1. Introduction

Laser with low threshold and high slope efficiency has been realized in Nd³⁺:Ca₅(PO₄)₃F (Nd³⁺:FAP) and Yb³⁺:Ca₅(PO₄)₃F (Yb³⁺:FAP) crystals [1,2]. However, the FAP crystal has poor thermomechanical properties and the growth of the single crystal is difficult and costly [3]. Therefore, rare earth doped transparent glass ceramic containing FAP nanocrystals could be a way for overcoming the demerits. This kind of materials, which are obtained from precursor glasses by thermal treatment close to the crystallization temperature, may combine the spectral characteristics of rare earth ions in crystalline environment with the high mechanical and thermal stabilities of host glass [4–7].

Recently, an Nd³⁺-doped transparent oxyfluoride glass ceramic containing Nd³⁺:FAP nanocrystals has been prepared in our lab [8]. All the spectroscopic properties of Nd³⁺-doped FAP glass ceramic are better than those of Nd³⁺-doped glass before thermal treatment, and these crystallization improvements are also better than or at least similar to those of the Nd³⁺-doped lithium aluminosilicate glass ceramic, in which laser operation at 1060 nm has been realized [8,9]. However, the site-selective excitation and absorption spectra of Nd³⁺ in FAP single crystal indicate that the Nd³⁺ ions appear to be distributed among one emitting and one non-emitting sites [10]. The distribution would affect the laser properties of the Nd³⁺-doped FAP glass ceramic. Therefore, a systematical investigation of the local structure and site symmetry of rare earth ions in the glass ceramic containing FAP nanocrystals is important for their further applications.

Nd³⁺ ion has a relatively complicated energy level, which is not suitable for structural analysis [11,12]. Eu³⁺ ion is known to be the most sensitive probe for the local structure [12–14]. The experimental results of Eu³⁺:FAP single crystal indicate that the emitting Eu³⁺ ions reside in the Ca(II) site with C_s symmetry [15]. Site-selective fluorescence studies of Eu³⁺:FAP powders prepared from solution indicate that the emitting Eu³⁺ ions reside in the Ca(I) site with C₃ symmetry [16]; but studies of Eu³⁺:FAP powders prepared by solid state reaction indicate that the emitting Eu³⁺ ions reside in the Ca(II) site with C_s symmetry [16]. Since the sites of Eu³⁺ ions in the FAP lattice are dependent on the conditions of synthesis, it is also interesting to investigate the local structure and site symmetry of Eu³⁺ ions in the glass ceramic containing FAP nanocrystals.

2. Experimental

A 29.4SiO₂ – 18.0Al₂O₃ – 12.0P₂O₃ – 20.0CaCO₃ – 18.0CaF₂ – 0.3La₂O₃ – 0.5Li₂CO₃ – 0.3B₂O₃ – 0.5ZrO₂:1Eu₂O₃ (mol%) precursor glass (denoted as Eu³⁺:PG) was prepared by melting a mixture of reagent grade chemical composition in an alumina crucible at 1450 °C for 2 h in air atmosphere. The melt was poured into a 280 °C preheated copper mold and then cooled down to room temperature naturally. The transparent glass ceramic (denoted as Eu³⁺:GC) was obtained through heating Eu³⁺:PG at 790 °C for 24 h. The sample densities were measured using the buoyancy method based on the Archimedes principle with the distilled water as the immersion liquid. The Eu³⁺ concentrations N can be evaluated by $N=2d{N}_A\rho /M$, where d is the weight percent of Eu₂O₃, N_A is the Avogadro’s constant, ρ is the sample density and M is the molar mass of Eu₂O₃. The Eu³⁺ concentration in Eu³⁺:PG and Eu³⁺:GC were estimated to be about 3.58 × 10²⁰ and 3.69 × 10²⁰ ions/cm³. Two samples with thickness of 1.86 and 1.92 mm were cut from Eu³⁺:PG and Eu³⁺:GC, respectively. All the surfaces of the samples were polished for spectral experiments.

To identify the crystallization phase, an X-ray diffraction (XRD) analysis was carried out with a powder diffractometer (DMAX2500, Rigaku). Sizes and morphologies of the Eu³⁺:GC were measured by a transmission electron microscope (TEM) (JEM-2010, JEOL). TEM specimen was prepared by dispersing fine powder ground from the bulk sample in ethanol, followed by ultrasonic agitation, and then depositing onto a carbon copper grid. Transmission spectra were recorded in a spectrophotometer (Lambda35, Perkin-Elmer) at room temperature. Excitation and emission spectra at 10K were recorded by a spectrometer (FL920, Edinburgh) equipped with a xenon lamp as the excitation source. Detection of the signals was achieved with a photomultiplier tube (R955, Hamamatsu).

