Structure and photoluminescence properties of red-emitting apatite-type phosphor NaY9(SiO4)6O2:Sm3+ with excellent quantum efficiency and thermal stability for solid-state lighting

A novel red-emitting phosphor NaY9(SiO4)6O2:Sm3+ (NYS:Sm3+) was synthesized and the X-ray diffraction and high-resolution TEM testified that the NYS compound belongs to the apatite structure which crystallized in a hexagonal unit cell with space group P63/m. The novel phosphor boasts of such three advantageous properties as perfect compatible match with the commercial UV chips, 73.2% quantum efficiency and 90.9% thermal stability at 150 °C. Details are as follows. NYS:Sm3+ phosphor showed obvious absorption in the UV regions centered at 407 nm, which can be perfectly compatible with the commercial UV chips. The property investigations showed that NYS:Sm3+ phosphor emitted reddish emission with CIE coordination of (0.563, 0.417). The optimum quenching concentration of Sm3+ in NYS phosphor was about 10%mol, and the corresponding concentration quenching mechanism was verified to be the electric dipole–dipole interaction. Upon excitation at 407 nm, the composition-optimized NYS:0.10Sm3+ exhibited a high quantum efficiency of 73.2%, and its luminescence intensity at 150 °C decreased simply to 90.9% of the initial value at room temperature. All of the results indicated that NYS:Sm3+ is a promising candidate as a reddish-emitting UV convertible phosphor for application in white light emitting diodes (w-LEDs).

class of apatite-type silicate compounds, NaY 9 (SiO 4 ) 6 O 2 is isostructural with natural oxyapatite, which consists of two cationic sites: one is in a distorted 3-fold capped trigonal prism (4 f sites with C 3 point symmetry), which is partially occupied by Y 3+ (the site occupancy of 75%)−Na + (the site occupancy of 25%) and nine oxygen ions, and the other is in a distorted pentagonal bipyramid (6 h sites with C s point symmetry) coordinated with Y 3+ and seven oxygen ions respectively. In 1982,Gunawardane first synthesized the apatite structure compound NaY 9 (SiO 4 ) 6 O 2 , led its chemical composition and detailed crystal structure 22 . Later, Redhammer's group found that the crystal structure of NaY 9 (SiO 4 ) 6 O 2 compound showed no symmetry change between room temperature and 100 K, and that the alterations in structural parameters were small 23 . Recently, apatite-type silicate compounds activated by rare earth have drawn great attention owing to their excellent luminescent characteristics 24-28 . You's team reported the color-tunable phosphors of Ce 3+ /Tb 3+ /Eu 3+ /Mn 2+ co-doped NaY 9 (SiO 4 ) 6 O 2 29 . However, to our best knowledge, the luminescence properties of Sm 3+ in NYS host have not yet been reported. As a member of efficient red-emitting trivalent rare earth ions, samarium (Sm 3+ ) ions with sharp 4f-4f emission peak have been receiving intense research interests, widely used in luminescence materials because of their unique luminescent properties. Customarily, Sm 3+ ions generate three dominant emissions located in reddish-emitting regions, which correspond to 4 G 5/2 → 6 H 5/2 4 , G 5/2 → 6 H 7/2 and 4 G 5/2 → 6 H 9/2 transitions respectively. However, the absorption efficiency of Sm 3+ is low in the n-UV region where the 4f-4f electric dipole transitions are absolutely forbidden. Therefore, it is necessary to develop an excellent phosphor with good quantum efficiency to decrease the effect of the poor absorption efficiency.
In view that the effective ionic radii of Sm 3+ ions is the closest to that of Y 3+ ions, it may serve as an activator for reddish-emitting luminescence materials when Sm 3+ ions are doped into the NYS matrix. Consequently, we demonstrate a red-emitting phosphor NYS:Sm 3+ with apatite structure pumped for UV-light emitting diodes via a traditional solid-state method. Accordingly, the crystal structure of the NYS has been investigated. In addition, the concentration quenching as well as lifetime of Sm 3+ in NYS phosphor was investigated. The excellent luminescence quantum efficiency and thermal stability indicate that as-prepared NYS:Sm 3+ phosphor can act as a red UV convertible phosphor for w-LEDs.

