Site occupancy and photoluminescence properties of Eu³⁺-activated Ba₂ZnB₂O₆ phosphor

Abstract

A series of Ba₂ZnB₂O₆:Eu³⁺ phosphors with a red-emitting band centered at 616 nm were prepared by traditional high temperature solid-state reaction methods. The site-preferred occupancy of Eu³⁺ in Ba₂ZnB₂O₆ and luminescence properties of Ba₂ZnB₂O₆:Eu³⁺ were studied combined with X-ray diffraction (XRD), photoluminescence excitation (PLE) spectra and emission (PL) spectra as well as temperature-dependent PL and decay curves. The Rietveld refinements indicate that the Eu³⁺ ions prefer to occupy Zn (1) (4a) and Zn (2) (4a) sites simultaneously. The PL intensity is improved with increasing Eu³⁺ content and the optimal dopant content is 0.05. The temperature-dependent PL spectra indicate that the emission intensity decreases with the temperature because of the enhancement of the non-radiative transition. The PL emission intensities of Ba₂ZnB₂O₆:0.05Eu³⁺ phosphors with Li⁺, Na⁺ and K⁺ as charge compensators are enhanced significantly, and the phosphor compensated by Li⁺ ions emits the strongest emission. The Commission Internationale de I'Eclairage (CIE) color coordinates of Ba₂ZnB₂O₆:0.05Eu³⁺ are very close to the CIE of standard red light.

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1. Introduction

The borate system comprises more than 500 kinds of structures due to the different combination modes of the B–O groups.^1,2 Because of the relative low synthesis temperature, stable physical and chemical properties, and excellent luminescence performance,^3,4 borates have been a hot research topic in the field of luminescent host materials recently.

Eu³⁺ is an important rare earth ion in synthesizing phosphors with good properties, for example Y₂O₃:Eu³⁺ is used as a red phosphor in wLEDs. Recently, many types of Eu³⁺-doped compounds have been reported as interesting candidates for potential red-emitting phosphors.^5–11 Because of the proper ionic radii of Ba²⁺ and Zn²⁺, a series of barium containing compounds and zinc containing compounds have been used as host materials to be doped by Eu³⁺, such as Ba_1−xB₈O₁₃:xEu³⁺,¹² Ba_5−2x(VO₄)₃Cl:xEu³⁺, xK⁺,¹³ Zn_1−xAlO₄:xEu³⁺¹⁴ and Zn_1−x−yB₂O₄:xBi³⁺, yEu³⁺,¹⁵ and so on. The radius of Ba²⁺ ion is larger than that of Eu³⁺ ion, and Zn²⁺ ion is smaller than that of Eu³⁺. However, Eu³⁺ can occupy Ba²⁺ sites or Zn²⁺ sites in different hosts. For example, Eu³⁺ ions occupy Ba²⁺ site in Ba_1−xB₈O₁₃ doped by Eu³⁺,¹⁰ but occupy Zn²⁺ site in Zn_1−x−yB₂O₄ codoped by Bi³⁺and Eu³⁺.¹⁵

As one of alkaline earth borates, Ba₂ZnB₂O₆ was firstly synthesized by Robert and Koliha in 1994.¹⁶ This compound crystallizes in Pca2₁ space group with a = 15.068(2), b = 8.720(2), c = 10.128(3) Å, and V = 1330.7 (9) ų. There are four different Ba²⁺ ions sites, two different Zn²⁺ ions sites and three different B³⁺ ions sites in this compound. Two different groups, vertex-sharing ZnO₄ tetrahedra and BO₃ triangles, constitute two-dimensional [Zn₃B₃O₆]_∞ layers which are perpendicular to the [100] direction in the Ba₂ZnB₂O₆ structure. The layers are linked by the additional BO₃ groups, forming a 3D framework. The Ba²⁺ ions fill the space of the 3D framework to balance the charge. All the BO₃ groups in the layers are parallel with each other throughout the structure. The structure of Ba₂ZnB₂O₆ indicates that this compound can offer a proper host structure environment for the doped Eu³⁺ ions. As mentioned above, the doped Eu³⁺ can occupy Ba²⁺ sites or Zn²⁺ sites in the Ba-based borates or Zn-based borates. However, Ba₂ZnB₂O₆ contains Ba²⁺ and Zn²⁺ ions. The site occupancy and the photoluminescence properties of Eu³⁺ ions in such Ba and Zn co-containing compounds are not studied before. In this study, Eu³⁺ activated Ba₂ZnB₂O₆ phosphors are prepared and the site occupancy of Eu³⁺ in Ba₂ZnB₂O₆ is studied as well as the photoluminescence properties of Eu³⁺ activated Ba₂ZnB₂O₆ phosphors. In order to eliminate the charge unbalance because of Eu³⁺ doped into Ba₂ZnB₂O₆, the effect of charge compensation is also studied in this paper.

