F Charpentier er al./ Materials Letters 101(2013)21-24 920 nm and an OPo (pumped at 832 nm by Ti: Sapphire laser) for SbS3] trigonal pyramids even if the EXAFS fit for Sb and Ga K-edge umping at 1300 nm were used to excite a 20 cm long Dyt doped are not absolutely perfect. In detail, the raman spectrum of the gla Ga-Ge-Sb-S(Se)fibers and bulk glasses. A homemade Tm: YAG is dominated by a broad band from 250 to 450 cm", which is formed laser pumped the F2 level of Pr+: Ga-Ge-Sb-S(Se)fibers and by an overlap of several bands as the symmetric stretching modes of bulk glasses absorbing around 2 um. The mid-IR light focused [GeSaz] Td centered at 342 cm". Gasap] Td at 320 cm"or [Sbs3p] the monochromator slit with a Sapphire lens was detected by a trigonal pyramids at 293 cm. The band located at 433 cm reveals cooled InSb detector. The structure of Ga-Ge-SbS(Se)glasses were that (Gesapl Td are mainly connected by their corners. The presence investigated using an HR800(Horiba-Jobin-Yvon) micro-Raman of a weak band at 476cm-l, attributed to the presence of S-s bonds spectrophotometer with 785 nm laser diode. Extended X-ray may explain the presence of a weak band at 269 cm-associated absorption fine structure(EXAFS)measurements were performed with the presence of homopolar bonds( S3 Ga-GaS3)[23]. In the case on Dy+: Gas Gez0Sb10S65 glasses. EXAFS of powdered glass and of selenide glasses, structural entities are similar according to raman several crystalline compounds, ie Dy2 S3. Ga2S3, Ba Ga2S4 CaGa2S4, spectroscopy although the homopolar (Se-Se and Ge-Ge)bonds Sb2S3, GeS2 were recorded at K-edge of Ge, Ga, Sb and Lur-edge of proportion and the presence of Td linked by edges appear to be Dy-at SAMBA beamline at SoLEIL The EXAFS measurements were greater [10. The EXAFS studies on pure and re doped selenide recorded in transmission or fluorescence modes at room tempera- glasses will be carried out to support this hypothesis. The [GaS(Se)a ture except for Ga K-edge and Dy Liur-edge for which a liquid N2 Td in stoichiometric glass are proposed to be distributed in the glass cryostat was used, respectively. Raw intensities were converted network to enable a good dispersion of REs forming bridges Ga-S into xk) curves by the viper program [14. Coordination numbers, (Se-Re to offset the partial negative charge of Gas(Se)4 Td [24, 25 bond lengths and Debye-Waller factors were obtained by fitting to the extent that the ga cone n is greater at least by an order the xk) curves with the Viper program. Backscattering amplitudes of magnitude with respect to RE ions. and phases needed to calculate the model curves were obtained by In sulfide glasses, our EXAFS results suggest that Dy+ is the Feff program [15 surrounded by 7.9+0.5 sulfur atoms for a concentration of 1 wt of Dy+( Fig. 1)whereas for a lower concentration(0.1 wt%) more representative for the doping of optical fibers, the coordina 3. Results and discussion tion number appears to be lower, 6.3+0.5. It is reasonable to Rare earth custering is expected to influence fluorescen efficiency by energy transfers Even if the Re concentration is 0.04 Purged set up relatively low, the clustering effect on fluorescence can occur Unpurged set up depending on matrix and rare earth [16]. For crystalline compounds, 3 it is well known that MGa2S4(M=Ca, Sr or Ba)thiogallates 0.03 of rare earths. Gallium atoms are in the center of tetrahedra(td linked by corners, for example [17]. In the case of CaGa2S4, the rE ions can occupy a single type of sites with a coordination number of ight. In the case of chalcogenide glasses, the presence of gallium facilitates the incorporation and dispersion of RE [18, 19 and the ri ions seem to be surrounded by x7 sulfur atoms [20, 21]. The role of o the gat ion in glassy matrix based on Ges2 can probably be 80.01 pared to that played by the apt ion in silicate glass the ionic-covalent