Synthesis, luminescence and energy transfer of Ca₃GdNa(PO₄)₃F: Ce³⁺, Tb³⁺ phosphor with novel apatite structure

Apatite compounds are regarded as important phosphor matrix materials due to their excellent thermal and chemical stability. Among them, Ca₅(PO₄)₃F is the simplest apatite structure and the first apatite compound proved to have P6₃/m space group. On the basis of Ca₃(PO₄)₃F, apatite fluorescent materials with very rich compositions were obtained by changing the types of cations, anion groups and channel atoms in the crystal [1]. In apatite compound crystals, there are usually two cationic crystal lattices, which are locally symmetric at C₃ point (4f lattices) and locally symmetric at Cs point (6h lattices). These two crystal lattice sites can be replaced by rare earth elements, because the 4f and 6h lattice sites have different requirements on the radius and valence state of substituted ions, which often make apatite phosphors with special spectral characteristics [2].

Green phosphors are useful to obtain white light source with high color rendering in the field of illumination [3]. It is also advantageous to obtain the display effect of high color purity in the display field. In recent years, many green phosphors based on apatite crystals have been reported. For example, the green phosphor Ca₆La₄(SiO₄)₂(PO₄)₄O₂:0.01Eu²⁺ prepared by Xia [4], its emission peak of 500 nm was emitted from the 4f and 6h positions occupied by Eu²⁺, and the quantum efficiency was 57.73%. Besides, Tian [5] prepared fluorescent powder Ca₉Sr(PO₄)₆Cl₂:Ce³⁺,Tb³⁺ by high temperature solid phase method. Based on the effective energy transfer between Ce³⁺ and Tb³⁺, the green emission peak of Tb³⁺ at 541nm was significantly increased, and the energy transfer efficiency of Ce³⁺ → Tb³⁺ was up to 76%. However, no relevant reports have been reported on the study of green phosphor with apatite structure Ca₃GdNa(PO₄)₃F as matrix. Based on the efficient energy transfer between Ce³⁺ and Tb³⁺, it is expected that new green fluorescent materials with apatite structure can be obtained by mixing Ce³⁺ and Tb³⁺ in the Ca₃GdNa(PO₄)₃F matrix. As the emission wavelength of semiconductor chips is gradually extended to short-wave UV and deep UV [6], this kind of fluorescent powder has the potential to be used in the solid-state lighting field in the future.

A series of phosphors Ca₃GdNa(PO₄)₃F: xCe³⁺,yTb³⁺(0 ≤ x ≤ 0.14, 0 ≤ y ≤ 0.28) were prepared by a high temperature solid-state reaction. Then the energy transfer process and mechanism of Ce³⁺, Tb³⁺ co-doped phosphor Ca₃Gd_0.92-yNa(PO₄)₃F:0.08Ce³⁺, yTb³⁺(0 ≤ y ≤ 0.28) were discussed, and the emission spectrum CIE chromaticity diagram was drawn.

2.1. Materials and synthesis

2.2. Characterization

As shown in figure 1(a), the peaks of all samples conform to the standard card of Ca₅(PO₄)₃F, and no additional peaks of other impurities are found in the diffraction patterns. The phenomenon indicates high purity of the samples and the doped ions will not change the matrix crystal phase. Figure 1(b) shows a partial amplification of XRD of the sample. With the increasing of the doping concentration of Ce³⁺, the diffraction peak shifts to the lower 2θ because the larger radius of Ce³⁺ is replaced by the smaller radius of Gd³⁺, when Tb³⁺ is not mixed. The offset profile of diffraction peak demonstrates that Ce³⁺ and Tb³⁺ have been successfully introduced into the matrix material Ca₃GdNa(PO₄)₃F [7].

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Ca1 forms a tetrahedral structure with 9O coordination points [8] in Ca₅(PO₄)₃F crystal (figure 2), which is a locally symmetric 4f lattice position at C₃ point. Ca2 coordinates with 6O and 1F to form a decahedral structure, which is a locally symmetric 6h grid position at Cs points. The O and P in the crystal coordinate to form the structure of (PO₄)³⁻ tetrahedron, which is connected with the common edges and angles of Ca1and Ca2 coordination polyhedra respectively.

