Sm³⁺ activated KSr₄(BO₃)₃ phosphors have not been reported. The site occupancy of Sm³⁺ in KSr₄(BO₃)₃, related to the crystal field afforded to the doping Sm³⁺ ions, is not clear yet. In this study, Sm³⁺ activated KSr₄(BO₃)₃ phosphors are prepared and the photoluminescence properties of Sm³⁺ activated KSr₄(BO₃)₃ phosphors are presented as well as the site occupancy of Sm³⁺.

2. Experimental

KSr₄(BO₃)₃:Sm³⁺ phosphors were synthesized by high temperature solid-state reaction method. Sm₂O₃(>99.99%),K₂CO₃(AR),Sr₂CO₃(AR),and H₃BO₃(AR) were used as starting materials. The raw materials were weighed out, mixed, and grounded thoroughly in an agate mortar. The mixtures were first heated at 600°C for 24 h in air to decompose the carbonate and eliminate the water; then heated to sintering temperature of 800 °C for 72 h.

Samples were first characterized by powder X-ray diffraction (XRD) using Panalytic X’Pert Pro diffractometer with CuKa radiation. The data for KSr₄(BO₃)₃:Sm³⁺ used for Rietveld refinement were collected over a 2$\theta$ range of 10-135° in the step scan mode with a step size of 0.017° and a measurement time of 1s per step at room temperature. The morphologies of the samples were measured by a scanning electron microscope (SEM; Hitachi S-4800). The excitation and emission spectra were recorded at room temperature on a fluorescence spectrophotometer (FLS920, instrumental resolution: 0.05 nm) equipped with a 450W Xenon lamp as the excitation source. For the measurements of fluorescence lifetime, the third harmonic (355 nm) of a Nd doped yttrium aluminum garnet laser (Spectra-Physics, GCR 130) was used as an excitation source, and the signals were detected with a Tektronix digital oscilloscope (TDS 3052). The internal quantum efficiency of optimized-composition phosphor KSr₄(BO₃)₃:0.02Sm³⁺ was determined on an FLS920 spectrometer under excitation of 376 nm.

3. Results and discussion

3.1 Site occupancy of Sm³⁺ in KSr₄ (BO₃)₃:Sm³⁺

Figure 1(a) shows the selected XRD patterns of KSr₄(BO₃)₃:Sm³⁺ phosphors. All XRD patterns are found to agree well with that reported in the Inorganic Crystal Stucture Database (ICSD 171423) regardless of the contents of dopants, indicating that the doped Sm³⁺ ions do not generate any impurity or induce significant changes in the host structure. The morphology of the KSr₄(BO₃)₃:0.02Sm³⁺ sample prepared by solid state reaction is presented in Fig. 1(b). The KSr₄(BO₃)₃:0.02Sm³⁺ sample consists of aggregated particles with sizes ranging from 2 to 5μm.

 

Fig. 1 (a) The XRD patterns of KSr₄(BO₃)₃:xSm³⁺ (x = 0.003, 0.02, 0.05) phosphors. (b) SEM morphology of KSr₄(BO₃)₃:0.02Sm³⁺ phosphor. (c) Final Rietveld refinement plots of the KSr₄(BO₃)₃:0.02Sm³⁺. Small circles (o) correspond to experimental values, and the continuous lines, the calculated pattern; vertical bars (|) indicate the position of Bragg peaks. The bottom trace depicts the difference between the experimental and the calculated intensity values. Inset is the coordination environments of Sr²⁺ with O^2-. The large white balls are Sr²⁺ ions, the small black balls are O^2- ions. (d) the dependence of cell volume of KSr₄(BO₃)₃:Sm³⁺ to the Sm³⁺ concentration.