3. Results and discussion

Figure 1 displays the XRD pattern of the Eu³⁺:GC. All diffraction peaks matched well with the standard data of hexagonal phase FAP (JCPDS 87-2462). The TEM image of the Eu³⁺:GC shown in Fig. 2 exhibits that the FAP nanocrystals have a 25-40 nm size range among the glassy matrix.

 

Fig. 1 XRD pattern of Eu³⁺:GC, the bars represent the diffraction pattern of the standard hexagonal phase FAP.

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Fig. 2 TEM micrograph of Eu³⁺:GC.

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The photographs of the Eu³⁺:PG and Eu³⁺:GC samples for spectral experiments are shown in the insets of Fig. 3, which display that the Eu³⁺:PG and Eu³⁺:GC are transparent in the visible range. The transmission spectra of Fig. 3 display the transmittances of the Eu³⁺:PG and Eu³⁺:GC reach 81.9% and 80.5%, respectively, at 633 nm without the absorption of Eu³⁺ ions. The transparency loss and the decline of the baseline at short wavelength for the Eu³⁺:GC are similar to the case for Nd³⁺:FAP transparent glass ceramic [8]. Meanwhile, the absorption bands are corresponding to the transitions from the ground multiplet ⁷F₀ to the final multiplets marked in the figure. The absorption for the transition of ⁷F₀→⁵D₀ is too weak to be seen for the Eu³⁺:PG sample, but it is stronger than that for the ⁷F₀→⁵D₁ transition after the crystallization process. Although the ⁷F₀→⁵D₀ transition of Eu³⁺ is forbidden by electric dipole and magnetic dipole mechanisms [17,18], the occurrence of the ⁷F₀→⁵D₀ transition in Eu³⁺:GC can be attributed to the J-mixing effect [19,20].

 

Fig. 3 Room temperature optical transmission spectra for Eu³⁺:PG and Eu³⁺:GC; the insets are the photographs of Eu³⁺:PG and Eu³⁺:GC samples for spectral experiments.

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Site-selective excitation and emission methods were used to identify the local structures of Eu³⁺ ions in the FAP nanocrystals for the GC. The excitation spectra related to the ⁷F₀→⁵D₂ transition of Eu³⁺ions in the GC were recorded at 10 K and are shown in Fig. 4. The monitored wavelengths of 571.8 and 576.4 nm were the positions of the emission peaks for the ⁵D₀→⁷F₀ transition of Eu³⁺ ions at two kinds of sites in the FAP nanocrystals (see Fig. 5). The emission spectra of the Eu³⁺:GC, corresponding to the transitions ⁵D₀→⁷F_0,1,2, were recorded at 10 K and are shown in Fig. 5. The exciting wavelengths of 458.2 and 461.8 nm were the positions of the peaks shown in Fig. 4. The peak positions in both the excitation and emission spectra of the Eu³⁺:GC at 10 K are shown in Table 1. It can be seen from Figs. 4 and 5 and Table 1 that the Eu³⁺ ions in the FAP nanocrystals occupy two types of sites, designated in the following by A and B, respectively.

 

Fig. 4 Excitation spectra of Eu³⁺:GC for emission at 571.8 and 576.4 nm at 10 K.

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Fig. 5 Emission spectra of Eu³⁺:GC under excitation at 458.2 and 461.8 nm at 10 K.

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Table 1. Positions of Excitation and Emission Peaks for Eu³⁺ Ions in FAP Nanocrystals of GC at 10 K

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In Fig. 4, the intensity of the peak emission at 571.8 nm under excitation at 458.2 nm for Eu³⁺ ions at site A is 10 times of that at 576.4 nm under excitation at 461.8 nm for Eu³⁺ ions at site B. In Fig. 5, the peak emission intensity at 571.8 nm corresponding to the ⁵D₀→⁷F₀ transition for Eu³⁺ ions at site A is 8 times of that at 576.4 nm for Eu³⁺ ions at site B. It indicates that site A is probably dominant in the site distributions for Eu³⁺:GC. This is similar to the case in bulk crystal [16]. The splittings of the crystal field sublevels for the ⁵D₀ and ⁷F_0,1,2 multiplets of Eu³⁺ ions in the FAP nanocrystals for Eu³⁺:GC are listed and compared with those of Eu³⁺:FAP single crystal [15] and Eu³⁺:FAP powder prepared by solid state reaction [16] in Table 2. It can be found from the table that the sublevels for Eu³⁺ at site A of Eu³⁺:FAP nanocrystals are similar to those of Eu³⁺:FAP single crystal and Eu³⁺ at site “A” of Eu³⁺:FAP powder. Therefore, the local structure and point symmetry of site A in the Eu³⁺:FAP nanocrystals are Ca(II) site and C_s, respectively [15,16]. And the crystal field may appear in the form of a ligand ion F^- being replaced by O^2- in accordance with the charge compensation scheme: Ca²⁺ + F^-→Eu³⁺ + O^2- [15,16]. In the same way, the sublevels for Eu³⁺ at site B of Eu³⁺:FAP nanocrystals are similar to those of Eu³⁺ at site “C” of Eu³⁺:FAP powder [16]. Therefore, the local structure and point symmetry of site A in the Eu³⁺:FAP nanocrystals are also Ca(II) site and C_s, respectively, but he possible charge compensation scheme is 3Ca²⁺→2Eu³⁺ + Vacancy [16]. The occupation of the site “B” in Eu³⁺:FAP powder does not happen in the Eu³⁺:FAP nanocrystals. The sites “A” and “B” in Eu³⁺:FAP powder exhibit very similar line positions and relative intensities in spectra as well as the same charge compensation scheme [16]. The site “B” is probably deformed from the site “A”, but this deformation may not exist in the Eu³⁺:FAP nanocrystals for GC.

