1. Introduction

In the past few years, energy-efficient solid-state lighting and high-power white light emitting diodes (LEDs), which are regarded as the next-generation light source, have been attracted increasing attention for use in display lighting sources and illuminating systems [1–6]. In the case of the phosphor converted white-light LEDs, the phosphor materials play an important role. For example, the most common and simple LED-based white light source is combined of a blue-emitting InGaN chip and a Ce³⁺-doped yttrium aluminium yellow phosphor (YAG:Ce³⁺) [7], which is very stable and exhibits high luminescence efficiency. However, in some respects YAG:Ce³⁺-based dichromatic systems often suffer from reduced thermal stability and exhibit a poor color-rendering index (CRI) caused by the color deficiency in the red- and blue-green of the phosphor. To overcome this weakness, one of the alternative ways is coupling a blue LED with long-wavelength yellow-emitting phosphors, which are with high down conversion efficiency. Xie’s group has made a great progress on these luminescent materials [8–11]. Another method is using an UV LED (λ_em = 350-410 nm) with RGB (red, green, and blue) phosphors or coupling blue LED with RG phosphors for a high CRI and a high power output. Therefore, the efficient red phosphors are required. Plenty of works has been devoted to find out a good red phosphor [12–18]. Though rare-earth-doped sulfides and oxysulfides have been used as the red phosphors [19–21], they cannot be used widely because these compounds are sensitive to moisture and not easy to be synthesized. Recently, nitrides and oxynitrides have been demonstrated to be good potential as red phosphors because of their good thermal stability [10,22,23]. However, the production cost of the phosphors is very high because the synthesis of the phosphors requires very high firing temperatures and high nitrogen pressures.

From the viewpoint of stability, environmental friendliness, convenient synthesis conditions and low cost, it is urgent to search for new red phosphors with high chemical stability. Borates, chemical stable and easily productive at temperatures below 800°C in air [24–27], have attracted much attention on exploring new phosphors because of being suitable host for rare earth ions [28–31]. Recently, luminescence properties of Ba₂Mg(BO₃)₂:Eu²⁺,Mn²⁺ and ZnB₂O₄:Bi³⁺,Eu³⁺ are investigated and conclude that they are highly efficient, red-emitting phosphors, which prove that borates are good candidates to find out potential new red phosphors [14,28].

KSr₄(BO₃)₃, synthesized by our group, crystallizes with space group Ama2 [32]. It contains [BO₃]^3- isolated triangles and 4 crystallographically independent cation sites. Jiang et al. have indicated that this compound is considered most suitable hosts for Ce³⁺ ion luminescence [33]. To our knowledge, the luminescence properties of Eu³⁺ doped KSr₄(BO₃)₃ have not been studied yet, though the red luminescence of Eu³⁺ ion has been extensively used in the lighting and displays. Therefore, motivated by the above investigations and the attempt to develop new red phosphors for the potential applications of white-light LED, we have investigated the luminescence and color chromaticity properties as well as the structure of KSr₄(BO₃)₃:Eu³⁺ phosphor in the present work. Our investigation has demonstrated that doping Eu³⁺ occupy Sr(2) (8c) site in the strucrture of KSr₄(BO₃)₃ and KSr₄(BO₃)₃:Eu³⁺ can emit red light with high color saturation. Besides, in order to eliminate the charge unbalance because of Eu³⁺ doped into KSr₄(BO₃)₃, The effect of charge compensation is also studied in this paper.

2. Experimental

Polycrystalline samples of KSr₄(BO₃)₃:Eu³⁺ and KSr₄(BO₃)₃:Eu³⁺,M⁺ (M = Li, Na, K) were prepared by a solid-state reaction at high temperature starting from analytical purity K₂CO₃, SrCO₃, H₃BO₃, Li₂CO₃, Na₂CO₃, and high purity Eu₂O₃(99.99%). The raw materials were weighed out, mixed and ground together in an agate mortar, and then sintered under ambient atmosphere at 600 °C for 24 h. After this step, the sintered powder mixtures were ground and sintered at 800 °C for 24 h twice.