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We can achieve the radius of each cation in different coordination environments [9] from table 1. Considering the difference of valence state and radius of each cation, the substitution of two Ca²⁺ by Gd³⁺ and Na⁺ had occurred [10]. Matrix crystal Ca₃GdNa(PO₄)₃F was obtained after being replaced by Gd³⁺ and Na⁺. In this matrix, Ce³⁺ and Tb³⁺ activators were introduced. Ce³⁺ and Tb³⁺ replaced Gd³⁺ in the matrix to form replacement solid solution [11] due to the need to keep valence balance. Figure 3. shows Ca₃Gd_0.72Na(PO₄)₃F:0.08Ce³⁺,0.20Tb³⁺ samples have irregular micromorphology with relatively smooth surface and particle size ranging from 6 ∼ 14μm.

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The emission spectra of the as-prepared samples Ca₃Gd_1-xNa(PO₄)₃F:xCe³⁺(0 ≤ x ≤ 0.14) as shown in figures 4(a), (b). Upon the excitation of 301nm, its emission spectral intensity enhances with the increasing of the doping concentration of Ce³⁺ [12]. The spectral intensity reaches the highest, when the doping of Ce³⁺ is 0.08. The doping amount of Ce³⁺ continues to increase, the concentration quenching phenomenon begins to appear, and the spectral intensity decreases accordingly. Figure 5(a) shows that when the single content of Ce³⁺ is 0.08, the phosphor has a broadband excitation spectrum from the 4f → 5d transition of Ce³⁺ at 250 nm–330 nm, and the optimal emission wavelength is 301nm, at which time the broadband emission presents an asymmetric peak [13]. Gaussian fitting was performed on the emission spectrum, and two peaks appeared at 334.47nm and 358.73nm. After Ce³⁺ is excited, a 5d → 4f transition emission occurs, and 4f splits into ²F_5/2 and ²F_7/2. Therefore, the emission spectrum comes from Ce³⁺ occupying the same crystal lattice position. High valence and low ion radius are the most likely to replace likely to replaces the lattice site of 6h crystals with shorter coordination bonds and smaller volume. According to the coordination ion radius in table 1, it can be seen that Ce³⁺ replaces Gd³⁺ and occupies the lattice site of 6h [14], and then emits light. Figure 5(b) shows that when the single dosage of Tb³⁺ is 0.20, the characteristic emission of Tb³⁺ at 542nm is monitored.

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In figure 5(c), the emission peak of 542 nm green light was monitored, and the excitation intensity of Ce³⁺ and Tb³⁺ co-doped phosphors at 301 nm was much higher than that of single doped Tb³⁺ phosphors at 301 nm. Then 301 nm ultraviolet light is the best excitation wavelength of Ce³⁺, and the spectra of figures 5(a) and (b) have a significant overlap, so it can be speculated that there is an energy transfer of Ce³⁺ → Tb³⁺ in phosphor Ca₃Gd_0.72Na(PO₄)₃F:0.08Ce³⁺,0.20Tb³⁺.

As the Tb³⁺ doping increases, the 355nm broadband emission peak intensity decreases (figure 6(a)). The narrow-band emission peaks in the emission spectrum come from the electron transition of Tb³⁺ [15], including the spectral peaks of 376 nm, 413 nm, 435 nm and 455 nm radiated by the electron transition of the high-excited energy level ⁵D₃ → ⁷F_J(J = 6, 5, 4, 3), and the spectral peaks of the low-excited energy level ⁵D₄ → ⁷F_J(J = 6, 5, 4, 3) radiated by the electron transition of 488nm, 542nm, 582nm and 620nm. Energy transfer efficiency (η_T) can be expressed by [16]:

$$\begin{eqnarray}&&{\eta }_{T}=1-{I}_{S}/{I}_{S0}\end{eqnarray} \tag{ 1 }$$

Among them, I_S and I_S0 represent the luminescence intensity of sensitizer Ce³⁺ when doped with Tb³⁺ and without Tb³⁺, respectively. The energy transfer efficiency of Ce³⁺ → Tb³⁺ can reach 78.6%, when the doping concentration of Tb³⁺ reaches 0.20.

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Dexter and Schulman suggest that the distance R_c between Ce³⁺ ion and Tb³⁺ can be estimated using the equation [17]:

$$\begin{eqnarray}&&{R}_{Ce-Tb}=2{[3V/(4\pi {x}_{c}N)]}^{1/3}\end{eqnarray} \tag{ 2 }$$

Where V is the volume of the unit cell volume, x_c is the critical concentration, which refers to the total concentration of Ce³⁺ and Tb³⁺ when the energy transfer efficiency between Ce³⁺ and Tb³⁺ reaches 50% (from figure 6). N is the number of sites that the Ce³⁺ ions can occupy in the unit cell (N = Z × 2) and of doped ions. In case of Ca₃GdNa(PO₄)₃F:xCe³⁺, yTb³⁺ phosphor, V = 0.52371 nm³, N = 2, and x_c = 0.18. Upon inserting the value in equation (2), R_c value is determined to be 1.41nm > 0.5nm, therefore the energy transfer mainly occurs through multiple dipole interactions from Ce³⁺ to Tb³⁺.