2. Experimental

Polycrystalline samples of Ba₂ZnB₂O₆:Eu³⁺ and Ba₂ZnB₂O₆:Eu³⁺, M⁺ (M = Li, Na and K) were prepared by a solid-state reaction at high temperature starting from analytical purity BaCO₃, ZnO, H₃BO₃ and Eu₂O₃, Li₂CO₃, Na₂CO₃, and K₂CO₃ (99.99%). The raw materials were weighed out, mixed and ground together in an agate mortar, and then sintered at 600 °C for 24 h in air to remove the H₂O and CO₂ in a muffle furnace. After cooled down to room temperature, the samples were ground again and further sintered at 800 °C for 72 h in air.

The X-ray diffraction (XRD) data for phase identification of the as-prepared powders were collected in the range of 10° to 80° on a PANalytical X'Pert Pro powder X-ray diffractometer with Cu Kα radiation (40 kV, 40 mA). The XRD for structure refinement were collected over a 2θ range from 10° to 140° at intervals of 0.017° with a counting time of 1 s per step. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples were measured by a spectrofluorometer (Edinburgh Instruments, FLS920) equipped with a Xe light source and double excitation monochromators. The powder samples were placed into circular sample cells with quartz plates and excited under 45° incidences. The emitted fluorescence was detected by a photomultiplier (R928P) that is perpendicular to the excitation beam. A cutoff filter was used to avoid the influence of the second-order emission of the source radiation. Diffuse reflectance spectra of the phosphors were measured by a UV-visible spectrophotometer (Hitachi U-4100). The lifetimes were recorded using a μF900 lamp (100 W) as a light source and a photomultiplier (R928P) was used as detector. The temperature-dependent luminescence properties were measured on a fluorescence spectrophotometer (F-4600, HITACHI, Japan) with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation lamp.

3. Results and discussion

3.1 Site-preferred occupancy of the Ba₂ZnB₂O₆:xEu³⁺

The phase purities of Ba₂ZnB₂O₆:xEu³⁺ (x = 0–0.08) were confirmed by X-ray diffraction (XRD) at room temperature, as shown in Fig. 1. The XRD patterns are found to be in good agreement with that was reported by Smith et al. in 1994,¹⁶ which indicates that the structure of host is not changed by doping Eu³⁺ ions regardless of Eu³⁺ contents.

Fig. 1 XRD patterns of Ba₂ZnB₂O₆:xEu³⁺ (x = 0.01–0.08).

As introduced above, Ba₂ZnB₂O₆ crystallizes in an orthorhombic system with space group Pca2₁, and offers two types of ions for Eu³⁺ to replace: four-coordinated Zn²⁺ ions and six- or seven-coordinated Ba²⁺ ions. Considering the ionic radius (r) of different coordination number (CN) reported by Shannon,¹⁷ the ionic radius changes with the coordination number. The radius of Zn²⁺ is r = 0.60 Å as CN = 4, and the radius of Ba²⁺ is r = 1.35 Å as CN = 6 and r = 1.38 Å as CN = 7, respectively. However, the radius of the doped Eu³⁺ is r = 0.89 Å as CN = 4, r = 0.947 Å as CN = 6, and r = 1.01 Å as CN = 7. Compared with the radii of Zn²⁺ and Ba²⁺, the doped Eu³⁺ will prefer to occupy Zn²⁺ site. The radius of Ba²⁺ is much larger than that of Eu³⁺ (the difference between of these two ions is 0.403 Å as CN = 6 and 0.37 Å as CN = 7), which will result in the crystal structure distortion of Ba₂ZnB₂O₆ too much to be stable in crystallography if the Ba²⁺ site is occupied by Eu³⁺. Therefore, it is expected that Eu³⁺ will not prefer to occupy Ba²⁺ sites in this compound.

