Eu-doped fluorosilicate apatites M2Y3[SiO4]3F (M = Sr, Ba) are prepared by solid state reaction. Unlike conventional Eu-doped materials, coexistence of Eu3+ and Eu2+ ions is found from the photoluminescence of Eu-doped apatites M2Y3[SiO4]3F (M = Sr, Ba) which were prepared in reducing atmosphere. Eu2+ ions are converted from Eu3+ ions in the reduction process. It is suggested that Eu2+ ions occupy A(I) (4f) or A(II) (6h) crystallographic site in the apatite lattices. Intense emission lines due to Eu3+ are observed at 600-630 nm, while broad emission band due to Eu2+ is observed at 450-650 nm. These emissions combined with blue emission from LED are suitable to obtain white light, i.e., white LEDs for lighting and display. Different luminescence characteristics are obtained between Sr2Y3[SiO4]3F:Eu and Ba2Y3[SiO4]3F:Eu, which were prepared in reducing atmosphere.
© 2014 Optical Society of America
Apatites have a large family of compounds with general formula M10(TO4)6X2 in which M is a mono-, di- or trivalent cation, TO4 a trivalent anion group, and X a monovalent anion . Its structure is hexagonal belonging to space group P63/m, with a single tetrahedral site (TO4) and two non-equivalent sites for the ten metallic ions (M). Apatite structure is a compositionally flexible network and can be widely modified by substitutions on cationic and/or anionic sites . Apatite compounds have been confirmed to be efficient luminescence materials for white LEDs (W-LEDs) .
Of various apatite compounds, Eu2+-doped alkaline earth halo-apatites are efficient blue-emitting phosphors, which have been used in tri-colour fluorescent lamps and LEDs . Eu2+-doped Sr5(PO4)3Cl and Sr5(PO4)3F are known as blue emitting materials used in highly efficient compact fluorescent lamp . Ba5(PO4)3Cl:Eu2+ is a promising material for X-ray imaging . Eu2+/Ce3+-codoped Ba5(PO4)3Cl is a persistent phosphor , while Eu2+/Gd3+-codoped Ba5(PO4)3Cl and Ba5(PO4)3F are photochromic materials . Eu3+ luminescence of apatites has been applied to investigate microstructure of fluoro-apatite [9,10]. Recently an attention was paid to Bi3+/Eu3+-codoped Ca10(PO4)6F2 , Ca10−x(PO4)6Cl2:xEu2+ , silica/apatite composite , and apatite-based colloids  because of medical imaging application. Unlike most europium-doped compounds, coexistence of divalent Eu2+ and trivalent Eu3+ is allowed in apatites .
In the present work, Eu3+-doped apatites M2Y3[SiO4]3F (M = Sr, Ba) are prepared by solid state reaction in both air and reducing atmospheres. The photoluminescence (PL) spectra, PL excitation (PLE) spectra, and the PL decay curves are investigated. It is found that different luminescence properties are obtained by growing in different atmospheres.
The powder samples of 10 mol% Eu3+-doped M2Y3[SiO4]3F (M = Sr, Ba) were prepared by conventional solid state synthesis. The raw materials are high-purity MCO3 (M = Sr, Ba), SiO2, MF2 (M = Sr, Ba), Y2O3, and Eu2O3. The excess of MF2 (M = Sr, Ba) was added in each of the reaction chemicals. Firstly the mixture was ground together in an agate mortar for enough time, which was then heated up to 850 °C and kept at this temperature for 2-4 h. After a second homogenization in the mortar, the samples were heated up to1250 °C for 5-10 h in crucibles with cover; four samples were obtained by finally heating in air (noted as A sample) and in reducing atmosphere (noted as R sample), i.e., Sr2Y3[SiO4]3F:Eu3+ (Sr-A), Sr2Y3[SiO4]3F:Eu2+ (Sr-R), Ba2Y3[SiO4]3F:Eu3+ (Ba-A), Ba2Y3[SiO4]3F:Eu2+ (Ba-R).
The crystal structure was checked by X-ray diffraction (XRD) analysis collected on a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Bragg–Brentano geometry using CuKα radiation (λ = 1.5405 Å). The PL and PLE spectra were recorded on a Perkin-Elmer LS-50B luminescence spectrometer. The quantum efficiencies (QE) were measured by Edinburgh Instruments FS-920 spectrometer equipped with an Edinburgh instruments integrating sphere, where the monochromator is connected with CCD sensor and a computer by light guides. QE values were calculated using a quantum-yield software.
