Blue-green tunable color of Ce3+/Tb3+ coactivated NaBa3La3Si6O20 phosphor via energy transfer

A series of color tunable phosphors NaBa3La3Si6O20:Ce3+, Tb3+ were synthesized via the high-temperature solid-state method. NaBa3La3Si6O20 crystallizes in noncentrosymmetric space group Ama2 with the cell parameters of a = 14.9226(4) Å, b = 24.5215(5) Å and c = 5.6241(2) Å by the Rietveld refinement method. The Ce3+ ions doped NaBa3La3Si6O20 phosphors have a strong absorption band from 260 to 360 nm and show near ultraviolet emission light centered at 378 nm. The Ce3+ and Tb3+ ions coactivated phosphors exhibit color tunable emission light from deep blue to green by adjusting the concentration of the Tb3+ ions. An energy transfer of Ce3+ → Tb3+ investigated by the photoluminescence properties and lifetime decay, is demonstrated to be dipole–quadrupole interaction. These results indicate the NaBa3La3Si6O20:Ce3+, Tb3+ phosphors can be considered as potential candidates for blue-green components for white light emitting diodes.


Results and Discussion
Crystal structure and phase formation. Figure 1(a) demonstrates the observed and calculated XRD patterns as well as their difference for the Rietveld refinement of NaBa 3 La 3 Si 6 O 20 . In the refinement, an initial structure model and atomic positions of NaBa 3 Eu 3 Si 6 O 20 were adopted for the structure refinement 35,36 . NaBa 3 La 3 Si 6 O 20 crystallizes in the noncentrosymmetric space group Ama2 and unit cell parameters are obtained as a = 14.9226(4) Å, b = 24.5215(5) Å and c = 5.6241(2) Å, which are slightly larger than those of NaBa 3 Eu 3 Si 6 O 20 due to large ionic radius of La 3+ ion 37 . As shown in Fig. 1(c), the basic structural units are distorted (SiO 4 ) 4− tetrahedra which are further linked by the Ba, La, and Na atoms to build a complex three-dimensional framework. The Na atoms which are surrounded by six oxygen adopt distorted pentagonal-pyramidal geometry, the Ba1 and Ba2 atoms coordinated to seven and eight oxygen are in distorted trigonal prism and cube configuration. In the structure of NaBa 3 La 3 Si 6 O 20 , there are two kinds of La sites, implying that there are two possible types of Ce 3+ ions in the NaBa 3 La 3 Si 6 O 20 :Ce 3+ samples. The La1 atoms are coordinated to seven oxygen atoms to form pentagonal bipyramid while the La2 surrounded by eight oxygen atoms are in square anti-prism environment ( Fig. 1(d)). Figure 1(b) shows the selected XRD patterns of the as-synthesized representative samples of NaBa 3 La 3 Si 6 O 20 and NaBa 3 La 3 Si 6 O 20 :0.007Ce 3+ , yTb 3+ (0 ≤ y ≤ 0.30) and the quantitative analysis of all the samples illustrate that the doping of Ce 3+ or/and Tb 3+ are successful (Supplementary Table S1). Also, it can be seen that all the diffraction peaks of the selected phosphors match well with the NaBa 3 La 3 Si 6 O 20 phase. Even at high doping concentration of the Tb 3+ ion (30%), the XRD patterns of phosphors are almost same with that of undoped phase, which illustrates the excellent stability and accommodation capacity for doped ions of crystal structure of the NaBa 3 La 3 Si 6 O 20 host. The XRD profiles for the Rietveld refinement of the single element doped and co-doped samples and the coordination, occupancy and isotropic displacement parameter for all samples are listed (Supplementary Figs S1-S7, Tables S2-S9).
Photoluminescence properties and energy transfer. As shown in Fig. 2(a), the PLE spectra of the NaBa 3 La 3 Si 6 O 20 :0.007Ce 3+ sample consist of three absorption bands centered at around 254, 282 and 331 nm, which arise from the electronic transitions between the ground state ( 2 F 5/2 and 2 F 7/2 ) and the levels of 5d excited split by crystal field of the Ce 3+ ion 38 . Under the excitation wavelength of 331 nm, the Ce 3+ ion doped NaBa 3 La 3 Si 6 O 20 sample shows an asymmetric emission band extending from 340 to 500 nm with the maximum at 378 nm, indicating a possible spectral overlap originating from different luminescence centers. It is obvious that one type of Ce 3+ ions gives rise to two emission band due to the transitions from the lowest 5d excited states to two ground states ( 2 F 7/2 and 2 F 5/2 ) respectively 39 . However, the emission band of the NaBa 3 La 3 Si 6 O 20 :0.007Ce 3+ sample can be decomposed into four Gaussian components A-D peaking at 364, 380, 394 and 410 nm with the energy gaps between A and C is 2092 cm −1 , that of B and D is 1926 cm −1 , which are close to the theoretical value of 2000 cm −1 40,41 . These results imply that there should be two kinds of Ce 3+ ions, which is consistent with the previous investigation on the crystal structure that there are two kinds of different chemical environment of La 3+ ions in the NaBa 3 La 3 Si 6 O 20 host.