Rietveld refinement was proposed by H. Rietveld in 1967. Refinement of the structure parameters from diffraction data can obtain the crystal structure properties, such as the lattice parameters, the atomic positions of the doped ions, and occupancies.^18,19 In order to prove the Eu³⁺ ions occupy Zn²⁺ sites in Ba₂ZnB₂O₆:Eu³⁺, the refinement of the XRD patterns of Ba₂ZnB₂O₆:Eu³⁺ by Rietveld method^20,21 within the Fullprof Program²² were performed. It is found that the Ba²⁺ sites cannot be occupied because of the final agreement factors are very high and the occupancy of Eu³⁺ on this site is far away from the nominal doping content. As for Zn²⁺ sites, it is found that all the doped Eu³⁺ ions are preferred to occupy Zn (1) (4a) site and Zn (2) (4a) sites during the refinement. Table 1 summarizes the lattice parameters and agreement factors for ZnBi₂B₂O₇:xEu³⁺ (x = 0.01–0.08) refined by Rietveld method. Fig. 2(a) shows the selected Rietveld refinement plot of Ba₂ZnB₂O₆:0.05Eu³⁺. Fig. 2(b) shows the structure of Ba₂ZnB₂O₆ and Fig. 2(c) shows two different Zn²⁺/Eu³⁺ coordination environment. The final agreement is converged to R_p = 5.97%, R_wp = 7.95%, and R_exp = 3.73%. The refinement results indicate that Ba₂ZnB₂O₆:0.05Eu³⁺ crystallizes in orthorhombic system with a space group Pca2₁ and the lattice parameters a = 15.1104(4) Å, b = 8.7218(2) Å, c = 10.1341(3) Å, and cell volume = 1336.010(2) ų. The longer lattice parameters of Ba₂ZnB₂O₆ doped by Eu³⁺ is because of the fact that r_Eu³⁺ > r_Zn²⁺. The atomic positions in the doped Ba₂ZnB₂O₆:0.05Eu³⁺ unit cell are shown in Table S1.† The refined concentration of Eu³⁺ is 5.3%, which is in good agreement with the original doping concentration of 5%. The refinement results confirm that Eu³⁺ ions prefer to occupy Zn 1 (4a) site and Zn 2 (4a) site, which means Eu³⁺ is coordinated by four O²⁻ with a tetrahedral crystal field environment.

Table 1 Lattice parameters and agreement factors for ZnBi₂B₂O₇:xEu³⁺ (x = 0.01–0.08) refined by Rietveld method

Lattice parameter	X = 0.01	X = 0.02	X = 0.03	X = 0.04	X = 0.05	X = 0.06	X = 0.07	X = 0.08
a (Å)	15.0762(2)	15.0842(2)	15.0941(1)	15.1000(3)	15.1095(4)	15.1172(1)	15.1260(3)	15.1337(4)
b (Å)	8.7203(2)	8.7207(1)	8.7211(1)	8.7214(2)	8.7213(1)	8.7223(2)	8.7225(1)	8.7231(2)
c (Å)	10.1290(3)	10.1305(1)	10.1317(1)	10.1324(2)	10.1333(3)	10.1354(2)	10.1360(2)	10.1377(3)
V (Å^3)	1331.61(3)	1332.67(2)	1333.69(1)	1334.35(3)	1335.57(4)	1336.42(2)	1337.31(3)	1338.30(4)
Rp (%)	5.43	5.68	5.88	5.84	5.97	6.12	6.22	6.21
Rwp (%)	7.57	7.67	7.80	7.76	7.95	8.22	8.37	8.40
Rexp (%)	3.70	3.70	3.73	3.73	3.73	3.73	3.74	3.74

