Rare earth ion Tb3+ doped natural sodium feldspar (NaAlSi3O8) Luminescent properties and energy transfer

In this study, Tb3+—doped natural sodium feldspar (NaAlSi3O8) phosphors have been successfully prepared using high−temperature solid—state method with natural sodium feldspar as a substrate. Energy—dispersive X—ray spectrometry analysis (EDX) of NaAlSi3O8 showed that 0.03 wt% of Eu element was present, and elemental distribution mapping analysis showed that the distribution of trace Eu in minerals was aggregated. The crystal structure and luminescence properties of the natural sodium Eu—containing feldspar and synthetic sodium feldspar NaAlSi3O8:Eu3+, Tb3+ phosphors are discussed in detail. The crystal structure analysis of the samples showed that the Na+ in the natural matrix was partly replaced by the doped Tb3+. Studies on the photoluminescence properties of the samples indicate that Eu does not form a luminescent center in the natural mineral, however, the strong characteristic peak of Eu3+ at 615 nm appears after doping with Tb3+ and the peak at 615 nm increases with the increase of Tb3+ concentration. According to the above spectral results, the energy transfer from Tb3+ to Eu3+ is obtained. Through the measurement and analysis of color coordinates, it is found that with the increase of Tb3+ concentration, the luminescence color of the samples can be regulated in the green to red region. NaAlSi3O8:Eu3+ Tb3+ phosphors has potential application value.


Results and Discussion
SEM-EDX analysis of natural sodium feldspar (NaAlSi 3 o 8 ). Energy-dispersive X-ray spectrometry (EDX) is a standard procedure for identifying and measuring the constituent elements of the samples. The minerals used in this experiment were analyzed by the EDX. Figure 1(a), Table S1 show the EDX analysis report of the natural sodium feldspar. Figure 1(a) and Table S1 showed that the natural matrix albite in this experiment contained small amounts of C, Ca, and Eu elements, except the main elements Na, Al, Si, O. The atomic percentage of the main elements is 0.85:1:2.6:8.3, close to the atomic percentage of albite 1:1:3:8, and the percentage of the weight of the Eu element was 0.03%. Figure S1 shows element distribution mapping of natural sodium feldspar, Na, Al, Si, O and a small amount of impurity Ca and Eu as impurity can be seen evenly in the map. However, the distribution density of the trace component Eu is uneven, which is concentrated in a certain part of the natural sodium feldspar. Therefore, assumed that Eu in natural minerals may not form luminescent center. Figure S2 shows element distribution mapping of thermally-treated natural sodium feldspar, after treatment, Eu elements are slightly dispersed. In the following passage, natural sodium feldspar in this experiment will be denoted by natural NaAlSi 3 O 8 :Eu 3+ .
In order to get deep insights into the natural sodium feldspar, the surface states of phosphor particles were analyzed in a detail. Figure 1(b-d) displays the surface morphology of natural sodium feldspar, thermally-treated natural NaAlSi 3 O 8 : Eu and NaAlSi 3 O 8 : Eu, 3%Tb 3+ respectively, it can be seen that samples are irregular particles. The sample before thermally-treated shows an irregular morphology with smooth surfaces. Contrast, the surface of thermally-treated sample seems to be etched and molten, and some small particles are observed, which are initially assumed to be the corrosion products falling off from the natural sodium feldspar particles during the treatment process crystal structure analysis. Figure 2 shows the crystal structure of NaAlSi 3 O 8 unit cell viewed in the z-direction. The four corners of each [SiO 4 ] tetrahedron are all shared with four adjacent [SiO 4 ] to forma Si-O frame structure in oxides, such as quartz. Because the Si in some tetrahedra is replaced by Al, there is excess negative charge, therefore the chemical formula of frame anion is generally written as (Al x Si n−x ) x − . Because of the existence of the tetrahedral structure, NaAlSi 3 O 8 has good chemical stability and rigidity 5,6 .