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As introduced above, the host crystal KSr₄(BO₃)₃ crystallizes in the noncentrosymmetric space group Ama2. There are four crystallographic positions of cations in the unit cell: eight-fold coordinated K⁺(4b) sites, eight-fold coordinated Sr(1) (4b) sites, eight-fold coordinated Sr(2) (8c) sites, and nine-fold coordinated Sr(3) (4a) sites. Based on the effective ionic radii (r) of cations with different coordination number (CN) reported by Shannon [15], it is proposed that Sm³⁺ ions are expected to occupy the Sr sites preferably, because the ionic radii of Sm³⁺ (r = 1.08 Å when CN = 8 and r = 1.13 Å when CN = 9) is very close to that of Sr²⁺ (r = 1.26 Å when CN = 8 and r = 1.31 Å when CN = 9). Though the ionic radii of K⁺ (r = 1.51 Å when CN = 8 and r = 1.55 Å when CN = 9) is also close to that of Sm³⁺, however, compared with the radii of Sr²⁺, the radii of K⁺ is much larger than that of Sm³⁺, which will result in the crystal structure distortion of KSr₄(BO₃)₃ too much to be stable in crystallography if the K⁺ site is occupied by Sm³⁺. Therefore, Sm³⁺ will not prefer to occupy K⁺ sites in this compound.

As shown in the inset of Fig. 1(c), the coordination environments of three Sr²⁺ sites with O^2- are different. They will afford different crystal fields for the doped Sm³⁺ and bring difference on the photoluminescence properties of Sm³⁺ in KSr₄(BO₃)₃:Sm³⁺. In order to determine the occupancy of Sm³⁺ ions in KSr₄(BO₃)₃:Sm³⁺, Rietveld refinements are performed. Rietveld refinement was proposed by H. Rietveld in 1967. This method uses a least squares approach to refine a theoretical line profile of XRD or neutron diffraction data until it matches the measured profile. Refinement of the structure parameters from diffraction data can determine the lattice parameters, atomic positions and occupancies, quantitative phase analysis, and so on. In this study, the structure of KSr₄(BO₃)₃:0.02Sm³⁺ is refined sing the structure of KSr₄(BO₃)₃ as the initial structure model by Rietveld method [16,17] within the Fullprof Program [18]. Because there are three different Sr sites, it is possible that the doped Sm³⁺ ions will occupy these three different Sr sites at the same time or one site or two sites among them. Therefore, we try to refine the XRD patterns supposing that Sm³⁺ will occupy the possible Sr sites, as shown in Table 1, in which A–G represents Sm³⁺ occupy Sr(1) site, Sr(2) site, Sr(3) site, Sr(1) and Sr(2) site, Sr(1) and Sr(3) sites, Sr(2) and Sr(3) sites, and Sr(1), Sr(2) and Sr(3) sites, respectively. Obviously, Sm³⁺ ions cannot occupy Sr(1), Sr(2), and Sr(3) sites at the same time because of the negative occupancy. As for the other cases, the lowest final agreement factors and the resonable final compositions which are in good agreement with the nominal compositions of the starting materials are obtained in case of F. Therefore, doped Sm³⁺ ions prefer to occupy both 8c (Sr2) and 4a (Sr3) sites. These refinement results also indicate that the doped Sm³⁺ ions do not change the structure of the host. Figure 1(c) is the Rietveld refinement plot of KSr₄(BO₃)₃:0.02Sm³⁺. In the final cycle of refinement, a total of 60 parameters were refined (42 structural parameters and 18 profile parameters), and the final agreement factors converged to R_p = 3.24%, R_wp = 4.24%, R_exp = 2.99% for KSr₄(BO₃)₃:0.02Sm³⁺. The fractional atomic coordinates and occupancies for KSr₄(BO₃)₃:0.02Sm³⁺ are listed in Table 2.The refined concentration of Sm³⁺ is 1.98mol % (Table 2), which is in good agreement with the original doping concentration of 2%.The relationship of the volumes of the as-synthesized phosphors and the contents of doped Sm³⁺ ions is shown in Fig. 1(d), in which a systematic relative decline of volume with increasing Sm³⁺ concentration can be found because of the smaller ionic radius of Sm³⁺ than that of Sr²⁺.