Experimental Section
The NYS:xSm 3+ (x = 0, 0.01, 0.05, 0.10, 0.15, 0.25, and 0.50) phosphors were prepared by a high temperature solid-state technique. The stoichiometric amounts of raw materials Na 2 CO 3 , SiO 2 with analytic grade purity and Y 2 O 3 (99.99%), Sm 2 O 3 (99.99%) were weighed and mixed by grinding in an agate mortar; to compensate for loss of Na source at high temperature, a slight excess of Na 2 CO 3 (5%wt beyond stoichiometry) was necessary, and the homogeneous mixture was transferred to an alumina crucible and annealed at 1400 °C for 4 h. The final products were cooled to room temperature and were reground for further measurements.
The structure of the final products was identified by using powder X-ray diffraction (XRD) analysis (XD-3, PGENERAL, China) in the 2θ range of 10° to 70°, with graphite monochromatized Cu Kα radiation (λ = 0.15406 nm) operating at 40 kV and 40 mA. The measurements of photoluminescence emission (PL) and photoluminescence excitation (PLE) spectra were performed by using 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. The room-temperature luminescence decay curves were obtained from a spectrofluorometer (Horiba, JobinYvon TBXPS) using a tunable pulse laser radiation as the excitation. Transmission electron microscope (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) analyses were checked by a JEOL JEM-2010 microscope with an accelerated voltage of 200 kV. The elemental analysis was carried out by energy dispersive spectroscopy (EDS) using an X-ray detector attached to the TEM instrument. The internal quantum efficiency was measured using the integrated sphere on the FLS920 fluorescence spectrophotometer (Edinburgh Instruments Ltd, UK), and a Xe900 lamp was used as an excitation source and white BaSO 4 powder as a reference to measure the absorption. The signals were detected by a Hamamatsu R928P photomultiplier tube. The temperature-dependence luminescence properties were measured on the same spectrophotometer, which was combined with a self-made heating attachment and a computer-controlled electric furnace. shows the enlarged spectrum profile in the range of 31.9 to 33.8°. It can be found that the sample spectra could be well indexed as pure crystalline NaY 9 (SiO 4 ) 6 O 2 phase (JCPDS no: 35-0404), which belongs to the hexagonal structure with the space group P6 3 /m, indicating that the introduction of Sm 3+ ion into the NaY 9 (SiO 4 ) 6 O 2 lattice does not cause any significant change to the crystal structure of the host. Based on the effective ionic radii and charge balance of cations with different coordination numbers (CN), the activators Sm 3+ ions are expected to occupy the Y 3+ sites randomly in the NYS host, because the effective ionic radii of Sm 3+ (r = 1.02 Å for CN = 7 and r = 1.132 Å for CN = 9) is the closest to that of Y 3+ ions (r = 0.96 Å for CN = 7 and r = 1.075 Å for CN = 9) 30 . In addition, according to the enlarged profile, we can find that the diffraction peaks shift to a larger angle with x increasing, indicating the increase of the cell volume with the substitution of Sm for Y ions. This phenomenon also indicates that the doped Sm 3+ are successfully incorporated in the host at Y 3+ sites. Figure 2 shows the observed (solid line), the calculated (red circles), and different (bottom) XRD profiles for the Rietveld refinement of NaY 9 (SiO 4 ) 6 O 2 with λ = 1.5406 Å by TOPAS program. Almost all peaks were indexed by hexagonal cell with parameters close to Cd 2 Nd 8 (SiO 4 ) 6 O 2 (apatite-type structure) 30 . Therefore crystal structure of Cd 2 Nd 8 (SiO 4 ) 6 O 2 was taken as starting model for Rietveld refinement. Sites of Cd/Nd ions were occupied by Na/Y ions and the occupation of sites was refined with assumption that their sum in each site equal to 1. Refinement was stable giving low R-factors (Table 1, Fig. 2).

