Optical Properties of Rare Earth Doped Transparent Oxyfluoride Glass Ceramics

Radiation Effects & Defects in Solids, 2003, Vol. 158, pp. 457–462

OPTICAL PROPERTIES OF RARE EARTH DOPED TRANSPARENT OXYFLUORIDE GLASS CERAMICS J. ME´NDEZ-RAMOSa, V. LAVI´Na, I. R. MARTI´Na, U. R. RODRI´GUEZ-MENDOZAa, ˜ EZb ´N V. D. RODRI´GUEZa,*, A. D. LOZANO-GORRI´Nb and P. NU a

Dpto. de Fisica Fundamental y Experimental, Electro´nica y Sistemas, Universidad La Laguna, 38206 La Laguna, Tenerife, Spain; bDpto. de Quı´mica Inorga´nica Universidad La Laguna, 38206 La Laguna, Tenerife, Spain

Optical properties of Eu3þ ions in oxyfluoride glasses and glass ceramics doped with low concentration (0.1 mol%) have been analysed and compared with previous results for high concentrated samples (2.5 mol%). The Eu3þ ions in the low dopant concentration glass ceramics are diluted into like crystalline environments with higher symmetry and lower coupled phonons energy than in the precursor glasses. Fluorescence line narrowing measurements indicate the presence of two main fluoride site distributions for the Eu3þ ions in these low concentrated glass ceramics. Keywords: Glass ceramics; Rare earths; Eu3þ; Fluorescence line narrowing; Site distribution

1

INTRODUCTION

A lot of work in different crystals and glasses doped with rare earth ions has been motivated by the interest in finding host materials for active optical devices, such as lasers and optical amplifiers, based in the optical transitions of these ions [1–3]. In the last decade a new kind of transparent materials has created much interest, these are the oxyfluoride glass ceramics. In these materials, fluoride nanocrystals are embedded in an oxide glass matrix, thus, when the rare earth ions are incorporated into the crystalline environment, the desirable optical properties of the fluoride hosts are combined with the elaboration and manipulation advantages of oxide glasses [4]. The optical properties of the rare earth ions in glass ceramics and, therefore, their interest for optical devices are ruled by the final environment of these ions. Most works in oxyfluoride glass ceramics suggest a PbxCd1
xF2 structure for the nanocrystals present in these samples, which is observed from X-ray diffraction patterns. Moreover, it has been assumed that the rare earth ions reside as dopants in these nanocrystals. However, it is difficult to conclude where the rare earth ions stay after the ceramming process [5, 6]. In this sense, it is useful to work with Eu3þ ions. They have been widely used as probes to investigate the local structure around rare earth ions in solid matrices. This is because the electronic transitions between the 7F0 and 5D0 non-degenerate levels are specially * Corresponding author.

ISSN 1042-0150 print; ISSN 1029-4953 online # 2003 Taylor & Francis Ltd DOI: 10.1080=1042015021000052160

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adequate to apply the fluorescence line narrowing (FLN) technique to analyse changes in the energy level diagram, lifetime, linewidth and energy transfer processes between ions in sites close in energy but spectrally different [7]. In a previous work [8], the present authors showed that the Eu3þ ions tend to form EuF3 clusters during thermal treatments in oxyfluoride glasses doped with 2.5 mol% of these ions. However, if the preparation of the samples is changed, the environment of the rare earth ions could also change. In this work, a study for low dopant concentration has been carried out in order to characterize the obtained glass ceramics and compare with the previous results for more concentrated samples.

2

EXPERIMENTAL

The samples used in this study were prepared with the following starting composition in mol%: 30 SiO2, 15 Al2O3, 29 CdF2, 22 PbF2, 3.9 YF3 and 0.1 EuF3; the composition of the matrix is similar to the ones used in references 4 and 9. The glasses were obtained by melting these mixed components at 1050  C for 2 hours and finally casting the melt into a slab on a stainless steel plate at room temperature. The transparent oxyfluoride glass ceramics were obtained just by thermal treatment of the glasses to precipitate nanocrystallites at 470  C for 36 hours. For comparison, some results for the same oxyfluoride composition, but doped with 2.5 mol% of Eu3þ [8], and for fluorozirconate [10] and calcium diborate [11] glasses are included. Broadband emission spectra were obtained by exciting the samples with light from a 250 W incandescent lamp passed through a 0.25 m monochromator. Fluorescence was detected through a 0.25 m double monochromator with a photomultiplier. The spectra were corrected by the instrumental response. For fluorescence line narrowing measurements a tunable dye laser was used as excitation source. The dye laser was pumped by the 532 nm light from a doubled Nd:YAG laser. For measurements at 13 K a helium continuous flow cryostat was used.

