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

A series of Ce3+/Tb3+ singly doped and Ce3+, Tb3+ co-doped Ca3GdNa(PO4)3F phosphors were synthesized via high temperature solid-state reaction in a reductive atmosphere. The crystal, luminescence properties, energy transfer mechanism and thermal stability of the samples were investigated in detail. The Ce3+ doped phosphors can be excited in the range from 250 to 330 nm. The Ce3+,Tb3+ co-doped phosphors exhibit characteristic emission of Ce3+ and Tb3+ simultaneously under the excitation of 301 nm. In addition, the energy transfer efficiency from Ce3+ to Tb3+ reaches as high as 78.6% when the doping amount of Tb3+ is 0.20. The mechanism of energy transfer between Ce3+ and Tb3+ ions is demonstrated to be a dipole−dipole interaction. The luminescence characteristics show that this phosphor can be a platform for modeling a new phosphor and application in the solid-state lighting field.


Introduction
Apatite compounds are regarded as important phosphor matrix materials due to their excellent thermal and chemical stability. Among them, Ca 5 (PO 4 ) 3 F is the simplest apatite structure and the first apatite compound proved to have P6 3 /m space group. On the basis of Ca 3 (PO 4 ) 3 F, 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 C 3 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 Ca 6 La 4 (SiO 4 ) 2 (PO 4 ) 4 O 2 :0.01Eu 2+ prepared by Xia [4], its emission peak of 500 nm was emitted from the 4f and 6h positions occupied by Eu 2+ , and the quantum efficiency was 57.73%. Besides, Tian [5] prepared fluorescent powder Ca 9 Sr(PO 4 ) 6 Cl 2 :Ce 3+ ,Tb 3+ by high temperature solid phase method. Based on the effective energy transfer between Ce 3+ and Tb 3+ , the green emission peak of Tb 3+ at 541nm was significantly increased, and the energy transfer efficiency of Ce 3+ →Tb 3+ was up to 76%. However, no relevant reports have been reported on the study of green phosphor with apatite structure Ca 3 GdNa(PO 4 ) 3 F as matrix. Based on the efficient energy transfer between Ce 3+ and Tb 3+ , it is expected that new green fluorescent materials with apatite structure can be obtained by mixing Ce 3+ and Tb 3+ in the Ca 3 GdNa(PO 4 ) 3 F 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. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Materials and synthesis
Powder samples of Ca 3 Gd 1-x-y Na(PO 4 ) 3 F:xCe 3+ ,yTb 3+ phosphors were prepared by a high temperature solidstate reaction. CaCO 3 (AR), Na 2 CO 3 (AR), (NH 4 ) 2 HPO 4 (AR), NH 4 F(AR) , CeO 2 (99.99%) and Tb 4 O 7 (99.99%) were used as the raw materials and weighed in a proper stoichiometric ratio with a 10% excess of fluorine for the loss at high temperature. The raw materials were fully mixed and ground in an agate mortar, and the mixture was placed into an alumina crucible and was heated at 1200°C for 1h in a reducing atmosphere with flowing gas.

Characterization
The phase purity of the phosphor was checked by powder x-ray diffraction (D/Max-rA 9kw, Japan) with Cu Kα radiation (λ=0.15406nm) from 15°∼80°(2θ). XRD data for the Rietveld refinement were performed using the computer software: general structure analysis system (GSAS). The particle morphology and microstructure of the samples were examined by scanning electron microscopy (SEM) using a JSM-7610F instrument with a voltage of 10kV. The photoluminescence spectra was recorded on a RF-6000 (Shimadzu corporation, Japan) luminescence spectrometer. The excitation light source was xenon arc lamp, and the spectral scanning step was 1nm.