3. Results and discussions
3.1 Crystal phase formation
Figure 1(a) shows the XRD patterns of M2Y3[SiO4]3F:Eu (M = Sr, Ba) prepared in air atmosphere (noted -A) and in reducing atmosphere (noted -R). The sharp diffraction peaks suggest a good crystalline. The XRD patterns are well indexed with PDF2 standard card No.15-0876 (Ca5(PO4)3F), indicating that all the samples have the crystal structure with space group of P63/m .
The crystal structure of Ba2Y3[SiO4]3F is shown in Fig. 1(b). M2Y3[SiO4]3F has two non-equivalent crystallographic sites for M and Y ions, i.e., A(I) in a column at z = 0 and 3/4, and A(II) in a screw axis at z = l/4 and 3/4 (see Fig. 1(b)) [17,18]. The A(I) site, 4(f) Wyckoff position, lies on ternary axes and is nine-fold coordinated with C3 symmetry, while the A(II) site, 6(h) Wyckoff position, is seven-fold coordinated with Cs symmetry . Eu3+ ions doped in apatite occupy mainly A(II) sites [17–19].
3.2 M2Y3[SiO4]3F:Eu (M = Sr, Ba) prepared in air atmosphere
Figure 2(a) presents the spectra of M2Y3[SiO4]3F:Eu (M = Sr, Ba) prepared in air atmosphere, which are excited at 390 nm (7F0 to 5L6 state of Eu3+). Unlike some apatites [17,18], the peak of the emission line due to the 5D0→7F0 transition at 577.5 nm is lower than those due to the 5D0→7F2 transition at 618 nm. The 5D0→7F0 transition is strictly forbidden by the selection rule of J = 0→J = 0. However, it is allowed for Eu3+ with Cs, Cn, and Cnv site symmetries. Therefore the observation of the 5D0→7F0 emission line indicates that Eu3+ ions occupy the sites with low symmetry and without an inversion center. This character is confirmed from the long decay times as shown inset Fig. 2(a). The decay lifetimes are estimated as 1350 and 1250 μs for Eu3+-doped Sr2Y3[SiO4]3F and Ba2Y3[SiO4]3F, respectively.
The luminescence of M2Y3[SiO4]3F:Eu (M = Sr,Ba) is different from the spectra of previously reported apatites as mentioned above. This might be due to the different occupancy of Eu3+ in the lattices. Eu3+ ions in M2Y3[SiO4]3F are suggested to substitute Y3+ ions because of the same valence, which occupy statistically on both A(I) and A(II) sites (Fig. 1(b)). The disordered occupancy of Eu3+ in these sites leads to the most intense intensity for the 618 nm emission due to the transition 5D0→7F2 (electric dipole), compared with emissions due to transition 5D0→7FJ (J = 1, 3, 4). This is favorite for color purity as a red-emitting phosphor. The CIE chromaticity coordinates of M2Y3[SiO4]3F:Eu3+ (M = Sr, Ba) are about (x = 0.636, y = 0.339) in the red region as shown in Fig. 2(b). This values are closer to the standard of NTSC (x = 0.67, y = 0.33) than those of a commercialized Y2O2S:Eu3+ (x = 0.622, y = 0.351) .
3.3 M2Y3[SiO4]3F:Eu (M = Sr, Ba) prepared in reducing atmosphere
Figure 3 presents the PL spectra of M2Y3[SiO4]3F:Eu (M = Sr, Ba) prepared in reducing atmosphere. The spectra consist of sharp lines superimposed on a broad band. The sharp lines are attributed to the 5DI→7FJ (I = 0, 1, 2, 3 and J = 0, 1, 2, 3) transitions of Eu3+, while the broad band is attributed to the 4f65d1→4f7 transition of Eu2+. Because, unlike 4f-4f transitions occurred in inner shell, the 5d orbital in the outer shell are sensitive to the surrounding of Eu2+ ion. The CIE coordinates for Eu2+,3+ doped M2Y3[SiO4]3F are shown in Fig. 2(b). They are in the orange zone. This is due to the emissions from Eu3+ and Eu2+ ions.