3 3.1

RESULTS AND DISCUSSION Broad Band Spectra

The changes in the Eu3þ environment in the oxyfluoride glass to glass ceramic transition can be monitored by optical spectroscopy. For this purpose, the emission spectra of the Eu3þ ions in the precursor glasses and in the glass ceramics have been obtained by exciting at 395 nm the 7 F0!5L6 transition and are presented in Figure 1, along with the energy level diagram of the Eu3þ ions. As a general feature, it can be observed that in the glass ceramics the emission bands become sharper and the components are better resolved for each transition, as expected for Eu3þ ions in a like crystalline phase. Moreover, X-ray diffraction measurements confirm that cubic fluoride nanocrystallites were successfully precipitated during thermal treatment. There are also important changes in the relative intensity of the different transitions. It is especially interesting the variation in the ratio of the hypersensitive electric dipole 5D0!7F2 transition to the magnetic dipole 5D0!7F1 transition (EMIR), which can be considered a measurement of how much close is the Eu3þ ion local environment to be centrosymmetric, for which the odd crystal field hamiltonian is null [12]. From the results for the EMIR for

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FIGURE 1 Emission spectra of Eu3þ ions in oxyfluoride glasses (G) and glass ceramics (GC) obtained by exciting at 395 nm the 7F0!5L6 transition at room temperature in 0.1 and 2.5 mol% doped samples. Spectra are normalized to the maximum intensity of the 5D0!7F1 transition. Energy level diagram of Eu3þ ions is also included.

the low doped samples, about 1.5 in the glass and about 0.6 in the glass ceramic, it can be concluded that the local symmetry for the Eu3þ ions is higher in the glass ceramic. As it can be seen in Figure 1, in addition to the emissions corresponding to the 5D0!7F1 transitions in the red region of the spectrum, blue and green fluorescence from the upper lying 5D1, 5D2, and 5D3 levels are also obtained in the 0.1 mol% Eu3þ samples. These emissions, which are observed in the glass sample, are strongly enhanced by the ceramming process. This result indicates that the Eu3þ ions in the glass ceramic are coupled to phonon modes with lower energies than in the glass. Moreover, the increase of these emissions in the glass to glass ceramic transition indicates that the Eu3þ ions are well dispersed and diluted into low phonon energy nanocrystals, without clustering. On the contrary, in glasses and glass ceramics with 2.5 mol% of Eu3þ, emissions from the 5D1–3 levels were not observed due to very efficient energy transfer, favoured for the shorter distances between the Eu3þ ions in these samples [8]. 3.2

Site Selective Excitation

Laser excitation at the pure electronic band (PEB) of the 7F0!5D0 transition allows to study selectively different sets of Eu3þ ions. In oxyfluoride glasses doped with 2.5 mol% of Eu3þ fluorescence line narrowing and dependence on the excitation wavelength were observed in the emission from the 5D0 level after selective laser excitation along the PEB, see inset in Figure 2. However, in the glass ceramics obtained from these glasses the fast energy migration between Eu3þ ions, due to the short distances between these ions in the EuF3 nanocrystals, prevents the observation of site selective emission [8]. In the 0.1 mol% of Eu3þ doped glass ceramics, the emission spectrum changes appreciably with the laser excitation wavelength, as can be observed in Figure 2 for the 5D0!7F1 transition. This is in agreement with the above-proposed idea of the partition of the Eu3þ ions into nanocrystals after the ceramming process without the formation of clusters. It would be remarked that this behaviour is opposite to that found for the 2.5 mol% Eu3þ glass ceramics [8]. In Figure 2, the inhomogeneous emission obtained by exciting at the phonon side band

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FIGURE 2 Site selective emission spectra of Eu3þ ions from the 5D0 level under selective excitation at the indicated wavelengths (in nm) in the oxyfluoride glass ceramic doped with 0.1 mol% at 13 K. Spectra are normalized to the maximum intensity of the high energy peak of the 5D0!7F1 transition. The spectrum obtained exciting within the phonon side band (PSB) is also included. Inset shows the excitation spectra of the 7F0!5D0 transition in the oxyfluoride glass doped with 2.5 mol% of Eu3þ ions at 13 K [8].