Results and discussion
As shown in figure 1(a), the peaks of all samples conform to the standard card of Ca 5 (PO 4 ) 3 F, 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 Ce 3+ , the diffraction peak shifts to the lower 2θ because the larger radius of Ce 3+ is replaced by the smaller radius of Gd 3+ , when Tb 3+ is not mixed. The offset profile of diffraction peak demonstrates that Ce 3+ and Tb 3+ have been successfully introduced into the matrix material Ca 3 GdNa(PO 4 Ca1 forms a tetrahedral structure with 9O coordination points [8] in Ca 5 (PO 4 ) 3 F crystal (figure 2), which is a locally symmetric 4f lattice position at C 3 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 (PO 4 ) 3− tetrahedron, which is connected with the common edges and angles of Ca1and Ca2 coordination polyhedra respectively.
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 Ca 2+ by Gd 3+ and Na + had occurred [10]. Matrix crystal Ca 3 GdNa(PO 4 ) 3 F was obtained after being replaced by Gd 3+ and Na + . In this matrix, Ce 3+ and Tb 3+ activators were introduced. Ce 3+ and Tb 3+ replaced Gd 3+ in the matrix to form replacement solid solution [11] due to the need to keep valence balance. Figure 3. shows Ca 3 Gd 0.72 Na(PO 4 ) 3 F:0.08Ce 3+ ,0.20Tb 3+ samples have irregular micromorphology with relatively smooth surface and particle size ranging from 6∼14μm.
The emission spectra of the as-prepared samples Ca 3 Gd 1-x Na(PO 4 ) 3 F:xCe 3+ (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 Ce 3+ [12]. The spectral intensity reaches the highest, when the doping of Ce 3+ is 0.08. The doping amount of Ce 3+ 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 Ce 3+ is 0.08, the phosphor has a broadband excitation spectrum from the 4f→5d transition of Ce 3+ 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 Ce 3+ is excited, a 5d→4f transition emission occurs, and 4f splits into 2 F 5/2 and 2 F 7/2 . Therefore, the emission spectrum comes from Ce 3+ 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 Ce 3+ replaces Gd 3+ and occupies the lattice site of 6h [14], and then emits light. Figure 5(b) shows that when the single dosage of Tb 3+ is 0.20, the characteristic emission of Tb 3+ at 542nm is monitored.
In figure 5(c), the emission peak of 542 nm green light was monitored, and the excitation intensity of Ce 3+ and Tb 3+ co-doped phosphors at 301 nm was much higher than that of single doped Tb 3+ phosphors at 301 nm. Then 301 nm ultraviolet light is the best excitation wavelength of Ce 3+ , and the spectra of figures 5(a) and (b)  As the Tb 3+ 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 Tb 3+ [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 5 D 3 → 7 F J (J=6, 5, 4, 3), and the spectral peaks of the low-excited energy level 5 D 4 → 7 F 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]: Among them, I S and I S0 represent the luminescence intensity of sensitizer Ce 3+ when doped with Tb 3+ and without Tb 3+ , respectively. The energy transfer efficiency of Ce 3+ →Tb 3+ can reach 78.6%, when the doping concentration of Tb 3+ reaches 0.20.
Dexter and Schulman suggest that the distance R c between Ce 3+ ion and Tb 3+ can be estimated using the equation [17]: Where V is the volume of the unit cell volume, x c is the critical concentration, which refers to the total concentration of Ce 3+ and Tb 3+ when the energy transfer efficiency between Ce 3+ and Tb 3+ reaches 50% (from figure 6). N is the number of sites that the Ce 3+ ions can occupy in the unit cell (N=Z×2) and of doped ions.  In case of Ca 3 GdNa(PO 4 ) 3 F:xCe 3+ , yTb 3+ phosphor, V=0.52371 nm 3 , 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 Ce 3+ to Tb 3+ .  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]:  figure 7, implying that the energy transfer mechanism from the Ce 3+ to Tb 3+ ions is a dipole-dipole mechanism in Ca 3 GdNa(PO 4 ) 3 F: Ce 3+ , Tb 3+ phosphor.
The measured CIE chromaticity coordinates of the Ca 3 Gd 0.92-y Na(PO 4 ) 3 F:0.08Ce 3+ , yTb 3+ (0y0.28) were presented in the inset of figure 8. Based on energy transfer from Ce 3+ to Tb 3+ , the emitting color can be varied gradually from blue to green region with the increasing doping amount of Tb 3+ . The purity of green light color continuously increases when the Tb 3+ doping amount is 0.2.
Among them, ΔX h and Δy h are the stability of color coordinates, X 0 and y 0 are the unheated color coordinates, and X h and y h are the heat processed color coordinate. After testing and calculation, ΔB h value is determined to be 2.56%<5%, and (ΔX h, Δy h )=(0.0003, 0.0006), which is also far less than national standard 0.0015. These data indicate that Ca3Gd0.72Na(PO4)3F:0.08Ce 3+ , 0.20Tb 3+ phosphors have good thermal properties and can be used in solid-state lighting.

Conclusions
To summarize, Ce 3+ and Tb 3+ singly doped and co-doped Ca 3 GdNa(PO 4 ) 3 F phosphors were successfully prepared and investigated. In Ca 3 GdNa(PO 4 ) 3 F: Ce 3+ , the Ce 3+ tent to replace Gd 3+ on 6h, and the broad emission band centered at 355nm was observed with optimal Ce 3+ concentration being 0.08. We observed the energy transfer in the Ca 3 Gd 0.92-y Na(PO 4 ) 3 F:0.08Ce 3+ , yTb 3+ . With the increasing of Tb 3+ doping, the emission spectrum intensity of Ce 3+ at 355nm decreased monotonously, while that of Tb3+at 542nm increased monotonously. When y=0.20, the energy transfer efficiency of Ce 3+ →Tb 3+ could reach 78.6%. Meanwhile, the energy transfer was about 1.41nm, and the energy transfer from the Ce 3+ to Tb 3+ ions was dipole-dipole interaction. The phosphor emitted bright green light and had good thermal stability. All the above results demonstrated that Ca 3 GdNa(PO 4 ) 3 F:Ce 3+ , Tb 3+ can be a platform for modeling a new phosphor and application in the solid-state lighting field.