Sr2Y3[SiO4]3F:Eu exhibits several emission lines at 570-700 nm which are due to the transitions from the 5D0 state of Eu3+, while the weak emission lines at 400-550 nm are attributed to the transitions from the 5DI (I = 1,2,3) states as follows. The emissions at 450-560 nm are due to the 5D1,2,3→7FJ transitions, i.e.,5D3 →7F2 (434 nm), 5D2→7F0 (468 nm), 5D2→7F2 (496 nm), 5D2→7F3 (514 nm), 5D1→7F1 (539 nm), 5D1→7F2 (555 nm), 5D1→7F3 (583 nm), and 5D1→7F4 (595 nm). Usually the emission from higher excited states 5D1, 5D2, and 5D3 of Eu3+ depends critically on the concentration of Eu3+ and the vibration frequencies of the host . If the doping concentration of Eu3+ is high (about 10 mol %) as the case of the present crystal, the 5D1,2,3 emission intensity is quenched by the cross-relaxation between the neighbour Eu3+ ions.
The emissions from the 5DI (I = 1,2,3) states are missing in Ba2Y3[SiO4]3F. This will be understood by difference of the neighboring environment of Eu3+ ions in Ba and Sr samples as follows. According to Tallant et al, interactions involving one activator in the 5D3, 5D2 or 5D1 state and another in the 7F0 ground state affect the kinetics of the relaxation of the upper 5DI (I = 1, 2, 3) manifolds, leading to suppression of emission from the 5DI states as the distance of two neighboring Eu3+ ions is decreased . Taking into account this result, we suggest that difference of the Eu3+-Eu3+ distance between Ba2Y3[SiO4]3F and Sr2Y3[SiO4]3F would lead to difference of the 5DI emissions between the two crystals although the exact distance isunknown. The distance will be examined in near future. As a result the emissions from the 5DI (I = 1, 2, 3) states are not observed in Ba2Y3[SiO4]3F.
The broad band due to Eu2+ is asymmetric. This band is decomposed to two Gaussian bands with maxima at 485nm (named Eu(I)2+ band) and 575nm (Eu(II)2+ band) in Sr-R sample, while at 460 and 540 nm in Ba-R sample. The Eu(II)2+ band is dominated in the two samples. A(II) site is more distorted than A(I) site. The bond-length of A(II)-O (2.4401Å) is shorter than that of A(I)-O (2.6076Å). Meanwhile, A(II) is coordinated by an F-anion, bringing a higher degree of covalency. Therefore the average covalency will be higher for the A(II) site, indicating that A(II) site is more stable for Eu2+ than A(I) site. As a result, taking into account the splitting into two components Eu(I)2+ and Eu(II)2+ by the distorted crystal field, the broad emission band of M2Y3[SiO4]3F is suggested to arise from Eu2+ ions on A(II) sites. This assignment agrees with the previous one for the luminescence observed in Eu2+-doped apatite .
We synthesized the apatites using various methods, e.g., heated in the reducing atmosphere of (N2 + H2) flow. All the times we confirmed the coexistence of Eu3+ and Eu2+ ions. This indicates that the two ions are stable in M2Y3[SiO4]3F:Eu (M = Sr, Ba).
As shown in the spectra in Fig. 3, M2Y3[SiO4]3F:Eu (M = Sr, Ba) contains two kinds of luminescence centres (Eu2+ and Eu3+). The time resolved spectra were measured to separate Eu2+ from Eu3+ ions. Under the time delay 0.3 μs, the dominated emissions are from Eu2+ ions as shown in Figs. 4(a) and 4(b). Enhancement of Eu3+ emission is observed with the decrease of Eu2+ emission at a long delay time. At the delay time of 100 μs, Eu2+ emission cannot be detected.
It is noted that the emissions due to the 5D0→7F3,4 transitions at 650-700 nm are observed in Sr-R sample but missing in Ba-R sample although the two samples have almost the same crystal structure (Figs. 3 and 4). Such a missing of the 5D0→7F3,4 emissions has been observed in several materials [22,23]. For example, Eu3+ emissions are studied by doping in the hybrid mesoporous materials Eu-SUASi-SBA-15 and phen-Eu-SUASi-SBA-15, where the 5D0→7F3,4 emissions are observed in the latter material but not in the former one . Here 1,10-phenanthroline, SBA-15, SUA, and APS mean mesoporous silica, 5-sulfosalicylic acid, and 3-aminopropyltrimethoxysilane, respectively. In this case, the introduction of phen gives influence on the luminescent property within the hybrid systems. Additionally it was found in  that the difference of local symmetry around Eu3+ leads to the non-appearance of the 5D0→7F3,4 emissions. This indicates that the reduced atmosphere has changed the local symmetry around Eu3+ differently between Sr-R and Ba-R samples.