As given in Fig. 2(b), the PL intensity of the NaBa 3 La 3 Si 6 O 20 :xCe 3+ samples increases gradually with the increase of the doping concentration of the Ce 3+ ions and reaches the maximum when the x value is 0.007, and then begins to decrease due to concentration quenching 42 . It is also indicated that the Ce 3+ ion is a sensitizer for the Tb 3+ ion and an energy transfer of Ce 3+ → Tb 3+ is crucial to enhance green emission of the Tb 3+ ion and achieve color tunable emission light. Therefore, the optimal concentration of the Ce 3+ ion in the NaBa 3 La 3 Si 6 O 20 :xCe 3+ samples is confirmed to be 0.007.
Generally, the critical distance R C between the Ce 3+ ions can be calculated with the following equation given by Blasse 43 : where V is the volume of unit cell, x is the critical concentration of doped ions, where the emission intensity of phosphors reaches the maximum, N is the number of host cations per unit cell. For the NaBa 3 La 3 Si 6 O 20 :0.007Ce 3+ sample, N = 12, V = 2057.989 Å 3 , R C is calculated to be about 25.00 Å. Dexter noted a non-radiative energy transfer usually was attributed to exchange or multipole -multipole interaction in oxide phosphors and the exchange interaction was valid only when the Rc was shorter than 5 Å 44 . In consequence, the concentration quenching mechanism of the Ce 3+ ions in the NaBa 3 La 3 Si 6 O 20 :xCe 3+ samples is dominated by the multipole -multipole interaction. Figure 3(a) depicts the PLE and PL spectra of the NaBa 3 La 3 Si 6 O 20 :0.20Tb 3+ sample. The PLE spectrum monitored at 542 nm exhibits a broad absorption band centered at 268 nm from 200 to 300 nm and several peaks within the scope of 300 to 400 nm. The former excited peak is ascribed to 4f 8 -4f 7 5d transition of the Tb 3+ ion, while the latter peaks are from the intra-4f 8 transitions 45,46 . Under the excitation wavelength of 268 nm or 378 nm, the NaBa 3 La 3 Si 6 O 20 :0.20 Tb 3+ sample emits green light with main peaks at 412, 435, 457, 488, 542, 581 and 622 nm, which can be ascribed to the 5 D 4 -7 F J (J = 6, 5, 4 and 3) transitions of the Tb 3+ ion. However, because the f-f absorption is a forbidden transition, only some narrow f-f transition lines locate in the excitation range of n-UV LED in spite of difficultly bumping the Tb 3+ ion 47 . There is an overlap between the emission band (magenta line in Fig. 3(a)) of the Ce 3+ ions and the f-f transition (olive line in Fig. 3(a)) absorption band of the Tb 3+ ions, therefore, it is potential that the Ce 3+ ions can be sensitizers to transfer energy to the Tb 3+ ions to enhance their absorption. As shown in Fig. 3(b), the PL spectrum of the NaBa 3 La 3 Si 6 O 20 :Ce 3+ , Tb 3+ phosphors exhibits broad emission bands corresponding to the allowed f-d transition of the Ce 3+ ions and the 5 D 4 -7 F J characteristic transitions of  the Tb 3+ ions. The emission intensity of the NaBa 3 La 3 Si 6 O 20 :Tb 3+ samples under excitation wavelength of 268 nm is larger than that under 378 nm, because the intensity of the absorption peak centered at 268 nm is more intense than that at 378 nm. However, the emission light intensity monitored at 268 nm is less than that at 374 nm in the NaBa 3 La 3 Si 6 O 20 :Ce 3+ , Tb 3+ phosphors. These results verify that it is the overlap between f-f transition (peaking at 374 nm) but not f-d transition (peaking at 268 nm) of the Tb 3+ ions and the emission band of the Ce 3+ ions induce the energy transfer. Figure 3(b) also shows the excitation spectrum of the NaBa 3 La 3 Si 6 O 20 :0.007Ce 3+ , 0.20Tb 3+ phosphor monitored at 378 nm (the Ce 3+ ions emission) is similar to that of at 542 nm (the Tb 3+ ions emission) except the difference of luminous intensity, which provides another evidence for energy transfer of Ce 3+ → Tb 3+ .
To further investigate the sensitized luminescence of the Tb 3+ ions by the Ce 3+ ions, the emission spectra of the NaBa 3 La 3 Si 6 O 20 :0.007Ce 3+ , yTb 3+ phosphors were measured (Fig. 4). Although the amount of the Ce 3+ ions is fixed, their emission intensity gradually decreases along with the increase of the concentration of the Tb 3+ ions. The result indicates that a lot of Tb 3+ ions as acceptors accelerate energy diffusion of donors, which speeds up the average transfer rate of Ce 3+ → Tb 3+ . Figure 5 and Table 1  Energy transfer mechanism. In general, the energy transfer from a sensitizer to an activator in oxide may take place via exchange interaction or electric multipolar interaction 48 . The separation distance R Ce-Tb can be also estimated from equation (1). Here, x is the total concentration of the Ce 3+ and Tb 3+ ions, where the  x Tb 3 is about 0.021 and 0.75 respectively, thus R Ce-Tb is calculated to be about 7.5 Å. Since exchange interaction was restricted to distances of about 4 Å, the energy transfer mechanism of Ce 3+ → Tb 3+ should may be electric multipolar interaction 43,44 .