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Fig. 2 (a) Rietveld refinement plot on the XRD pattern of Ba₂ZnB₂O₆:0.05Eu³⁺, small black circles and the red continuous lines represent the experimental and the calculated values respectively; vertical bars (|) indicate the position of Bragg peaks. The blue bottom trace depicts the corresponding residuals between the experimental and the calculated intensity values. (b) Crystal structure projection of Ba₂ZnB₂O₆ along c axis. (c) The coordination environments of two different Zn²⁺/Eu³⁺ ions.

3.2 Diffuse reflectance spectra

Because the powder of Ba₂ZnB₂O₆:Eu³⁺ is opaque, diffuse reflectance spectra (DRS) of the parent Ba₂ZnB₂O₆ and doped Ba₂ZnB₂O₆:0.05Eu³⁺ phosphor were measured instead of absorption spectra. As shown in Fig. 3, the diffuse reflectance spectrum of undoped sample shows obvious decrease from 240 nm to 370 nm near UV-region, which is due to the boron to oxygen charge transition (CT) in Ba₂ZnB₂O₆. When introducing the Eu³⁺ ions, the Ba₂ZnB₂O₆:0.05Eu³⁺ sample reveals stronger absorption than the undoped sample in this range. The stronger absorption is possibly caused by the energy transfer from borate groups to Eu³⁺ ions and CT transition of O²⁻–Eu³⁺ in Ba₂ZnB₂O₆:0.05Eu³⁺ phosphor. In careful observation, the absorption in the range of 390–470 nm has a slight decrease compared with the undoped sample (upper inset in Fig. 3), which is ascribed to the typical 4f–4f transition absorption of Eu³⁺. The characteristic absorption bands of Eu³⁺ are not observed besides of the weak band centered at 466 nm (⁷F₀–⁵D₂ transition).

Fig. 3 Diffuse reflectance spectra of the parent and doped Ba₂ZnB₂O₆:0.05Eu³⁺ phosphors. Inset in the upper is the enlarged drawing on the peak centered at 465 nm. Inset in the below shows the absorption spectra (K/S) of Ba₂ZnB₂O₆ and Ba₂ZnB₂O₆:0.05Eu³⁺ derived with the Kubelka–Munk function.

The absorption spectra of undoped and Ba₂ZnB₂O₆:0.05Eu³⁺ samples are also calculated by the Kubelka–Munk function:²³

F(R) = (1 − R)²/2R = K/S (1)

where R, K and S represent the reflection, absorption, and scattering coefficient, respectively. The absorption spectra are displayed in the below inset of Fig. 3. The Ba₂ZnB₂O₆:0.05Eu³⁺ sample shows strong absorption from 240 nm to 370 nm, which is consistent with the diffuse reflectance spectra.

3.3 PLE and PL spectra

The PLE and PL spectra of Ba₂ZnB₂O₆:0.05Eu³⁺ are shown in Fig. 4(a). As can be seen in this figure, the PLE spectrum (recorded at 616 nm emission, black line) contains a broad band and a series of narrow bands locating in the range of ∼240–480 nm, which is associated with the absorption spectrum (Fig. 3). The broad band at 240–320 nm originates from the charge transfer (CT) transition of O²⁻–Eu³⁺.²⁴ Those narrow bands belong to the typical 4f–4f transition absorption bands of Eu³⁺, in which the two intense bands centered at 393 nm and 466 nm are attributed to ⁷F₀–⁵L₆ and ⁷F₀–⁵D₂ transition. This demonstrates that the phosphor can be excited efficiently by near-ultraviolet and blue light. The other excitation peaks centered at 319 nm, 362 nm, 376 nm, 383 nm, 398 nm, and 415 nm correspond to the transitions from ⁷F₀ to ⁵H₆, ⁵D₄, ⁵G₃, ⁵L₇, ⁵L₆, and ⁵D₃ levels, respectively.