The X-ray diffraction (XRD) data for phase identification and structural refinements were collected at ambient temperature by a PANalytical powder X-ray diffractometer X’Pert Pro with Cu Kα radiation (40 kV, 40 mA). The data were collected over a 2θ range from 10þ to 135þ at intervals of 0.017þ with a counting time of 1 s per step. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured by a spectrofluorometer (Edinburgh Instruments, FLS920) equipped with a Xe light source and double excitation monochromators. The powder samples were compacted and excited under 45° incidence and emitted fluorescence was detected by a photomultiplier (R928P) perpendicular to the excitation beam. To eliminate the second-order emission of the source radiation, a cutoff filter was used in the measurement. A μF900 lamp (100 W) was used as a light source for the emission lifetimes measurement. The slits were set at 2 nm, and a photomultiplier (R928P) was used as detector.

3. Results and discussion

3.1 Structure of KSr₄ (BO₃)₃:Eu³⁺ phosphors

The XRD patterns of KSr₄(BO₃)₃:Eu³⁺ phosphors with different doping Eu³⁺ contents are found to be in good agreement with that reported of KSr₄(BO₃)₃ [32] regardless of the contents of dopants as certificated in the following Rietveld refinement (Fig. 1 is the Rietveld refinement plot with the XRD pattern of KSr₄(BO₃)₃:0.03Eu³⁺ phosphor). This indicates that no impurity phase is present in the as-prepared powders and the host crystal structure is not changed by doping Eu³⁺ ions.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 [34], it is proposed that Eu³⁺ ions are expected to occupy the Sr sites preferably, because the ionic radii of Eu³⁺ (r = 1.25 Å when CN = 8 and r = 1.30 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 Eu³⁺, however, compared with the radii of Sr²⁺, the radii of K⁺ is much larger than that of Eu³⁺. The compound structure will not be stable in crystallography when Eu³⁺ occupy K⁺ site because the difference of effective ionic radii between these two ions is too large. Therefore, Eu³⁺ will not prefer to occupy K⁺ sites in this compound. In order to prove the Eu³⁺ ions occupy Sr²⁺ sites and also further to make clear which Sr sites are occupied by Eu³⁺, refinement of the XRD patterns of KSr₄(BO₃)₃:Eu³⁺ are performed by Rietveld method [35,36] within the Fullprof Program [37]. The K⁺ sites are found could not be occupied because of the final agreement factors is very high and the occupancy of Eu³⁺ on this site is far away from the nominal doping content. As for Sr sites, it is found that all the doping Eu³⁺ ions are preferred to occupy Sr(2) (8c) sites during the refinement. Setting the structure of KSr₄(BO₃)₃ as the initial structure, we try to refine the XRD patterns supposing that Eu³⁺ occupy the three different Sr sites, as shown in Table 1 (selected XRD pattern of KSr₄(BO₃)₃:0.03Eu³⁺ as example), in which A–G represents Eu³⁺ 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, Eu³⁺ 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 reasonable final compositions which are in good agreement with the nominal compositions of the starting materials is obtained in case of B. Therefore, doped Eu³⁺ ions prefer to occupy Sr(2) (8c) site. Figure 1 is the final Rietveld refinement plot of KSr₄(BO₃)₃:0.03Eu³⁺, and the inset is the structure projection of KSr₄(BO₃)₃. The selected refinement results are listed in Tables 2 and 3 . It is found that the distance between Sr(2) (8c) sites is much shorter than that of the other two sites, which might be the reason for the prefer occupancy. Therefore, we conclude that doped Eu³⁺ ions prefer to occupy Sr(2) (8c) sites.

 

Fig. 1 Final Rietveld refinement plots of the KSr₄(BO₃)₃:0.03Eu³⁺. 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 structure projection of KSr₄(BO₃)₃.