Results and Discussion
As mentioned previously, NYS belongs to apatite-type compound. In this compound, the Na (1) Fig. 4a. It can be seen that the PLE spectrum of NYS:0.10Sm 3+ monitored by 603 nm presents some observable band features ranging from 300 to 500 nm, which is in consistent with that monitored at 566 and 650 nm except for the tiny difference in the relative intensity. It is obvious that these sharp excitation peaks are located in the wavelength region of 300-500 nm appearing at 326, 345, 364, 378, 388, 407, 420, 441, 465, and 478 nm, which could be assigned to transitions from the 6 Fig. 4a,b. It is obvious that the emission intensity of Sm 3+ ion first increases and reaches a maximum at x = 0.10, and then the emission intensity decreases as a result  of concentration quenching effect. The concentration quenching may be induced by cross relaxation processes due to proximity in Sm 3+ ions. With increasing the Sm 3+ ions doping concentration, there is more possibility of enhancing the energy transfer within Sm 3+ ions beyond the optimal doping concentration through non-radiative process 34 . If we consider energy transfer between two identical centers, the critical distance (Rc) is defined as the distance for which the probability of energy transfer equals the probability of radiative emission of Sm 3+ , as pointed out by Blasse 35 . The average separation R c can be estimated according to the following equation [36][37][38] : where V is the volume of the unit cell, x c is the critical concentration, and N is the number of available sites for the dopant in the unit cell. In our host of NYS, N equals to 1, and V is estimated to be 508.98 Å 3 , and the critical doping content x c = 0.10. According the Equation (1), the critical distance was determined to be about 21.34 Å. It is of little possibility for energy transfer through the exchange interaction mechanism with longer distance. Thus, the electric multipolar interaction will take place for energy transfer between two Sm 3+ ions. In order to further describe internal concentration quenching effect, interaction between sensitizers or between a sensitizer and an activator can be expressed by the following equation [39][40][41] : where x is the activator concentration which is higher than the critical concentration, I/x is the emission intensity (I) per activator concentration (x), K and β are constants for the same excitation condition, and θ is an indication of electric multipolar character, θ = 6, 8, 10 for dipole-dipole (d-d), dipole-quadrupole (d-q), or quadrupole-quadrupole (q-q), respectively 42 . The insert of Fig. 4b shows the fitting lines of log(I/x) vs log(x) in NYS:xSm 3+ phosphors at 603 nm beyond the quenching concentration. It is clear that the fitting curves of lg(I/x) vs lg(x) can be well matched as linear dependence. The slope of the straight line was determined to be −2.115.
Herein, the value of θ can be calculated as 6.345, which is close to 6 consistent with the quenching results arising from dipole-dipole interactions in the NYS:xSm 3+ phosphor. The decay curve of Sm 3+ emission for NYS:xSm 3+ (x = 0.01, 0.05, 0.10, 0.15, 0.25, 0.35 and 0.50) excited at 407 nm was measured as shown in Fig. 5. It can be found that all of the decay curve can be fitted successfully, based on a typical second-order exponential decay equation as follows 43 : where I is the luminescence intensity at time t and I 0 are the luminescence intensity initially, A 1 and A 2 are constants,τ 1 and τ 2 are the lifetimes for the exponential components. Furthermore, the effective lifetime constant (τ * ) can be calculated as: The average decay times (τ * ) were calculated to be 1.69, 1.53, 1.47, 1.22, 1.03, 0.78 and 0.64 ms for NYS:xSm 3+ with x = 0.01, 0.05, 0.10, 0.15, 0.25, 0.35 and 0.50, respectively. An obvious phenomenon is obtained that the decay time begins to decrease gradually with Sm 3+ concentration increasing. Considering the distance between Sm 3+ -Sm 3+ ions decreases in pace with increasing Sm 3+ doped concentrations, the probability of energy transfer to