As shown, the XRD patterns of these samples match well with the standard JCPDS card (No. 9-0466) and no impurity phases were observed, which indicates that slight doping would not lead to a large change in the crystal structure, That is to say, the observed small particles in the thermally-treated sample (Fig. 1b) is either natural sodium feldspar or newly-formed amorphous impurity. The existence of trace elements and increase of the doping concentration of Tb 3+ lead to a diffraction peak located at 27.9° in 2θ, corresponding to the (002) crystal plane, shifted slightly to a higher angle direction (Fig. 3b), suggesting the substitution of larger Na + (CN = 6, www.nature.com/scientificreports www.nature.com/scientificreports/ r = 0.102 nm) with smaller Tb 3+ (CN = 6, r = 0.092 nm). This causes a change in the lattice constant of the host lattice 7,8 . According to the Bragg formula: The decrease of radius r leads to the decrease of surface spacing d, displacing the peak to a large angle Figure S3 shows the calculated lattice constants and unit cell volume as a function of Tb 3+ content. Lattice constants a, b and c calculated by Jade. It is clearly seen that the lattice constants a, b, and c decrease with increasing Tb 3+ content, and the unit cell volume can be calculated as: Absorption spectrum analysis. Figure 4 exhibits the UV-vis absorption spectra of thermally-treated natural NaAlSi 3 O 8 : Eu and NaAlSi 3 O 8 : Eu, 3%Tb 3+ . At x = 0, the absorption spectra show a strong absorption band around 232 nm. When the Tb 3+ ions are incorporated into the host lattice, the absorption spectra show a slight red-shift, which indicates that the Tb 3+ ions are doped successfully into the host lattice, and it is consistent with the XRD results. The cause of the red-shift was already reported by Ahemen·I et al. The red-shift of absorption spectra is mainly attributed to the electro-negativity difference between Natrium and Terbium. Since, the electro-negativity of Terbium is higher than that of Natrium (Pauling electro-negativity of Terbium is 1.1 and that of Natrium is 0.98), and thus the electro-negativity difference between Na and O is larger than that of Tb and O.  www.nature.com/scientificreports www.nature.com/scientificreports/ In our case, the introduction of Tb 3+ may lower the O 2− ligand to cations bond energy in comparison with that of pristine sample 9,10 . photoluminescence properties. Figure 5a shows the PLE spectrum of NaAlSi 3 O 8 Eu 3%Tb 3+ monitored at 545 nm, 704 nm. When the monitoring wavelength is 545 nm, a broadband strong excitation band with a peak of 248 nm (220-300 nm) is observed. There are many explanations for the excitation of 220-300 nm containing Eu 3+ and Tb 3+ system, such as studies of terbium-europium Co-doped ZrO 2 system explained that the excitation peaks at 247 nm and 278 nm belong to Eu 3+ -O 2− charge transfer zone (CTB) and 4f → 5d transitions of Tb 3+ . In the study of NaGd (MoO 4 ) 2 :5% Tb 3+ , 1% Eu 3+ system, Yao D et al. explained that the excitation peaks at 200-300 nm were attributed to the O 2− Eu 3+ and O 2− Mo 6+ transitions 11 . Figure 5b shows the PLE spectrum of natural NaAlSi 3 O 8 : Eu and NaAlSi 3 O 8 : Eu, 3%Tb 3+ monitored at 615 nm. No excitation peaks were found at the 257 nm, 273 nm, and 284 nm sites of undoped Tb 3+ , and only the 232 nm centric broadband excitation peak was observed. After Tb 3+ incorporation, the main peak showed a red-shift, and the excitation peak appeared at 257 nm, 273 nm, and 284 nm. Therefore, we determined the broadband strong excitation band (220-300 nm) with the peak of 248 nm in Fig. 5a, which was caused by Eu 3+ -O 2− CTB and allowable transition 4f 8 → 4f 7 5d 1 of Tb 3+ Within the range of 300-500 nm, a series of very weak excitation peaks caused by f → f transition of Tb 3+ ions can be observed. The emission peaks of Tb 3+ and Eu 3+ may be located at 589 nm and 613 nm, respectively, in order to exclude the interference from the Tb 3+ emission, the emission wavelength of 704 nm is selected as the monitoring wavelength, the strong excitation peaks at 248 nm and a series of excitation peaks caused by the superposition of the f → f transition from Tb 3+ and the 4f → 4f transition of the Eu 3+ are also observed 12,13 . The excitation peaks corresponding to Tb 3+ were observed when the Eu 3+ characteristic emission was monitored. This result confirms that the energy transfer of Tb 3+ to Eu 3+ may be effective. Figure 6 shows the PL (λ ex = 351 nm) spectra of thermally-treated natural NaAlSi 3 O 8 :Eu, x%Tb 3+ (x = 3, 5, 7, 9, 11). The emission peaks at 545 nm, 586 nm, 591 nm, 621 nm, and 626 nm were observed, corresponding to the 5 D 4 → 7 F 5,4,4,3,3 transitions of Tb 3+ . The emission peaks at 578 nm 586 nm, 591 nm, 615 nm, 621 nm, 626 nm, www.nature.com/scientificreports www.nature.com/scientificreports/ 652 nm, correspond to the 5 D 0 → 7 F 0,1,1,2,2,2,3 transitions of Eu 3+ 14 . Therefore, there is a superposition of emission spectra between Tb 3+ and Eu 3+ in the range of 586-626 nm. The inset of Fig. 6 shows the concentration dependence of Tb 3+ luminescence intensity of 545 nm (Tb 3+ ) and 615 nm (Eu 3+ ). With the increase of Tb 3+ concentration, the peak at 545 nm decreases, while the peak at 615 nm increases. The concentration of Tb 3+ increases and the energy is transferred to Eu 3+ simultaneously. Therefore, the emission peak at 615 nm continues to increase. We also measured the photoluminescence properties of thermally-treated natural NaAlSi 3 O 8 : Eu. Figure S4 shows the PL spectra of thermally-treated natural NaAlSi 3 O 8 :Eu and NaAlSi 3 O 8 :Eu, 3%Tb 3+ . When natural NaAlSi 3 O 8 :Eu absence of Tb 3+ , we observed very weak peaks (Eu 3+ ) at 615 nm, 626 nm and 652 nm. Natural sodium feldspar containing Eu does not emit light under ultraviolet lamp, which may be due to low Eu 3+ content and uneven distribution (Figs S1 and 2). We also measured the quantum efficiency of the sample. When the Tb doping concentration is 3%, 5%, 7%, 9%, 11%, the quantum efficiency is 29%, 33%, 40%, 51%, 67%, respectively.
