New insight into the crystal structure of Sr4Ca(PO4)2SiO4 and the photoluminescence tuning of Sr4Ca(PO4)2SiO4:Ce3+,Na+,Eu2+ phosphors

A new single phase based on the substitution of a Sr cation by a Ca cation in the apatite-type Sr5(PO4)2(SiO4) has been fabricated with the nominal chemical composition of Sr4Ca(PO4)2(SiO4), which appears as a definite compound rather than a solid solution between (Sr,Ca)3(PO4)2 and (Sr,Ca)2SiO4. The crystal structure of Sr4Ca(PO4)2(SiO4) has been firstly analysed by the difference electron map, and further resolved by the Rietveld refinement, and the final composition has been determined as Sr4Ca(PO4)(2+x)(SiO4)(1−x)(OH)x (x = 0.64) with a hexagonal cell (P63/m). The Ce3+/Eu2+ codoped Sr4Ca(PO4)2SiO4 phosphors have been designed and prepared by the solid state method, and the photoluminescence tuning from blue to green upon 365 nm ultraviolet (UV) radiation can be realized, which is ascribed to the energy transfer from Ce3+ to Eu2+ ions. The luminescence properties and the energy transfer mechanism in Ce3+/Eu2+ codoped Sr4Ca(PO4)2SiO4 phosphors have been discussed, which might act as potential candidates for blue-green components in UV-pumped white light emitting diodes (WLEDs).

In the present study, a new single phase based on the substitution of a Sr cation by a Ca cation in Sr 5 (PO 4 ) 2 (SiO 4 ) has been prepared with the nominal chemical composition of Sr 4 Ca(PO 4 ) 2 (SiO 4 ). On the basis of the difference electron map and Rietveld refinement from the XRD pattern, the final composition of Sr 4 Ca(PO 4 ) 2 (SiO 4 ) has been determined as Sr 4 Ca(PO 4 ) (2+x) (SiO 4 ) (1Àx) (OH) x (x = 0.64) with a hexagonal cell (P6 3 /m), and the crystal structure has been carefully described in this paper. Moreover, Ce 3+ /Eu 2+ codoped Sr 4 Ca(PO 4 ) 2 SiO 4 phosphors have been prepared by the solid state method, and the photoluminescence tuning from blue to green has been realized based on the energy transfer of Ce 3+ to Eu 2+ ions, which might act as potential candidates for blue-green components in UV-pumped WLEDs.

Characterization
The phase structures of the as-synthesized samples were checked using a D8 Advance diffractometer (Bruker Corporation, Germany) operating at 40 kV and 40 mA under Cu Ka radiation (l = 1.5406 Å). The photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra were recorded using a fluorescence spectrophotometer (F-4600, HITACHI, Japan) equipped with a photomultiplier tube operating at 400 V, and a 150 W Xenon lamp as the excitation source. The temperature dependent photoluminescence spectra have been measured by the same spectrophotometer, and it was combined with a self-made heating attachment and a computer-controlled electric furnace (Tianjin Orient KOJI Co., Ltd, TAP-02). The luminescence decay curves were obtained using a FLSP9200 fluorescence spectrophotometer (Edinburgh Instruments Ltd, UK), and an nF900 flash lamp was used as the excitation source.