Table 1. Occupancy of Sm³⁺ occupying different sites for KSr₄(BO₃)₃:0.02Sm³⁺ refined by Rietveld method, in which A–G represents Sm³⁺ occupy Sr(1) site, Sr(2) site, Sr(3) site, Sr(1) and Sr(2) site, Sr(1) and Sr(3) sites, Sr(2) and Sr(3) sites, and Sr(1), Sr(2) and Sr(3) sites, respectively.

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Table 2. Fractional atomic coordinates and occupancy for KSr₄(BO₃)₃:0.02Sm³⁺.

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3.2 Luminescence properties of KSr₄ (BO₃)₃:Sm³⁺ phosphor

Figure 2(a) shows the selected PLE and PL spectra of KSr₄ (BO₃)₃:0.02Sm³⁺. The blue curve on the left is the PLE spectra monitored by a fluorescence spectrophotometer at 598 nm. A wide band around 240 nm belongs to charge transfer band (CTB) O^2--Sm³⁺. Because the energy levels of Sm³⁺ above about 20000 cm⁻¹ are very crowded and therefore intermediate coupling is essential, which means each of the levels among this range cannot be given by a unique LS-term. Therefore, the sharp excitation peaks are associated with 4fⁿ-4fⁿ intraconfiguration transitions from ground state ⁶H_5/2 of Sm³⁺ to excited state levels (⁴K, ⁴L)_17/2 (346 nm), (⁴D, ⁶P)_15/2 (364 nm), ⁴D_1/2 (376 nm), ⁶P_3/2 (404 nm), ⁴M_19/2 (⁶P, ⁴P)_5/2 (416 nm), (⁴G_9/2, ⁴I_15/2) (443 nm), (⁴F_5/2, ⁴M_17/2) (463 nm), and (⁴I_11/2, ⁴I_13/2) (479 nm) transitions, in which the transition ⁶H_5/2→⁶P_3/2 at about 404 nm shows the maximum intensity [19]. From the excitation spectra of KSr₄(BO₃)₃:Sm³⁺ phosphor, it can be found that KSr₄(BO₃)₃:Sm³⁺ phosphor shows a very wide excitation band from ultraviolet, near ultraviolet band to blue band of visible light. This indicates that KSr₄ (BO₃)₃:Sm³⁺ can be used as a potential phosphor for UV/NUV LED lighting.

 

Fig. 2 (a) PL/PLE spectra of as–synthesized KSr₄(BO₃)₃:0.02Sm³⁺ (blue, λ_em = 598 nm; black, λ_ex = 376 nm). The PL spectrum shows the terminal levels of the transitions from the ⁴G_5/2 state. (b) The energy levels diagram for Sm³⁺ ions in KSr₄(BO₃)₃.