According to Dexter's multi-dipole interaction energy transfer formula and Reisfeld's approximation theory, the energy-transfer mechanism of multi-dipole interaction which can be givens as [18]:

$$\begin{eqnarray}&&{I}_{S0}/{I}_{S}\propto {{C}_{Ce+Tb}}^{n/3}\end{eqnarray} \tag{ 3 }$$

where I_S0 and I_S have the same meaning as equation (1). Different n values mean different mechanisms of multiple dipole interactions. The dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions correspond to curves (I_S0/I_S) and C^n/3 with n = 6, 8, and 10. The linear relationship reached the optimal one for (I_S0/I_S) versus C^6/3 by comparing the fitting factors of R² values in figure 7, implying that the energy transfer mechanism from the Ce³⁺ to Tb³⁺ ions is a dipole-dipole mechanism in Ca₃GdNa(PO₄)₃F: Ce³⁺, Tb³⁺ phosphor.

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The measured CIE chromaticity coordinates of the Ca₃Gd_0.92-yNa(PO₄)₃F:0.08Ce³⁺, yTb³⁺(0 ≤ y ≤ 0.28) were presented in the inset of figure 8. Based on energy transfer from Ce³⁺ to Tb³⁺, the emitting color can be varied gradually from blue to green region with the increasing doping amount of Tb³⁺. The purity of green light color continuously increases when the Tb³⁺ doping amount is 0.2.

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The relative brightness stability and color coordinate stability of the phosphors were determined according to GB/T 23595.4-2009 national standard for Ca3Gd0.72Na(PO4)3F: 0.08Ce3+, 0.20Tb3 + prepared under optimal conditions (1200 °C, 1 h). Calculated according to formula (4)–(6):

$$\begin{eqnarray}&&{B}_{h}={B}_{h}-{B}_{0}/{B}_{0}\times 100\end{eqnarray} \tag{ 4 }$$

Where ∆B_his relative brightness thermal stability(%), B₀ and B_h represent the relative brightness(%) of the sample after heat treatment and heat treatment.

$$\begin{eqnarray}&&{\rm{\Delta }}{X}_{h}={X}_{h}-{X}_{0}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{\rm{\Delta }}{y}_{h}={y}_{h}-{y}_{0}\end{eqnarray} \tag{ 6 }$$

Among them, ∆X_h and ∆y_hare the stability of color coordinates, X₀ and y₀ are the unheated color coordinates, and X_h and y_h are the heat processed color coordinate. After testing and calculation, ∆B_h value is determined to be 2.56% < 5%, and (∆X_h, ∆y_h) = (0.0003, 0.0006), which is also far less than national standard 0.0015. These data indicate that Ca3Gd0.72Na(PO4)3F:0.08Ce³⁺, 0.20Tb³⁺ phosphors have good thermal properties and can be used in solid-state lighting.

To summarize, Ce³⁺ and Tb³⁺ singly doped and co-doped Ca₃GdNa(PO₄)₃F phosphors were successfully prepared and investigated. In Ca₃GdNa(PO₄)₃F: Ce³⁺, the Ce³⁺ tent to replace Gd³⁺ on 6h, and the broad emission band centered at 355nm was observed with optimal Ce³⁺ concentration being 0.08. We observed the energy transfer in the Ca₃Gd_0.92-yNa(PO₄)₃F:0.08Ce³⁺, yTb³⁺. With the increasing of Tb³⁺ doping, the emission spectrum intensity of Ce³⁺ at 355nm decreased monotonously, while that of Tb3 + at 542nm increased monotonously. When y = 0.20, the energy transfer efficiency of Ce³⁺ → Tb³⁺ could reach 78.6%. Meanwhile, the energy transfer was about 1.41nm, and the energy transfer from the Ce³⁺ to Tb³⁺ ions was dipole-dipole interaction. The phosphor emitted bright green light and had good thermal stability. All the above results demonstrated that Ca₃GdNa(PO₄)₃F:Ce³⁺, Tb³⁺ can be a platform for modeling a new phosphor and application in the solid-state lighting field.

The authors would like to acknowledge the financial support in part from the National Key R&D Program of China (Grant Nos. 2017YFB0404300, 2017YFB0404301) and the Natural Science Foundation of Jiangsu Province (Grant No. BK20171128).