Phase formation and crystal structure
The diffraction data of the as-prepared nominal Sr 4 Ca(PO 4 ) 2 (SiO 4 ) compound for Rietveld analysis were collected at room temperature with a step size of 2y being 0.013131, and the counting time was 5 s per step. Rietveld refinement was performed by using TOPAS 4.2. Almost all peaks were indexed to the hexagonal cell (P6 3 /m) with parameters close to Sr 5 (PO 4 ) 3 (OH) (strontium-apatite structure). 21 Therefore, the crystal structure of Sr 5 (PO 4 ) 3 (OH) was taken as a starting model for Rietveld refinement. Sites of Sr ions were equally occupied by Sr and Ca ions, the site of P was also occupied by P and Si randomly. All these occupancies were refined in an assumption that the sum of occupancies in each site is equal to 1. The OH group was deleted at the first stage of refinement due to the suggested formula Sr 4 Ca(PO 4 ) 2 SiO 4 . Refinement gave a low R-factor (R B = 1.99%). However, as given in Fig. 1, it demonstrated the difference electron map calculated using the F obs À F calc difference of observed and calculated structural amplitudes (herein, F obs are the observed structure-factor amplitudes, F calc are the calculated structure-factor amplitudes which were calculated from the atom coordinates of the model). The results showed maxima at (0, 0, x) (x B 0.2) and several symmetry equivalent maxima. It was known that these peaks corresponded to OH À ions because (0, 0, 0.2) is close to the position of the OH À ion (0, 0, 0.1856 (14)) in Sr 5 (PO 4 ) 3 (OH). Therefore it was suggested to return the OH group in the model. It is believed that the residual H 2 O molecules combined with starting materials and the H 2 O in the air can promote the phase formation of Sr 4 Ca(PO 4 ) (2+x) (SiO 4 ) (1Àx) (OH) x , which enabled the presence of the OH À group. According to this observation and the difference electron map analysis in Fig. 1, the chemical formula should be rewritten as Sr 4 Ca(PO 4 ) (2+x) (SiO 4 ) (1Àx) (OH) x in order to keep the sum of charges to be zero. On the basis of such a model, Fig. 2 gives the Rietveld analysis patterns for X-ray powder diffraction data of the nominal Sr 4 Ca(PO 4 ) 2 (SiO 4 ) compound, and the value of x was determined; the final refinement was stable and gave low R-factors (Table 1 and Fig. 2), and the R B factor dropped to 1.93%. The inset in Fig. 2 shows the representative crystal structure of Sr 4 Ca(PO 4 ) (2+x) (SiO 4 ) (1Àx) (OH) x , which crystallized in the hexagonal cell (space group P6 3 /m) with lattice constants a = b = 9.67202 (8), c = 7.25393 (7), V = 587.68 (1), Z = 2, as shown in Table 1, and the crystallographic information file (CIF) is presented in the ESI. † We have also checked the phase purity of the as-prepared Ce 3+ /Eu 2+ codoped Sr 4 Ca(PO 4 ) 2 SiO 4 phosphors. Fig. 3 shows the representative XRD patterns of Sr 3.8Àx Ca(PO 4 ) 2 SiO 4 :0.1Ce 3+ ,-0.1Na + ,xEu 2+ (x = 0, 0.03) samples. Firstly, the diffraction peaks agree well with the phase of Sr 4 Ca(PO 4 ) (2+x) (SiO 4 ) (1Àx) (OH) x , x = 0.64(3) refined by the Rietveld analysis, as shown in Fig. 2. Secondly, it is obvious that all the diffraction peaks of these samples can also be indexed to the pure hexagonal phase of Sr 5 (PO 4 ) 2 SiO 4 (JCPDS . No other phase is detected after doping, indicating that Ce 3+ or Eu 2+ ions were completely dissolved in the nominal Sr 4 Ca(PO 4 ) 2 SiO 4 host without leading to any significant changes in the crystal structure. However, as discussed above, Sr 4 Ca(PO 4 ) (2+x) (SiO 4 ) (1Àx) (OH) x and title phase Sr 4 Ca(PO 4 ) 2 SiO 4 are isostructural. Only one difference -the presence or absence of OH groups in the void of the structure can be found. The X-ray diffraction method is an effective approach to check the presence of OH groups using difference electron density maps mentioned above. It is clear that the addition of some amount of OH À ions to the structure enables charge imbalance. In order to make the charge of unit cell equal to zero the following mechanism was suggested: . Therefore the chemical formula of such compounds should be Sr 4 Ca(PO 4 ) (2+x) (SiO 4 ) (1Àx) (OH) x . Therefore, we still used the chemical formula of the nominal Sr 4 Ca(PO 4 ) 2 SiO 4 phase in the following section in order to discuss the photoluminescence properties of Ce 3+ /Eu 2+ codoped samples.

Luminescence properties of Ce 3+ /Na + singly doped Sr 4 Ca(PO 4 ) 2 SiO 4 phosphors
In order to investigate the luminescence properties of Ce 3+ activated Sr 4 Ca(PO 4 ) 2 SiO 4 phosphors, we firstly prepared a series of samples Sr 3.8À2x Ca(PO 4 ) 2 SiO 4 :xCe 3+ ,xNa + (x = 0.01, 0.03, 0.05, 0.07, 0.10 and 0.15). Fig. 4a depicts the PLE and PL spectra of the selected Sr 3.8 Ca(PO 4 ) 2 SiO 4 :0.1Ce 3+ ,0.1Na + phosphor. The PLE spectrum monitored at 428 nm exhibits a broad band from 250 to 400 nm, which is ascribed to the transitions from the ground state of the Ce 3+ ions to the field splitting levels of the 5d state. 22 The PL spectrum consists of an asymmetric broad band peaking at 428 nm under the excitation of 365 nm. As we know, such an asymmetric broad band should be ascribed to the characteristics of double band emission of Ce 3+ , which is due to the transition of Ce 3+ ions from the 5d excited state to the 2 F 7/2 and 2 F 5/2 ground states. Fig. 4b shows the PL spectra of Sr 3.8À2x Ca(PO 4 ) 2 SiO 4 :xCe 3+ ,xNa + depending on different Ce 3+ doping content x. All the PL spectra exhibited a similar broad band emission centered at 428 nm, which was also ascribed to the 5d 1 -4f 1 of Ce 3+ ions. The optimal Ce 3+ dopant content was found to be 0.1 mol per formula unit and the PL intensity was observed to increase with increasing x when x o 0.1. That is to say, with the Ce 3+ dopant content being higher than 0.1, concentration quenching was observed and the PL intensity was found to decrease with increasing Ce 3+ .
Accordingly, the inset of Fig. 5 presents the PL emission intensities as a function of Ce 3+ content for Sr 3.8À2x Ca(PO 4 ) 2 SiO 4 : xCe 3+ ,xNa + , and it is obvious that the optimal doping concentration is x = 0.1 and then decreased, resulting from the concentration quenching effect. It is known that the