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Table 1. Occupancy and Lattice Parameters of Eu³⁺ Occupying Different Sites for KSr₄(BO₃)₃:0.03Eu³⁺ Refined by Rietveld Method, in which A–G Represents Eu³⁺ 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 Equivalent Isotropic Displacement Parameters (Ų) for KSr₄(BO₃)₃:0.03Eu³⁺

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Table 3. Interatomic Distances (Å) and Angles (deg) for KSr₄(BO₃)₃:0.03Eu³⁺

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3.2 Photoluminescence properties of KSr₄(BO₃)₃:Eu³⁺

The PLE and PL spectra of KSr₄(BO₃)₃:0.03Eu³⁺ are shown in Fig. 2 . The excitation spectrum detected at 612 nm consists of a broad band which is attributed to the O^2--Eu³⁺ charge transfer band (CTB) and several linear peaks centering at 318 nm, 361 nm, 366 nm, 377 nm, 382 nm, 394 nm, 416 nm, and 465 nm, corresponding to the typical f-f transitions, i.e., the transitions from the ground state ⁷F₀ to the exciting state of 4f⁶ configuration. The strongest peak in the excitation spectrum locates at 394 nm which comes from the ⁷F₀-⁵L₆ transition.

 

Fig. 2 PL/PLE spectra of as–synthesized KSr₄(BO₃)₃:0.03Eu³⁺ (red, λ_em = 612 nm; black, λ_ex = 394 nm). The inset is the effect of Eu³⁺ concentration on the PL intensity.

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The emission spectrum is composed of a series sharp peaks from 560 nm to 720 nm, which are attributed to the transitions of ⁵D₀ →⁷F_J (J = 0–5). It is accepted 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, the highly intense line at ⁵D₀ → ⁷F₁ is because of magnetic dipole transition. The main emission bands centered at 612 nm and 701 nm are attributed to the electric dipole transition of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ respectively, which are ascribed to the lifting of the spin and parity prohibitions of Eu³⁺ f – f transitions in KSr₄(BO₃)₃ host lattice in the asymmetric C_2v point group [28,38]. It is consisted with the structural study that Eu³⁺ occupies the noncentrosymmetric position of Sr2 (8c). Because of the crystal field effect, ⁷F_J energy level will appear Stark levels. Therefore, the energy level transition ⁵D₀→⁷F₁ is split into 589 nm and 593 nm emission peaks and ⁵D₀ → ⁷F₂ is split into 612 nm, 617 nm, 620 nm and 623 nm; Moreover, a transition of ⁵D₀ → ⁷F₀ (579 nm) is observed in the PL spectra. It does not belong to the selection rule of ED transition. This transition is coming from the relaxation of selection rule somewhat because the crystal field plays a role in mixing states.

Normally, with different amounts of Eu³⁺ doped into the host lattice, the local surroundings around a substituted site will be changed, which eventually makes it possible to tune the luminescence properties. By investigating the emission intensity of KSr₄(BO₃)₃ doped with different Eu³⁺ contents excited at 394 nm, we get the dependence of the corresponding PL intensity as a function of Eu³⁺ concentration, which is shown in the inset of Fig. 2. Obviously, the optimal dopant concentration of Eu³⁺ is 3 mol.%, which means the PL intensity of emission peaks decrease while the doped content is above 3 mol.% because of the quenching concentration of Eu³⁺.

Based on the Dexter and Schulman theory [39], concentration quenching is due to energy transfer from one activator to another until an energy sink in the lattice is reached. Critical distance (R_c) is the critical separation between donor (activator ion) and acceptor (quenching ion), where the nonradiative rate equals that of the internal single ion relaxation. Thus the critical distance (R_c) is an important parameter which can be estimated from the concentration quenching data. Blasse proposes that the average shortest distance between the nearest activator ions for the critical concentration is equal to the critical distance (R_c) [40]. The R_c can be expressed by

Dexter theory indicates that the concentration quenching of inorganic materials is due to the electric multipolar interaction or magnetic dipolar interaction among the activator ions [39]. In this study, the concentration quenching of the KSr₄(BO₃)₃:Eu³⁺ phosphor is probably from the non-radiative transition among the Eu³⁺ ions. It is well know that the crystal field in non-centrosymmetric C_2v sites mixes some 5d character into the 4f wavefunction, which will make electric multipolar interactions as the probable transfer mechanism. In addition, from the data of fluorescence lifetime (discussed later), there is no any evidence of lengthening lifetime with increasing concentration. Therefore, the process of energy transfer of Eu³⁺ ions in KSr₄(BO₃)₃ would be due to electric multipolar interaction [39,41,42].