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Under the excitation of 376 nm, the characteristic emission peaks of Sm³⁺ ions can be observed (red curve in Fig. 2(a)). The emission spectra of Sm³⁺ show reasonable emission at 558 nm due to ⁴G_5/2-⁶H_5/2 transition, strong and efficient emission at 598 nm due to ⁴G_5/2 - ⁶H_7/2 transition, and weak emission at 645 nm due to ⁴G_5/2-⁶H_9/2 transition. The emission band centered at 710 nm is attributed to the transition of ⁴G_5/2→⁶H_11/2, which is ascribed to the lifting of the spin and parity prohibitions of Sm³⁺ f – f transitions in KSr₄(BO₃)₃ host lattice in the asymmetric C_2v point group [19]. It is well know that the selection rule of electric dipole (ED) transition is △J$\le$6 when J or J^’ = 0, △J = 2, 4, 6, and the selection rule of magnetic dipole (MD) transition is △J = 0, ± 1. Thus, according to the selection rule, ⁴G_5/2-⁶H_5/2 (△J = 0), ⁴G_5/2-⁶H_7/2 (△J = 1) mainly arise from magnetic dipole, while the electric dipole mechanism play a key role in the transitions of ⁴G_5/2-⁶H_9/2 transition and ⁴G_5/2-⁶H_11/2 transition. One can also find that the emission lines of Sm³⁺ are broadened somewhat and split into several lines because there are several Stark levels for the ⁶H_J levels. The energy level transition ⁴G_5/2→⁶H_5/2 is split into 558nm, 561nm and 567 nm emission peaks; ⁴G_5/2→⁶H_7/2 is split into 598 and 609 nm emission peaks, and ⁴G_5/2→⁶H_9/2 is split into 645 and 653 nm emission peaks. These splits are resulted from the crystal field effects, and their extents are related to the structure property of KSr₄(BO₃)₃. The phenomenon of one transition featuring two emission peaks is considered to be caused by crystal field splitting, which shows that the Sm³⁺ ions occupy low symmetry sites. Figure 2(b) presents a simplified Sm³⁺ energy level diagram that indicates the observed excitation and emission transitions. For excitation at 376 nm, the Sm³⁺ ions are promoted from the ⁶H_5/2 ground state to the excited ⁴D_1/2 manifold and subsequent nonradiative and radiative decays originate the Stokes emissions that are recorded, and they are corresponding to ⁴G_5/2 to ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) transitions, respectively.

Emission spectra of KSr₄(BO₃)₃:Sm³⁺ with different Sm³⁺ concentrations are shown in Fig. 3(a).The intensity of the emission peak at 598 nm is the strongest one in the emission band, which can be identified easily. So this peak is selected to study the concentration dependence of emission intensity of Sm³⁺. Figure 3(b) summarizes the concentration dependence of the emission integrated intensity of Sm³⁺ ions doped in KSr₄(BO₃)₃ at 598 nm. The emission intensity of Sm³⁺ increases with the increase of the concentration of the doped Sm³⁺ until a maximum intensity at 2 mol.% is reached, and then decreases with increasing Sm³⁺ content due to concentration quenching. Thus the optimum concentration for Sm³⁺ in KSr₄(BO₃)₃ host is 2 mol.%. The energy transfer from one Sm³⁺ ion to another Sm³⁺ ion is shown in Fig. 4.The energy between ⁴G_5/2-⁶F_9/2 and ⁶H_5/2 -⁶F_9/2 matches well. By cross relaxation, the energy of ⁴G_5/2 will transfer to ⁶F_9/2, and the energy of ⁶F_9/2 will be consumed by relaxation, resulting in the decrease of emission intensity.

 

Fig. 3 (a) Emission spectra of KSr₄(BO₃)₃:Sm³⁺ with different Sm³⁺ concentrations, and (b) Emission intensity vs. Sm³⁺ concentration (x) of KSr₄ (BO₃)₃:Sm³⁺ phosphors. The inset in (b) shows the dependence of lg(I/x) on lgx.

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Fig. 4 Schematic diagram of cross relaxation process in the self-concentration quenching of Sm3 + ions in KSr₄ (BO₃)₃:Sm³⁺.

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In order to further estimate the critical energy transfer distance (R_c) between the activators in the host, Blasse proposed that the average shortest distance between the nearest activator ions for the critical concentration is equal to the critical distance (R_c) [20]:

Dexter theory indicates that the concentration quenching of inorganic materials is due to the electric multipolar interaction or magnetic dipoles interaction among the activator ions [21]. In this study, the concentration quenching of the KSr₄(BO₃)₃:Sm³⁺ phosphor is mainly from the non-radiative transition among the Sm³⁺ ions. Because the exchange interaction takes place generally in forbidden transition (the R_c is typically ~5 Å) as well as the PLE and PL spectra overlap, we can infer that the nonradiative concentration quenching among two nearest Sm³⁺ centers occurs via electric multipolar interactions based on the Dexter theory. Besides, energy transfer may happen from a percolating Sm³⁺ cluster to a killer centers. The mechanism of interaction between Sm³⁺ ions can be expressed by the following equation [22]:

In order to obtain additional information on the luminescence properties of Sm³⁺ ions in KSr₄(BO₃)₃ host, the decay curves of the Sm³⁺ emission at 558, 598, 645, and 710 nm corresponding to the ⁴G_5/2 to ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) transitions lines upon 376 nm excitation for KSr₄(BO₃)₃:0.02Sm³⁺ phosphor. As shown in Fig. 5, the luminescence decay curves for ⁶H_5/2, ⁶H_7/2, ⁶H_9/2, and ⁶H_11/2 can be well fitted by double exponential equation as follows [23]:

 

Fig. 5 Photoluminescence decay curve for Sm³⁺ emission at 558, 598, 645, and 710 nm corresponding to the ⁴G_5/2-⁶H_J (J = 5/2, 7/2, 9/2, 11/2) emission lines upon 376 nm excitation for KSr₄(BO₃)₃:0.02Sm³⁺ phosphor. Black circles and red solid lines represent the experimental data and fitting results, respectively.

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Table 3. Constants (A) and Decay times (τ) of KSr₄(BO₃)₃:0.02Sm³⁺ as a function of emission wavelength of Sm³⁺.

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Furthermore, the effective decay lifetimes (t) can be calculated using following equation:

Based on the Eq. (4) and the fitting data listed in Table 3, the average lifetime value is calculated to be 0.69, 0.74, 0.70, and 0.69 ms for the transition of ⁴G_5/2 to ⁶H_J (J = 5/2, 7/2, 9/2, 11/2), respectively. The result is consistent with the fact that the decay time of the Sm³⁺ ions is in the order of milliseconds [24,25]. In the present cases, the variation of the measured lifetimes has the same trend as the luminescence intensities for the corresponding ⁴G_5/2 to ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) transitions. It indicates that the big lifetime value of the ⁶H_7/2 level will induce a high-energy transfer efficiency, and then it will produces a high emission intensity at 598 nm.

Quantum efficiency is requested for LED phosphor, which is related to the overall electrical-to-optical conversion efficiency of the entire LED-phosphor package. To determine the absolute quantum efficiency of photo-conversion for the KSr₄(BO₃)₃:Sm³⁺ phosphor, the integrated sphere method is applied for the measurements of optical absorbance (A) and quantum efficiency (η_int) of the phosphors. The absorbance can be calculated by using the following equation:

Conclusion

In conclusion, the red phosphors KSr₄(BO₃)₃:Sm³⁺ are synthesized through a solid state reaction. The site occupancy of Sm³⁺ ions in KSr₄(BO₃)₃:Sm³⁺ are studied. The doped Sm³⁺ ions do not change the basic structure, but decrease the volume of the unit cell. Sm³⁺ ions incline to occupy 8c (Sr2) and 4a (Sr3) sites in the structure of KSr₄(BO₃)₃ according to the Rietveld refinement results. The excitation and emission spectra of KSr₄(BO₃)₃:Sm³⁺ phosphor indicate that the phosphor can be efficiently excited by UV- LED, and emit reddish orange light with the wavelength at 598 nm. The emission intensity of KSr₄(BO₃)₃:Sm³⁺ increases with the increase of the doped Sm³⁺concentration until reaching a maximum value at 2 mol.% doped Sm³⁺, and then decreases with increasing Sm³⁺ content because of concentration quenching. The concentration quenching mechanism occurs via electric multipolar interactions according to Dexter theory. The fluorescence lifetime τ of Sm³⁺ in KSr₄(BO₃)₃ host is 0.69, 0.74, 0.70, and 0.69 ms for the transition of ⁴G_5/2 to ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) respectively and the critical distance is 17.6 Å.