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interaction type between sensitizers or between the sensitizer and the activator can be calculated using the following eqn (1) and it can be used to predict the mechanism of energy transfer and enable concentration quenching. Therefore, we can get the detailed information of multipolar interaction from the variation of the emission intensity depending on the concentration of the activators according to the report of Van Uitert. The emission intensity (I) per activator concentration (x) follows the following equation: 23 where I is the emission intensity, x is the concentration of the activator ions above the concentration quenching point, b and K are constants under the same conditions, and y is the function of multipole-multipole interaction. When the value of y is 6, 8 or 10, the form of the interaction corresponds to dipole-dipole (d-d), dipole-quadrupole (d-q), or quadrupole-quadrupole (q-q), respectively. To obtain a correct y, the dependence of lg(I/x) on lg(x) is plotted, and it yields a straight line with a slope of Ày/3. The fitting result for Ce 3+ emission centers, which is corresponding to the Sr 3.8À2x Ca(PO 4 ) 2 SiO 4 :xCe 3+ ,xNa + phosphor compositions beyond the quenching concentration of Ce 3+ , is shown in Fig. 5. The slope is determined to be À1.80, through which the value of y can be calculated as 5.4. Therefore, the values are approximately equal to 6, which means that the quenching process is ascribed to the dipole-dipole interaction in the present system. 24 Under the irradiation of 365 nm, the co-doped phosphor shows a broad bluish-green emission band containing the superimposed emission peaks that originated from the Eu 2+ ions and Ce 3+ ions. When monitoring at 493 nm and 429 nm, the two sets of PLE spectra give similar spectral profiles, which agree well with the PLE spectrum of the singly Ce 3+ doped sample, as shown in Fig. 6a.   The above comparative analysis on the PL and PLE spectra of the Ce 3+ /Na + and Eu 2+ singly doped and Ce 3+ /Na + /Eu 2+ -co-doped Sr 4 Ca(PO 4 ) 2 SiO 4 phosphors proves the occurrence of the energy transfer from the Ce 3+ to Eu 2+ ions. 27 Therefore, the photoluminescence tuning originating from the energy transfer process can be expected.
In order to further investigate the energy transfer process between the Ce 3+ and Eu 2+ ions in the Sr 4 Ca(PO 4 ) 2 SiO 4 host, we have studied the luminescence properties of a series of samples with designed compositions. Fig. 7 displays the PL spectra of Sr 3.8Àx Ca(PO 4 ) 2 SiO 4 :0.1Ce 3+ ,0.1Na + ,xEu 2+ samples under 365 nm excitation with a fixed Ce 3+ content of 0.1 and a varying Eu 2+ content x in the range of 0-0.06. As shown in Fig. 7, the emission peak is red-shifted from 429 nm to 493 nm with increasing concentration of Eu 2+ . The results verified that the superimposed emission peaks originated from the Eu 2+ ions and Ce 3+ ions, and the observed photoluminescence tuning should be ascribed to the energy transfer from the Ce 3+ to Eu 2+ ions.
Normally, the energy transfer from the sensitizer to the activator may be via a multipolar interaction or an exchange interaction that occurs at a higher concentration. On the basis of the Dexter's energy transfer expressions of multipolar interaction, the following relation can be obtained: 28,29 where I s0 and I s are the luminescence intensities of the sensitizer Ce 3+ with and without the activator Eu 2+ ; C is the concentration of the sum of Ce 3+ and Eu 2+ ; and n = 6, 8 and 10 corresponding to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively. The I S0 /I S p C n/3 plots are further illustrated in Fig. 8. when n = 6 we can observe a linear behavior with the optimum fitting factor of R 2 = 0.9987, indicating that energy transfer from Ce 3+ to Eu 2+ took place via the dipoledipole mechanism.
In order to validate the energy transfer from Ce 3+ to Eu 2+ , we investigated the lifetime values of Ce 3+ emission, which are calculated by analyzing decay curves of Sr 3.8Àx Ca(PO 4 ) 2 SiO 4 : 0.1Ce 3+ ,0.1Na + ,xEu 2+ phosphors. Fig. 9 shows the fluorescence decay curves of Ce 3+ emission under excitation at 354 nm and by monitoring the emission peak at 426 nm. It is found that all the decay curves can be fitted well with a second-order exponential decay, which can be obtained using the equation: 30 where I(t) is the luminescence intensity, t is the time, A 1 and A 2 are constants, and t 1 and t 2 are rapid and slow decay times for the exponential components, respectively. According to the parameters in eqn (3), the average lifetime t* can be obtained using the formula: Therefore, the decay lifetime values at 426 nm are determined to be 37. 13, 32.28, 24.42, 14.42, and 9.14 ns, respectively. Obviously, the decay lifetime values decreased monotonically as the Eu 2+ concentration increases, which also strongly demonstrated the energy transfer from Ce 3+ to Eu 2+ . As shown in Fig. 10, the energy transfer efficiency (Z T ) between the Ce 3+ and Eu 2+ ions can also be obtained from the decay lifetime values by using the following eqn (5): 31 where t x and t 0 represent the lifetime values of sensitizer Ce 3+ ions with and without the activator Eu 2+ , respectively.    The better thermal quenching luminescence behavior will be useful for the high temperature and high power application when this kind of phosphor is used as the blue-green components in UV-pumped WLEDs. Fig. 12 shows the chromaticity coordinates of the as-reported Sr 3.8Àx Ca(PO 4 ) 2 SiO 4 :0.1Ce 3+ ,0.1Na + ,xEu 2+ phosphors on the Commission Internationale de l'Eclairage (CIE) chromaticity diagram. The CIE chromaticity coordinates for different samples in Sr 3.8Àx Ca(PO 4 ) 2 SiO 4 :0.1Ce 3+ ,0.1Na + ,xEu 2+ phosphors were measured based on the corresponding PL spectra upon 365 nm excitation. As shown in Fig. 12, we can clearly see that the emission colors of the phosphors can be easily modulated from blue to green by simply changing the value of x from 0 to 0.06. Accordingly, the corresponding CIE coordinates change from (0.156, 0.0717) to (0.233, 0.346), due to the variation of the emission composition of the Ce 3+ and Eu 2+ ions. The inset in Fig. 12 illustrates the digital photos of this series of phosphors under 365 nm UV lamp excitation. These results indicate that the as-reported and composition-optimized phosphor might act as a potential candidate for blue-green components in UV-pumped WLEDs.