Fig. 4 (a) PLE/PL spectra of the Ba₂ZnB₂O₆:0.05Eu³⁺ sample. (b) The energy level diagram for Eu³⁺ in Ba₂ZnB₂O₆.

The PL spectrum (excited at 393 nm, red line) in Fig. 4(a) shows a series of sharp bands from 579 nm to 621 nm corresponding to the ⁵D₀–⁷F_J (J = 0, 1, 2) transitions of Eu³⁺. According to the magnetic dipole (MD) transition selection rule (ΔJ = 0, ±1), the emission band belonging to ⁵D₀–⁷F₁ transition (ΔJ = 1) is attributed to the magnetic dipole transition and the emission band belonging to ⁵D₀–⁷F₂ is attributed to the electric dipole (ED) transition on the basis of the selection rule of electric dipole transition (ΔJ ≤ 6, when J or J′ = 0, ΔJ = 2, 4, 6).¹⁵ The ⁵D₀–⁷F₀ transition is forbidden and sensitive to the crystal field.²⁵ The weak emission peak centered at 579 nm belongs to the ⁵D₀–⁷F₀ transition that is induced by role of crystal field in odd. The ⁵D₀–⁷F₀ transition exists only when Eu³⁺ occupies sites with local symmetries of C_n, C_nv or C_s. So we can deduce that Eu³⁺ occupies one site of C_n, C_nv or C_s symmetries.²⁶ The emission bands of Eu³⁺ in Fig. 4(a) are broadened and split into several lines, indicating that the ⁷F_J levels appear Stark levels because of crystal field effect.^27,28 The ⁵D₀–⁷F₁ transition splits into two emission peaks centered at 590 nm and 594 nm, the ⁵D₀–⁷F₂ transition splits into three emission peaks centered at 610 nm, 616 nm and 621 nm, and the ⁵D₀–⁷F₃ transition splits into three emission peaks centered at 649 nm, 656 nm and 666 nm, respectively. Fig. 4(b) illustrates the partial energy diagram of Eu³⁺ ions in Ba₂ZnB₂O₆ host. For the excited state of Eu³⁺, a fast non-radiation (NR) transition happened to the ⁵D₀ level, resulting the ⁵D_3,2,1–⁷F_J transitions are restrained, so the emission of Eu³⁺ ions is thought to come from the ⁵D₀–⁷F_J transitions.²⁹

3.4 Concentration quenching

As seen in Fig. 5(a), the quenching concentration of Ba₂ZnB₂O₆:xEu³⁺ phosphors is 5 mol%. The emission intensity increases with increasing Eu³⁺ dopant content when x < 0.05, while it decreases after that doping concentration. Based on the Dexter and Schulman theory,³⁰ concentration quenching is due to energy transfer from one activator to another until an energy sink is reached in the lattice. The concentration quenching in solid systems is because of the role of electric multipole or magnetic dipole interaction. On the basis of energy transfer formula:

I/x = k[1 + β(x)^θ/3]⁻¹ (2)

where x is the mole fraction of activator ions; k and β are constants; I is the luminous intensity, and θ = 6, 8, 10 corresponding to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. As illustrated in the inset of Fig. 5(b), the relationship of lg(I/x) versus lg(x) is linear and the slope of the line is about −2. So the value of θ is approximately equal to 6, which clearly indicates that the concentration quenching mechanism of Eu³⁺ in Ba₂ZnB₂O₆:0.05Eu³⁺ is the dipole–dipole interaction.

Fig. 5 (a) PL spectrum of the Ba₂ZnB₂O₆:xEu³⁺ sample. (b) The emission intensity as a function of Eu³⁺ concentration in Ba₂ZnB₂O₆:xEu³⁺. The inset is the dependence of lg(I/x) on lg(x).