Because the lifetimes are nearly the same for the all samples and the curves are overlapped, the selected decay properties by time-resolved measurement of the KSr₄(BO₃)₃:0.03Eu³⁺ phosphor are shown in Fig. 3 as an example (excited at 394nm, monitored at 612 nm). The curve fits a first-order exponential decay model described by equation [43]:

 

Fig. 3 Selected decay curve of KSr₄(BO₃)₃:xEu³⁺ (x = 0.03) (excited at 394 nm, monitored at 612 nm).

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Charge compensation is an important factor to the emission efficiency of the luminescent materials. In this study, alkali metal ions, Li⁺, Na⁺ and K⁺ (1 mol.%), are codoped into the KSr₄(BO₃)₃:0.01Eu³⁺ to compensate the charge unbalancing brought by Eu³⁺. Examining the phosphors doped with Eu³⁺ and M⁺ (M = Li, Na, or K) by X-ray diffraction, no other peaks appear besides KSr₄(BO₃)₃ (as shown in Fig. 4(a) ), which means the doped alkali ions will not change the host structure. Figure 4(b) is the dependence of volume of KSr₄(BO₃)₃:0.01Eu³⁺ to the type of 1 mol.%-doped alkali metal ions, in which the volumes of samples are obtained from XRD data. A slightly difference is observed for the volumes in accordance with the change of the ion radii of Li⁺ (0.96 Å), Na⁺ (1.18 Å), and K⁺ (1.38 Å). When K⁺ ions doped into host compounds, might not all of them substitute the Sr²⁺ sites. A part of them will occupy some vocations of K⁺ formed during crystallization, and the bonding environment is changed because K⁺ ions will combine with O^2- tightly. That is why the volume of KSr₄(BO₃)₃:0.01Eu³⁺ doped by K⁺ ions shrinks compared with that of host compound. Figure 4(c) is the PL spectra and 4(d) is the intensities of peaks at 612 nm for the emission spectra of KSr₄(BO₃)₃:0.01Eu³⁺, 0.01M⁺(M = Li, Na and K). Obviously the codoping Li⁺, Na⁺, or K⁺ significantly enhances the luminescence of in KSr₄(BO₃)₃:Eu³⁺. It yields 3.9, 2.3, and 4.5 times of the emission intensity of the sample for the KSr₄(BO₃)₃:Eu³⁺ doping 1 mol.% Li⁺, Na⁺ and K⁺, respectively.

 

Fig. 4 (a) Selected X-ray diffraction pattern of KSr₄(BO₃)₃:0.01Eu³⁺,0.01Li⁺; (b) volume of KSr₄(BO₃)₃:0.01Eu³⁺ doped by 0.01M⁺ (M = Li, Na, K), none in the figure refers to the volume of KSr₄(BO₃)₃:0.01Eu³⁺. (c) the emission spectra of KSr₄(BO₃)₃:0.01Eu³⁺, 0.01M⁺ (M = Li, Na, K) excited at 394 nm. (d) relation between the intensities of peaks at 612nm of the emission spectra in (c) and the compensation charge of alkali ions M⁺ (M = Li, Na, K).