In order to further confirm the energy transfer mechanism between the Tb 3+ and the trace element Eu 3+ in the NaAlSi 3 O 8 : Eu, xTb 3+ phosphor, it is necessary to test and analyze the transient spectrum in addition to the steady-state spectrum. The fluorescence decay curves of theNaAlSi 3 O 8 : Eu, x%Tb 3+ sample are shown in Fig. 7(a)  www.nature.com/scientificreports www.nature.com/scientificreports/ where I (t) is the luminescence intensity at time t. The calculated average lifetimes for different samples are2.29, 2.17, 1.82, 1.53, 1.12 ms, which correspond to the concentrations of x = 3, 5, 7, 9, 11 respectively. The decreased life-time of Tb 3+ with increased Tb 3+ concentration demonstrates an effective energy transfer from Tb 3+ to Eu 3+ . The contributions of the different ions in the corresponding PL spectra have been calculated and are shown in Fig. 7(b). It is obvious that the G/R (green to red) ratio continuously increases 15 .
The energy transfer of Tb 3+ to Eu 3+ is very effective because their energy level distributions have a large overlap. Figure 8 shows the possible process of energy transfer between Tb 3+ and Eu 3+ . Under ultraviolet light, the 4f 8 electrons of Tb 3+ transition from the ground state to the excited state 4f 7 5d, then, these electrons relax to the excited state 5 D 4 level. Thereafter, the blue-green light ( 5 D 4 → 7 F 6, 5, 4, 3 ) are emitted from the gound state by the polychromatic relaxation, and the energy is transferred to the 5 D 1 and 5 D 0 levels of Eu 3+ by cross relaxation. Eu 3+ absorbs energy from Tb 3+ and emits orange-red light. The energy transfer of Tb 3+ to Eu 3+ is effective 16,17 . chromaticity coordinates. Table 1 and Fig. 9 show the color coordinates and chromaticity diagrams of the NaAlSi 3 O 8 : Eu, x%Tb 3+ (x = 3, 5, 7, 9, 11) respectively. With the increase of Tb 3+ concentration, the color coordinates change from (0.3894, 0.3834) to (0.5033, 0.373) which corresponds to the color change of green-yellow to orange under the ultraviolet radiation of 254 nm. It is found that when the concentration of Tb 3+ is higher, the color of the sample is closer to the red emission (615 nm). The increasing concentration of Tb 3+ will transfer energy to Eu 3+ , which will increase the emission peak at 615 nm. These results show that the obtained samples exhibit the advantages of polychromatic emission in the visible region and have potential applications in the field of solid illumination.

conclusions
In this study, Tb 3+ doped natural sodium feldspar (NaAlSi 3 O 8 ) phosphors were been successfully prepared by using high-temperature solid-state method with natural sodium feldspar as a substrate. EDX analysis showed that the natural matrix albite in this experiment contained a small amount of C, Ca, and Eu elements except the main elements Na, Al, Si, and O. The percentage of the weight of the Eu element is 0.03%, and the distribution is not uniform. Spectral analysis of phosphors shows that natural albite containing Eu does not form any luminescent centers after heat treatment without Tb 3+ . After doping Tb 3+ , the emission peak 615 nm (Eu 3+ ) increases with the increase of Tb 3+ concentration. The excitation peaks corresponding to Tb 3+ were observed when the Eu 3+ characteristic emission was monitored. According to the above spectral results, the energy transfer from Tb 3+ to Eu 3+ was obtained. The fluorescence lifetime of the sample confirmed the existence of energy transfer between Sample no.