According to Dexter's energy transfer expressions of multipolar interaction and Reisfeld's approximation, the following relation can be given as 42,[48][49][50][51] : Ce Tb n 0 /3 3 3 where ƞ 0 and ƞ are the luminescence quantum efficiency of the Ce 3+ ions in absence and presence of the Tb 3+ ions, n = 6, 8 and 10 are corresponding to dipole -dipole, dipole -quadrupole and quadrupole -quadrupole interactions, respectively. The value ƞ 0 / ƞ is approximately estimated by the ratio of related luminescence intensity I 0 /I, I 0 is the intrinsic luminescence intensity of the Ce 3+ ions, and I is the luminescence intensity of the Ce 3+ ions in presence of the Tb 3+ ions. Figure 6(a-d) illustrates the relationships between I 0 /I and The R 2 value is reasonable in Fig. 6(b,c), implying the energy transfer of Ce 3+ → Tb 3+ may occur via dipole-dipole or dipole-quadrupole interaction. However, Sommerdijk stated the probability of energy transfer of Ce 3+ → Tb 3+ via electric dipole-dipole interaction was less likely, therefore, dipole -quadrupole interaction should mainly contribute to energy transfer of Ce 3+ → Tb 3+ 52 .  In order to further validate the energy transfer process, the room temperature decay curves for the 4f-5d (centered at 378 nm) transition of the Ce 3+ ions in NaBa 3 La 3 Si 6 O 20 :0.007Ce 3+ , yTb 3+ (y = 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) excited at 330 nm are shown in Fig. 7 For existing two types of Ce 3+ ions in topic phosphors, the decay curves should be well fitted with a typical two exponential function 53 : where I(t) and I 0 are the luminescence intensity at time t, A 1 and A 2 are the fitting constants, τ 1 and τ 2 represent the decay time for the exponential components. Then the average lifetime (τ *) can be calculated to be 24.5, 22.6, 21.7, 20.5, 20.0, 19.2 and 17.3 by the following formula 40,54 : The decay time of the Ce 3+ ions decreases as increase of the concentration of the Tb 3+ ions, which strongly demonstrates the energy transfer of Ce 3+ → Tb 3+ .
Subsequently the energy levels model for the energy transfer processes of Ce 3+ → Tb 3+ was investigated. As given in Fig. 8(a), the Ce 3+ ion absorbs light firstly, then it jumps from the ground states ( 2 F 5/2 ) to the excited states (5d energy levels), subsequently the excited state Ce 3+ ion returns to the lowest level of 5d levels by giving off excess energy to its surroundings, eventually goes back to the 2 F 7/2 or 2 F 5/2 ground states by a radiative process. The energy transfer efficiency of Ce 3+ → Tb 3+ should increase as the increase of the concentration of the Tb 3+ ions due to more neighboring Tb 3+ ions around the Ce 3+ ions. Finally the energy level transitions of 5 D 4 to 7 F J (J = 3, 4, 5 and 6) produce the characteristic emission of the Tb 3+ ions.
The energy transfer efficiency ƞ T from the Ce 3+ ions to the Tb 3+ ions can be calculated according to the following equation 55   where I 0 and I y are the emission light intensity of the sensitizer with and without an activator, respectively. In the NaBa 3 La 3 Si 6 O 20 :0.007Ce 3+ , yTb 3+ samples, the Ce 3+ ion is a sensitizer and the Tb 3+ ion is an activator.  Fig. 8(b)). The energy transfer of Ce 3+ → Tb 3+ is consistent with the conclusion that the energy transfer efficiency increases as the increase of the concentration of the Tb 3+ ions due to more neighboring Tb 3+ ions around the Ce 3+ ions and is equivalent to that of the reported K 2 MgSiO 4 :Ce 3+ , Tb 3+ silicate phosphor 56 . Material characterization. The powder XRD measurements were taken on a Bruker D8 X-ray diffractometer with a Cu Kα source (λ = 1.5418 Å) in the angular range from 5° to 80° with a scanning step of 0.15. The structure refinement was carried out with the General Structure Analysis (GSAS) and EXPGUI software 57,58 . XRD Rietveld profile refinements of the structural models were performed using the General Structure Analysis (GSAS) software. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were obtained by an FLS-980 fluorescence spectrophotometer equipped with a 450 W Xe light source. The photoluminescence lifetime curves were measured on an FLS-920 fluorescence spectrophotometer equipped with a laser as light source. All measurements were performed at room temperature. The element analyses of samples were performed by the (X-ray fluorescence) XRF method on a thermo ARL ADVANTXP+ apparatus.