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When a trivalent cation Eu³⁺ is incorporated into a KSr₄(BO₃)₃ lattice and occupies the divalent Sr²⁺ site, charge balance is destroyed. In order to maintain the charge balance, the host has to capture O₂ in the air. That is, not all Eu³⁺ ions go into the lattice and occupy Sr sites. In this study, 1 mol.% Eu³⁺ ions are doped into the host, the sites of 3 Sr²⁺ ions will be occupied by 2Eu³⁺ ions. Thus, only about 0.67 mol.% Eu³⁺ ions occupy the sites of Sr²⁺ ions, and about 0.16 mol.% Eu³⁺ ions exists in the sample as Eu₂O₃ (which is below the detection limit of XRD, so it cannot be shown in the XRD data), and the sample is possible denoted as KSr_3.96(BO₃)₃:0.0067 Eu³⁺/0.0016 Eu₂O₃. Then the existence of Eu₂O₃ phase will restrain the KSr₄(BO₃)₃ grains growth during the sintering process, leading to the decrease of luminescence intensity. However, in the charge compensated phosphors KSr₄(BO₃)₃:0.01Eu³⁺, 0.01M⁺(M = Li, Na and K), the charge unbalance will be compensated directly by this possible mechanisms: 2Sr²⁺→Eu³⁺ + K⁺. Therefore, suitable amounts of Eu³⁺ (and M⁺) are able to occupy Sr sites. For example, in KSr₄(BO₃)₃:0.01Eu³⁺, two Sr²⁺ ions are replaced by one Eu³⁺ and one alkali ion, resulting in the decrease of Eu₂O₃ content. In consequence, the phosphors with efficient charge compensation exhibit higher emission [44,45]. Because the radius of Li⁺ ion is smaller than that of Na⁺ and K⁺, the impurity phase is formed hardly in the KSr₄(BO₃)₃:0.01Eu³⁺,0.01Li⁺ phosphor and therefore the KSr₄(BO₃)₃:0.01Eu³⁺, 0.01Li⁺ phosphor shows comparatively good photoluminescence. As for the KSr₄(BO₃)₃:0.01Eu³⁺,0.01K⁺, in addition to occupy Sr²⁺ site, some doped K⁺ will occupy some vacancy caused by K⁺ loss during the sintering process, which will promote the growth of KSr₄(BO₃)₃. As a result, the phosphor charge compensated by K⁺ show an intensive emission.The Commission International de I’Eclairage (CIE) chromaticity coordinates for KSr₄(BO₃)₃:Eu³⁺ excited at 394 nm are measured and shown in Fig. 5 . The CIE chromaticity coordinates of Eu³⁺ activated KSr₄(BO₃)₃ are (0.64, 0.35), which is very close to that of NSTC standard red color (0.66, 0.33). This means that the red phosphor found in this study is high purity and high color saturation. Furthermore, charge compensation with Li⁺, Na⁺, or K⁺ does not change the chromaticity coordinates of KSr₄(BO₃)₃:Eu³⁺, as shown in Table 4 . This discloses that charge compensation will just enhance the luminescence intensity. The narrow emission band, high color saturation, and short decay time make KSr₄(BO₃)₃:Eu³⁺ a very attractive red phosphor in white LEDs.

 

Fig. 5 (color online) CIE chromaticity diagram for KSr₄(BO₃)₃:Eu³⁺ excited at 394 nm.

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Table 4. Chromaticity coordinates of KSr₄(BO₃)₃:Eu³⁺ and KSr₄(BO₃)₃:0.01Eu³⁺,0.01M⁺ (M = Li, Na, and K)

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4. Conclusion

A new red-emitting phosphor, KSr₄(BO₃)₃:Eu³⁺, is synthesized by solid state reactions and the photoluminescence is studied for the first time. The doped Eu³⁺ ions prefer to occupy Sr(2) (8c) crystallographic sites by Rietveld refinement on XRD patterns. The phosphor can be effectively excited by a near UV light of 394 nm. The emission spectra exhibit the transition from ⁵D₀ level to ⁷F_J (J = 0-5) with a main emission at 612 nm, which comes from the electro-dipole transition because of the asymmetric point group. The quenching concentration of Eu³⁺ is about 3%, and the critical distance is about 15.3 Å. The codoped alkali ions (Li⁺. Na⁺, K⁺) increase the intensities of emission spectra at 612 nm effectively but do not change the chromaticity coordinates. The present results indicate that the novel red emitting phosphor is a suitable candidate for the application on white LEDs combine with a near UV chip.