Table 2. Relative Energies of the Crystal Field Sublevels for the ⁵D₀ and ⁷F_0,1,2 Multiplets of Eu³⁺ Ions of the Eu³⁺:FAP Single Crystal, Eu³⁺:FAP Powder Prepared by Solid State Reaction, and Eu³⁺:GC (in unit of cm⁻¹)

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The ⁵D₀→⁷F₂ transition is hypersensitive and the ⁵D₀→⁷F₁ transition is insensitive to site symmetry [21,22]. Therefore, the integrated intensity ratio for the emission bands of the transitions ⁵D₀→⁷F₂ to ⁵D₀→⁷F₁ of Eu³⁺ ions is usually a good measure for the symmetry. The intensity ratios of Eu³⁺ ions at sites A and B in the FAP nanocrystals are 6.03 and 5.77, respectively. The higher intensity ratio, as well as the larger splitting of the ⁷F₂ and ⁷F₁ multiplets shown in Table 2, indicates that the crystal field around Eu³⁺ ions at site A is more deformed and stronger.

It can be found from Fig. 4 that when the emission peak at 576.4 nm related to the ⁵D₀→⁷F₀ transition for Eu³⁺ ions at site B is monitored, the excitation peak at 458.2 nm corresponding to the ⁷F₀→⁵D₂ transition for Eu³⁺ ions at site A also exists. Meanwhile, the peak at 618 nm corresponding to the ⁵D₀→⁷F₂ transition of Eu³⁺ ions at site B can be found in the emission spectrum when Eu³⁺ ions at site A are excited at 458.2 nm to the ⁵D₂ multiplet (see Fig. 5). Therefore, energy transfer from Eu³⁺ at site A to Eu³⁺ at site B exists in the Eu³⁺:FAP nanocrystals. Figure 6 is a schematic of the energy transfer process from Eu³⁺ at site A to Eu³⁺ at site B. Eu³⁺ ions in the ground state are excited to the ⁵D₂ multiplet and then populate the ⁵D₀ multiplet via nonradiative relaxation. It can be found from Table 2 that the energy position of ⁵D₀ multiplet for Eu³⁺ at site A is 140 cm⁻¹ higher than that at site B. Therefore, the energy transfer from Eu³⁺ at site A to Eu³⁺ at site B can occur with emission of phonon. However, the energy transfer from Eu³⁺ at site B to Eu³⁺ at site A with absorption of phonon cannot occur in the Eu³⁺:FAP nanocrystals at 10 K.

 

Fig. 6 Schematic diagram of the energy transfer process from Eu³⁺ at site A to Eu³⁺ at site B of Eu³⁺:GC at 10 K.

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4. Conclusion

An Eu³⁺-doped transparent oxyfluoride glass ceramic containing FAP nanocrystals was prepared by thermal treatment at the crystallization temperature for the precursor glass. The transmittance of Eu³⁺:GC with a thickness of 1.92 mm is up to 80.5% in the visible range at room temperature. Site-selective excitation and emission spectra indicate that Eu³⁺ ions in the FAP nanocrystals occupy two types of sites, A and B, with a same point symmetry C_s. For Eu³⁺ ions at site A, the crystal field is more deformed and stronger. The distribution of Eu³⁺ at site A is probably dominant in the Eu³⁺:GC. The crystal field may appear in the form of a ligand ion F^- being replaced by O^2- in accordance with the charge compensation scheme: Ca²⁺ + F^-→Eu³⁺ + O^2-. As for Eu³⁺ ions at Site B, the possible charge compensation scheme is 3Ca²⁺→2Eu³⁺ + Vacancy. Furthermore, it was experimentally evidenced that an energy transfer process from Eu³⁺ at site A to Eu³⁺ at site B exists in Eu³⁺:GC at 10K.