Eu3+ ions, which had occupied the A(I) or A(II) sites in M2Y3[SiO4]3F:Eu (M = Sr, Ba) before the reduction process, are partially converted to Eu2+ ions in reducing atmosphere. Simultaneously oxygen vacancies are created in this non-stoichiometric process. On the other hand, Eu2+ ions are partially oxidized as Eu2+→Eu3+ + e-. As a result, both Eu2+ and Eu3+ ions coexist in reducing atmosphere. The substitution of Eu2+ by Eu3+ leads to creation of negative vacancy for the compensation in Sr-R and Ba-R samples as suggested in Sr2Y8(SiO4)6O2:Eu2+,3+ apatite by Zuev et al .
3.4 The properties for luminescence application
The PL excitation (PLE) spectra of Eu-doped M2Y3[SiO4]3F (M = Sr, Ba) phosphors prepared in air atmosphere (-A) and in reducing atmosphere (-R) are displayed in Fig. 5.The PLE spectra for 618 nm emission due to the 5D0→7F2 emission of Eu3+ in A-samples consist of a broad band and several lines. The broad excitation band with maximum at 265 nm can be attributed to the CT band of Eu3+-O2−. In the range from 350 to 500 nm, A-samples present sharp lines due to the 4f–4f transitions of Eu3+ at 394 nm (7F0→5L6) and 464 nm (7F0→5D2).
The PLE spectra for 550 nm emission of Eu2+ in Sr-R and Ba-R samples consist of two broad bands, whose peaks are at about 350 and 300 nm for Sr-R and at 420 and 300 nm for Ba-R. Taking into account that Eu2+ has two broad absorption bands , the broad bands are attributable to the 4f→5d transitions in Eu2+. The PLE spectrum for 618 nm emission of Eu3+ in Sr-R (Fig. 5 (a)) consists of two broad bands with maxima at 350 and 265 nm and several sharp lines due to Eu3+. The 265 nm band is the same as the 265 nm CT band observed in Sr-A. Since the 350 nm PLE band is located at almost same position as the PLE band for Eu2+ emission in Sr-R, it is attributable to Eu2+. This indicates that energy transfer from Eu2+ to Eu3+ occurs in Sr-R. The PLE spectrum for 618 nm Eu3+ emission in Ba-R (Fig. 5 (b)) consists of two broad bands with maxima at about 420 and 300 nm and several sharp lines due to Eu3+. Since the two broad bands are located close to the two PLE bands for 550 nm Eu2+ emission in Ba-R, they are attributable to Eu2+, indicating that the energy transfer from Eu2+ to Eu3+ also occurs in Ba-R.
Figure 5 indicates that all the samples have intense broad excitation bands and sharp lines in the near UV-blue range. This matches well with the output wavelength of near-UV or blue LED chips in phosphor-converted W-LEDs. The absolute quantum efficiency (QE) in the emission range from blue to red is summarized for all the samples in Table 1.The maximum QE value of Sr2Y3[SiO4]3F:Eu was measured to be 33.5% (under excitation at 380 nm) at room temperature. From these results, this phosphor is expected to be useful as W-LED material for lighting and display.
Eu-doped M2Y3[SiO4]3F (M = Sr, Ba) were prepared via a solid state reaction method by finally heating in air or in reducing atmosphere. M2Y3[SiO4]3F:Eu (M = Sr, Ba) prepared in air atmosphere exhibits the emission from Eu3+ ions with intense red emission due to the 5D0→7F2 transition. The conversion of Eu3+ to Eu2+ cannot be fully achieved in M2Y3[SiO4]3F (M = Sr, Ba) host prepared in reducing atmosphere. The Eu3+ ions in Sr2Y3[SiO4]3F prepared in reducing atmosphere show emission lines from the 5D0 state as well as from upper states of 5D1,2,3. However, only emission lines from the 5D0 state are observed in reduced Ba2Y3[SiO4]3F:Eu. Eu2+ ions give a broad band in a spectral range from blue to red and two broad excitation bands below 400 nm in Sr2Y3[SiO4]3F and 450 nm in Ba2Y3[SiO4]3F. These excitation bands well match with the output wavelength of near-UV or blue LED chips which are currently used in phosphor-converted W-LEDs. The maximum QE of Sr2Y3[SiO4]3F:Eu was measured to be 33.5% (under excitation at 380 nm) at room temperature. This material is suggested to be a candidate as phosphor for W-LED to be used in lighting and display.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013-R1A1A2009154).
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