luminescent killer sites increased, thereby the lifetimes of Sm 3+ ions are shortened due to favorable non-radiative energy transfer processes as Sm 3+ concentration increases, which is similar to some previous results discussed in other systems [44][45][46] . In order to further investigate the possible practical application in high power w-LEDs of the NYS:Sm 3+ phosphor, the temperature dependent emission spectra of NYS:0.10Sm 3+ phosphor ranging from 25 to 300 °C monitored by 407 nm is shown in Fig. 6a. It depicts that the position and shape of the emission spectra do not change with increasing temperatures. While the temperature increases, the intensity of the emission spectrum decreases. As given in the inset of Fig. 6b, when the temperature turns up to 150 °C, the emission intensity is 90.9 percent of that at 25 °C, which indicates this phosphor has excellent thermal stability for potential application in w-LEDs. In order to understand the temperature dependence behavior better, an Arrhenius fitting of the emission intensity of NYS:0.10Sm 3+ phosphor and the calculated activation energy (ΔE) for thermal quenching were carried out and the results were given in Fig. 6b. The activation energy (ΔE) can be expressed by Equation (5)  The chromaticity diagram and CIE coordinates are very important to disclose the exact emission color and color purity of the sample. The CIE chromaticity diagram for the NYS:0.10Sm 3+ phosphor under 365 nm UV excitation is shown in Fig. 7a. It can be seen that the emission color of the as-prepared samples located in the reddish light  region. The calculated color coordinate is found to be (0.563, 0.417), which indicates the phosphor can be used as a reddish-emitting candidate phosphor for w-LEDs application as the obtained CIE chromaticity coordinate is very much close to the Nichia corporation developed amber LED NSPAR 70BS (0.575, 0.425) 48 . A digital photo of the NYS:0.10Sm 3+ phosphor under 365 nm UV lamp is shown in the inset of Fig. 7a revealing an intense reddish light. Additionally, we have also measured the internal quantum efficiency (QE) of NYS:0.10Sm 3+ phosphor according to the reported method. On the basis of the result of Fig. 7b, the internal QE value can be calculated by following formula 49,50 : where L S is the luminescence emission spectrum of the sample; E S is the spectrum of the light used for exciting the sample; E R is the spectrum of the excitation light without the sample in the sphere. The measured internal QE of NYS:0.10Sm 3+ phosphor is determined as 73.2% under 407 nm excitation. As a reference, we have also measured the QE value of the commercial red phosphor Y 2 O 2 S:Eu 3+ , and the value is 80.9% under 393 nm excitation. It is believed that as-prepared NYS:0.10Sm 3+ phosphor can act as a potential reddish-emitting UV convertible phosphor for w-LEDs with high luminescence quantum efficiency.

Conclusion
In summary, a red-emitting apatite-type phosphor NaY 9 (SiO 4 ) 6 O 2 :Sm 3+ has been synthesized and its properties have been reported in this paper. The phase purity and fine local structure were examined by XRD and HRTEM. This phosphor showed strong absorption in the UV regions and the optimum excitation band was located at 407 nm region, which matched well with the commercial UV chips (350-410 nm). The emission spectrum demonstrated that the characteristic emission consists of three bands, which corresponded respectively to the transitions from the 4 G 5/2 excited state to the 6 H 5/2 (566 nm) 6 , H 7/2 (603 nm), and 6 H 9/2 (650 nm). The critical Sm 3+ quenching concentration (QC) was estimated to be about 10%mol, and the corresponding QC mechanism was verified to be based upon dipole-dipole interactions. The fluorescence decay curves, temperature dependence photoluminescence and CIE value of NYS:Sm 3+ phosphors have been discussed in detail. The excellent thermal stability and internal quantum efficiency indicated that the as-prepared NYS: Sm 3+ phosphors are potential red-emitting UV convertible phosphor for applications in the next generation w-LEDs.