Blasse³¹ suggested that the critical distance (R_c) of energy transfer can be expressed by eqn (3):

R_c ≈ 2(3V/4πx_cZ)^1/3 (3)

where x_c is the critical concentration of dopants, V is the volume of the unit cell, Z is the number of formula units per unit cell. As for Ba₂ZnB₂O₆ host, V = 1336.010(2) ų, Z = 8, and x_c = 0.05, the calculated value of R_c is about 18.55 Å.

3.5 Temperature-dependent PL properties

Temperature-dependent PL spectra of Ba₂ZnB₂O₆:0.05Eu³⁺ are measured and displayed in Fig. 6(a). Because of the resolution of the fluorescence spectrophotometer used for temperature-dependent PL measurement, the splits of the emission peak ⁵D₀–⁷F₁ and the ⁵D₀–⁷F₂ cannot be distinguished clearly. With the temperature increasing, the emission intensities decrease fast, and the intensity at 150 °C only remains 30% of that at room temperature. The fast decrease of emission intensity with the increasing of temperature is possibly because of the non-radiative process. The non-radiative transition probability is strongly dependent on temperature. With the temperature increase, the non-radiative transition probability enhances greatly, which will result in the decrease of the emission intensity. It can also be found that the main emission peak wavelength shifts from 616 nm to 623 nm. To better understand the temperature dependence of photoluminescence and to determine the activation energy for thermal quenching, the Arrhenius equation is fitted to the thermal quenching data:³²

I_T = I₀/[1 + exp(−E_a/kT)] (4)

where I₀ and I_T are the luminescence intensities of Ba₂ZnB₂O₆:0.05Eu³⁺ at room temperature and the testing temperature, respectively. E_a is activation energy and k is the Boltzmann constant (8.617 × 10⁻⁵ eV K⁻¹). Fig. 6(b) plots ln(I₀/I_T − 1) vs. 1/kT and the slope of the line is calculated to be −0.27. Therefore, E_a is obtained to be 0.27 eV.

Fig. 6 (a) Temperature-dependent PL spectra of Ba₂ZnB₂O₆:0.05Eu³⁺ phosphor (λ_ex = 393 nm). (b) The ln(I₀/I_T − 1) vs. 1/kT activation energy graph for thermal quenching of Ba₂ZnB₂O₆:0.05Eu³⁺.

3.6 Charge compensation

In Ba₂ZnB₂O₆:Eu³⁺, the charge is not balanced because of the trivalent Eu³⁺ ions occupy the sites of the bivalent Zn²⁺ ions, which will introduce Zn vacancies or extra O²⁻ ions at nearby interstitial position. Each of these defects leads to the distortion of local environment symmetries of optical centers.³³ Therefore, alkali metal ions Li⁺, Na⁺ and K⁺ are co-doped with Eu³⁺ in the matrix to balance the charge and reduce the variety of the activator symmetry sites. The PL spectra of Ba₂ZnB₂O₆:0.05Eu³⁺ and Ba₂ZnB₂O₆:0.05Eu³⁺, 0.05M (M = Li⁺, Na⁺ and K⁺) under 393 nm excitation are shown in Fig. 7. No significant difference is observed between the emission peaks of Ba₂ZnB₂O₆:0.05Eu³⁺ and Ba₂ZnB₂O₆:0.05M, 0.05Eu³⁺ (M = Li⁺, Na⁺ and K⁺) except the enhanced emission intensity. As shown in Fig. 7, the emission intensity compensated by Li⁺ is strongest. The intensities of the emission peak centered at 616 nm for the phosphors doped by Li⁺ and Na⁺ as charge compensators are enhanced 1.61 and 1.41 times compared with the corresponding peak intensity for the phosphor without charge compensators, respectively. However, the emission intensity of the peak centered at 616 nm is almost unchangeable when the K⁺ ions are added as charge compensation, which is considered to be caused by the huge difference between the K⁺ and Zn²⁺ ionic radii. In Ba₂ZnB₂O₆:Eu³⁺, Eu³⁺ ions have been proved to occupy the two four-coordinated Zn²⁺ sites. The ionic radii of Li⁺, Na⁺ and K⁺ (CN = 4) reported by Shannon¹⁷ are 0.59 Å, 0.99 Å and 1.37 Å, respectively. Obviously, the ionic radius of Li⁺ is very close to the radius of Zn²⁺ (r = 0.60 Å, CN = 4), while K⁺ is too large, which will be difficult to occupy Zn²⁺ sites. Therefore Ba₂ZnB₂O₆:0.05Eu³⁺ singly doped Li⁺ as compensator shows the strongest emission intensity.