Conclusion
A new apatite-type phase that originated from the substitution of a Sr cation by a Ca cation in the Sr 5 (PO 4 ) 2 (SiO 4 ) has been prepared with the nominal chemical composition of Sr 4 Ca-(PO 4 ) 2 (SiO 4 ). The crystal structure of Sr 4 Ca(PO 4 ) 2 (SiO 4 ) has been analysed by the difference electron map, and further resolved by the Rietveld refinement, and the final composition has been determined as Sr 4 Ca(PO 4 ) (2+x) (SiO 4 ) (1Àx) (OH) x (x = 0.64), which crystallized in the hexagonal cell (space group P6 3 /m) with   lattice constants a = b = 9.67202 (8), c = 7.25393 (7), V = 587.68 (1), Z = 2. The photoluminescence properties of Ce 3+ /Na + or Eu 2+ singly doped and Ce 3+ /Na + /Eu 2+ -co-doped Sr 4 Ca(PO 4 ) 2 SiO 4 phosphors have been investigated in detail. When Ce 3+ and Eu 2+ were codoped in Sr 4 Ca(PO 4 ) 2 SiO 4 , the photoluminescence spectra displayed tunable blue-green emission by varying their relative concentrations. The effective energy transfer from the Ce 3+ to Eu 2+ has been discussed and verified based on the measured spectra and the decay curves, and the dipole-dipole interaction mechanism should be mainly responsible for the energy transfer process. The as-developed Sr 4 Ca(PO 4 ) 2 SiO 4 : Ce 3+ ,Na + ,Eu 2+ phosphor might act as a potential candidate for blue-green components in UV-pumped WLEDs.