(PSB) of the 5D0!7F0 transition at about 565 nm, where site selection is not expected [10, 13], is also included. In the 5D0!7F1 emission spectra, in Figure 2, more than three peaks are observed for excitation wavelengths up to 578 nm. It would be considered that the crystal field does not split the non-degenerated 5D0 level and can split the 7F1 level at most in three Stark components. Then it would be concluded that Eu3þ ions in different sites are simultaneously excited at near every laser excitation wavelength. In order to study the possible environments for the Eu3þ ions in this glass ceramic, the energy of the 7F1 Stark levels obtained from the emission spectra are presented in Figure 3. The emissions in the high and low energy region of the spectra are more easily analysed than the intermediate ones. It is remarkable the double peak in the high energy side of the spectra (the lowest Stark levels in Fig. 3) along nearly the whole excitation range, which clearly indicates the existence of two site distributions for the Eu3þ ions in this glass ceramics, hereafter referred as Site I and Site II. In Figure 3 the 7F1 Stark levels in a fluoride [10] and an oxide [11] glass have also been included for comparison. It is found that the results for the glass ceramic are more similar to the results for the fluoride glass, indicating that in the two site distributions proposed the Eu3þ ions are in fluoride environments. In the spectra of Figure 2 corresponding to the lowest excitation wavelengths, three peaks are observed similar to those in the spectrum obtained by exciting at the PSB, this indicates that for these low excitation wavelengths non selective excitation occurs due to the overlapping of the PSB and the PEB of the 7F0!5D0 transition. Furthermore, the existence of two main site distributions for the Eu3þ ions in the low concentrated glass ceramics is confirmed by excitation spectra of the 7F0!5D0 transition under selective detection in the high-energy side of the 5D0!7F1 emission spectra. Some selected

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FIGURE 3 7F1 Stark levels as a function of excitation wavelength obtained by selective laser excitation in an oxyfluoride glass ceramic doped with 0.1 mol% of Eu3þ. Data corresponding to oxyfluoride, fluoride and oxide glasses are also shown for comparison [8, 10, 11].

FIGURE 4 Excitation spectra of the 7F0!5D0 transition in the oxyfluoride glass ceramic doped with 0.1 mol% of Eu3þ ions at 13 K by selective detection at the indicated wavelengths (in nm). The position of the maxima of these excitation spectra are shown on the right side of the Figure.

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spectra and the position of the excitation peaks are shown in Figure 4. Two peaks can be clearly observed throughout the studied detection range, except for detection wavelengths over 588 nm where an increasingly intense and broad peak appears at 578 nm, which corresponds to inhomogeneous excitation due to loss of selectivity in the detection for this range. The lowest 7F1 Stark level of the Eu3þ ions in the precursor glass has also been included in Figure 3 for different excitation wavelengths, only one site distribution is observed for this sample. It seems that the Eu3þ ions Site I observed in the glass ceramics is similar to that present in the precursor glass, whereas the Site II would be obtained by the ceramming process.

4

CONCLUSIONS

In oxyfluoride glass ceramics doped with 0.1 mol% of Eu3þ ions, optical spectroscopy indicates that the rare earth ions are well partitioned into low phonon energy nanocrystals without clustering. Two site distributions for the Eu3þ ions corresponding to fluoride environments have been found through fluorescence line narrowing measurements. One of them is similar to the distribution found in the precursor glass. These results are different to those previously found in high concentrated samples (2.5 mol%) where clusters of EuF3 were observed in the glass ceramics. Acknowledgments We would like to thank ‘Gobierno Auto´nomo de Canarias’ (PI 2001=048), ‘Comisio´n Interministerial de Ciencia y Tecnologı´a’ (MAT 2001-3363) and ‘Ministerio de Educacio´n, Cultura y Deportes’ (Beca FPU: AP2000-1801) for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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