1. Introduction

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 [1]. 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 [2]. Apatite compounds have been confirmed to be efficient luminescence materials for white LEDs (W-LEDs) [3].

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 [4]. Eu²⁺-doped Sr₅(PO₄)₃Cl and Sr₅(PO₄)₃F are known as blue emitting materials used in highly efficient compact fluorescent lamp [5]. Ba₅(PO₄)₃Cl:Eu²⁺ is a promising material for X-ray imaging [6]. Eu²⁺/Ce³⁺-codoped Ba₅(PO₄)₃Cl is a persistent phosphor [7], while Eu²⁺/Gd³⁺-codoped Ba₅(PO₄)₃Cl and Ba₅(PO₄)₃F are photochromic materials [8]. 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 [11], Ca_10−x(PO₄)₆Cl₂:xEu²⁺ [12], silica/apatite composite [13], and apatite-based colloids [14] because of medical imaging application. Unlike most europium-doped compounds, coexistence of divalent Eu²⁺ and trivalent Eu³⁺ is allowed in apatites [15].

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.

2. Experimental

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 [16].

 

Fig. 1 (a) XRD patterns of M₂Y₃[SiO₄]₃F:Eu (M = Sr, Ba) prepared in air atmosphere (labeled as A) and reducing atmosphere (labeled as R); the patterns were compared with the indexed PDF2 standard card No.15-0876 (Ca₅(PO₄)₃F); (b) schematic view of crystal structure of Ba₂Y₃[SiO₄]₃F along c-direction.

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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 C_s symmetry [18]. 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 C_s, C_n, and C_nv 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.

 

Fig. 2 (a) Emission spectra (λ_ex = 390 nm) M₂Y₃[SiO₄]₃F:Eu³⁺ (M = Sr, Ba) prepared in air atmosphere ; inset is the luminescence decay curve of ⁵D₀→⁷F₂ transitions. The spectra of Ba-A and Sr-A are shown in the upper and lower parts, respectively. (b): CIE chromaticity coordinates of M₂Y₃[SiO₄]₃F:Eu (M = Sr, Ba) prepared in air atmosphere (-A) and in reducing atmosphere (-R).

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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→⁷F_J (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) [20].

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 ⁵D_I→⁷F_J (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 Eu^2+,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.

 

Fig. 3 (a): The emission spectra (λ_ex = 390 nm) of M₂Y₃[SiO₄]₃F:Eu, M = Sr (a), M = Ba (b) prepared in reducing atmosphere. The emission bands were decomposed into two Gaussian components at Eu(I) and Eu(II), W and A denote FWHM and intensity, respectively.

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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 ⁵D_I (I = 1,2,3) states as follows. The emissions at 450-560 nm are due to the ⁵D_1,2,3→⁷F_J 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 [19]. If the doping concentration of Eu³⁺ is high (about 10 mol %) as the case of the present crystal, the ⁵D_1,2,3 emission intensity is quenched by the cross-relaxation between the neighbour Eu³⁺ ions.

The emissions from the ⁵D_I (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 ⁵D_I (I = 1, 2, 3) manifolds, leading to suppression of emission from the ⁵D_I states as the distance of two neighboring Eu³⁺ ions is decreased [21]. 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 ⁵D_I 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 ⁵D_I (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 [15].

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.

 

Fig. 4 The time resolved spectra of M₂Y₃[SiO₄]₃F:Eu, M = Sr (a), M = Ba (b) prepared in reducing atmosphere under excitation of 355 nm of a pulsed Nd:YAG laser with pulsed delay times of 100 μs, 3 μs, and 0.3 μs after the laser excitation.

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It is noted that the emissions due to the ⁵D₀→⁷F_3,4 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→⁷F_3,4 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→⁷F_3,4 emissions are observed in the latter material but not in the former one [22]. 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 [22] that the difference of local symmetry around Eu³⁺ leads to the non-appearance of the ⁵D₀→⁷F_3,4 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:Eu^2+,3+ apatite by Zuev et al [15].

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).

 

Fig. 5 the excitation spectra of Eu³⁺-doped M₂Y₃[SiO₄]₃F M = Sr (a), M = Ba (b) phosphors prepared in air atmosphere (-A) and in reducing atmosphere (-R).

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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 [24], 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.

Table 1. QE values of M₂Y₃[SiO₄]₃F:Eu (M = Sr, Ba) prepared in air atmosphere (A) and in reducing atmosphere (R)

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5. Conclusions

Eu-doped M₂Y₃[SiO_4]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 ⁵D_1,2,3. 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[SiO_4]3F and 450 nm in Ba₂Y₃[SiO_4]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.