Fig. 7 PL spectra of Ba₂ZnB₂O₆:0.05Eu³⁺ and Ba₂ZnB₂O₆:0.05M, 0.05Eu³⁺ (M = Li⁺, Na⁺ and K⁺) (λ_ex = 393 nm).

3.7 Decay properties and CIE coordinate

The decay curves of Ba₂ZnB₂O₆:0.05Eu³⁺ and Ba₂ZnB₂O₆:0.05Li⁺, 0.05Eu³⁺ were measured and shown in Fig. 8(a) and (b). The corresponding luminescence decay times of Eu³⁺ emission at 616 nm can be best expressed by the double exponential equation:²⁵

I = I₀ + A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (5)

where I₀ and I represent the luminescence intensities when time is 0 and t, τ₁ and τ₂ are the fast and slow components of luminescent lifetime respectively, and A₁ and A₂ are the fitting parameters respectively. When there is no interaction between rare earth ions, the decay curve is usually a single exponential function. But both the decay curves of Ba₂ZnB₂O₆:0.05Eu³⁺ and Ba₂ZnB₂O₆:0.05Eu³⁺, 0.05Li⁺ exhibit obvious deviations from the single exponential decay. As displayed in Fig. 8(a) and (b), the decay curves of Ba₂ZnB₂O₆:0.05Eu³⁺ and Ba₂ZnB₂O₆:0.05Eu³⁺, 0.05Li⁺ are fitted well by the double exponential equation, which is one of the evidences that Eu³⁺ ions occupy the sites of both Zn (1) and Zn (2) and charge compensator does not change the site occupancy of Eu³⁺ in Ba₂ZnB₂O₆. The related fitting parameters are listed in Table 2.

Fig. 8 (a) Decay curve of Eu³⁺ fluorescence in Ba₂ZnB₂O₆:0.05Eu³⁺; (b) decay curve of Eu³⁺ fluorescence Ba₂ZnB₂O₆:0.05Li⁺, 0.05Eu³⁺ (excited at 393 nm, monitored at 616 nm).

Table 2 Constants (A) and decay times (τ) of Ba₂ZnB₂O₆:0.05Eu³⁺ and Ba₂ZnB₂O₆:0.05Li⁺, 0.05Eu³⁺

	A1	A2	τ1 (ms)	τ2 (ms)	τ (ms)
Ba2ZnB2O6:0.05Eu^3+	0.963	2.148	0.619	1.077	0.98
Ba2ZnB2O6:0.05Li^+, 0.05Eu^3+	1.506	2.130	1.565	1.360	1.45

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The effective decay lifetime is calculated by the following equation:

τ = (A₁τ²₁ + A₂τ²₂)/(A₁τ₁ + A₂τ₂) (6)

Based on eqn (6), the average lifetimes of Ba₂ZnB₂O₆:0.05Eu³⁺ and Ba₂ZnB₂O₆:0.05Li⁺ are 0.98 ms and 1.45 ms, respectively. The results indicate the decay times of Ba₂ZnB₂O₆:0.05Eu³⁺ and Ba₂ZnB₂O₆:0.05Li⁺, 0.05Eu³⁺ are in the order of milliseconds.

The CIE coordinates of Ba₂ZnB₂O₆:0.05Eu³⁺ phosphors are calculated to be x = 0.655, y = 0.345, as shown in Fig. 9. The CIE coordinate of Ba₂ZnB₂O₆:0.05Eu³⁺ is very close to the CIE coordinate of standard red light (x = 0.67, y = 0.33), and is considered to be better than that of the commercial Y₂O₂S:Eu³⁺ (x = 0.622, y = 0.351).³⁴

Fig. 9 CIE chromaticity diagram of Ba₂ZnB₂O₆:0.05Eu³⁺ phosphor under 393 nm excitation.

Table 3 summarizes the photoluminescence properties of some Ba-based and Zn-based hosts doped by Eu³⁺ as well as the data reported in this study. For those phosphors listed in Table 3, there is no big difference for their photoluminescence properties. The reported critical distance is around 15 nm for all the phosphors and the decay time is the magnitude of millisecond. The quenching mechanism are all dipole–dipole interaction for the phosphors listed in Table 3, which indicate that the different site occupancy for Eu³⁺ will not bring a big effect on the quenching mechanism of phosphors. However, emission color is different for these two different group phosphors. Compared with the red color emitted by Zn-based phosphor, the Ba-based phosphors emit the light from orange to red color (⁵D₀–⁷F₁ and ⁵D₀–⁷F₂ transitions), which indicate that crystal environment will bring effect on the emission wavelength. When the large Eu³⁺ ion occupy Zn site, the volume of the crystal structure will be expanded and the polyhedron will be distorted, which will result in the increase the crystal field strength. As a consequence, a red shift will be observed in Zn-based phosphors.

Table 3 Luminescence properties including the position of emission peak, optimal concentration, critical distance and quenching mechanism for Eu³⁺ in some Ba-based and Zn-based hosts as well as that in Ba₂ZnB₂O₆

Host	Strongest excited peak (nm)	Main transition	Optimal concentration	Critical distance (Å)	Quenching mechanism	Life time (ms)	Ref.
Ba5−2x(VO4)3Cl	466	^5D0–^7F2 (614 nm)	0.22	—	Dipole–dipole	0.56	13
BaB2O4	394	^5D0–^7F2 (615 nm)	0.06	14.56	Dipole–dipole	—	35
KBaBP2O8	394	^5D0–^7F1 (594 nm)	0.1	15.2	Dipole–dipole	2.54	36
ZnB2O4	393	^5D0–^7F2 (621 nm)	0.1	—	—	—	15
Ba2ZnB2O6	393	^5D0–^7F2 (616 nm)	0.05	18.55	Dipole–dipole	1.45	This work

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4. Conclusion

In summary, the Ba₂ZnB₂O₆:Eu³⁺ phosphors were synthesized and investigated. The site-preferred occupancy of Eu³⁺ in Ba₂ZnB₂O₆ is studied by the Rietveld refinement. The doped Eu³⁺ ions prefer to occupy two Zn²⁺ sites. These phosphors can be excited efficiently by near-ultraviolet and blue light. The concentration quenching of the Ba₂ZnB₂O₆:xEu³⁺ phosphors in the PL emission spectra optimized at 616 nm is x = 0.05. The PL emission intensities of Ba₂ZnB₂O₆:0.05Eu³⁺ phosphors compensated by Li⁺, Na⁺ and K⁺ as charge compensators are enhanced, and the phosphors compensated by Li⁺ ions show the strongest emission. The critical distance (R_c) and the lifetime value are determined to be 18.55 Å and 0.98 ms, respectively. The CIE coordinate is calculated to be x = 0.655, y = 0.345.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (51372121, 61274053, U146020005), Natural Science Foundation of Tianjin (14JCYBJC17800), the Program for New Century Excellent Talents in University of China (NCET-11-0258), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0149). We thank Mrs Xu of N01 group, Institute of Physics, Chinese Academy of Science for her great help in collecting the powder X-ray diffraction data and Prof. Dr Z. Xia for the great help of in the temperature-dependent PL spectra measurement.

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Footnote

† Electronic supplementary information (ESI) available: Rietveld refinement results for Ba₂ZnB₂O₆:0.05Eu³⁺ phosphor. See DOI: 10.1039/c4ra11516c

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