nature of bonds and crystal field around the H12H132 ons are not equivalent for these two matrices. Adding this element 000 enables to insert more rE in the glass while limiting the formation of 40004100420043004400450046004700 aggregates by the formation of RE-Al complexes [ 22]. In the case of GasGezoSb1oS65 glasses, the Raman and EXAFS studies confirm that wavelength [nm glassy network is mainly composed of [Gesa and Gasa Td and Fig. 2. Fluorescence spectra of Dy+: GaGeSbS fiber(0.1 wt%) 71250661%coy Gas Gez0 Sb10 Se65 Dy L edge EXAFS data =00137A b3四≥0cooo3o Gas Ge20 Sb10 S6 00051.015202530354045505560 r闪A Wavenumber [cm-1 Fig. 1.(a) Radial distribution function curves of Dy ions in Gas Gezo Sb1oSas(phase shifts not corrected) and results of the fitting of the first coordination shell around Dy+920 nm and an OPO (pumped at 832 nm by Ti:Sapphire laser) for pumping at 1300 nm were used to excite a 20 cm long Dy3þ doped Ga–Ge–Sb–S(Se) fibers and bulk glasses. A homemade Tm:YAG laser pumped the 3 F2 level of Pr3þ:Ga–Ge–Sb–S(Se) fibers and bulk glasses absorbing around 2 mm. The mid-IR light focused on the monochromator slit with a Sapphire lens was detected by a cooled InSb detector. The structure of Ga–Ge–SbS(Se) glasses were investigated using an HR800 (Horiba–Jobin-Yvon) micro-Raman spectrophotometer with 785 nm laser diode. Extended X-ray absorption fine structure (EXAFS) measurements were performed on Dy3þ:Ga5Ge20Sb10S65 glasses. EXAFS of powdered glass and several crystalline compounds, i.e., Dy2S3, Ga2S3, BaGa2S4, CaGa2S4, Sb2S3, GeS2 were recorded at K-edge of Ge, Ga, Sb and LIII-edge of Dy-at SAMBA beamline at SOLEIL. The EXAFS measurements were recorded in transmission or fluorescence modes at room tempera￾ture except for Ga K-edge and Dy LIII-edge for which a liquid N2 cryostat was used, respectively. Raw intensities were converted into χ(k) curves by the Viper program [14]. Coordination numbers, bond lengths and Debye–Waller factors were obtained by fitting the χ(k) curves with the Viper program. Backscattering amplitudes and phases needed to calculate the model curves were obtained by the Feff program [15]. 3. Results and discussion Rare earth clustering is expected to influence fluorescence efficiency by energy transfers Even if the RE concentration is relatively low, the clustering effect on fluorescence can occur depending on matrix and rare earth [16]. For crystalline compounds, it is well known that MGa2S4 (M¼Ca, Sr or Ba) thiogallates compounds have characteristics quite favorable for the fluorescence of rare earths. Gallium atoms are in the center of tetrahedra (Td) linked by corners, for example [17]. In the case of CaGa2S4, the RE ions can occupy a single type of sites with a coordination number of eight. In the case of chalcogenide glasses, the presence of gallium facilitates the incorporation and dispersion of RE [18,19] and the RE ions seem to be surrounded by ∼7 sulfur atoms [20,21]. The role of the Ga3þ ion in glassy matrix based on GeS2 can probably be compared to that played by the Al3þ ion in silicate glasses although the ionic–covalent nature of bonds and crystal field around the RE ions are not equivalent for these two matrices. Adding this element enables to insert more RE in the glass while limiting the formation of aggregates by the formation of RE-Al complexes [22]. In the case of Ga5Ge20Sb10S65 glasses, the Raman and EXAFS studies confirm that glassy network is mainly composed of [GeS4] and [GaS4] Td and [SbS3] trigonal pyramids even if the EXAFS fit for Sb and Ga K-edge are not absolutely perfect. In detail, the Raman spectrum of the glass is dominated by a broad band from 250 to 450 cm−1 , which is formed by an overlap of several bands as the symmetric stretching modes of [GeS4/2] Td centered at 342 cm−1 , [GaS4/2] Td at 320 cm−1 or [SbS3/2] trigonal pyramids at 293 cm−1 . The band located at 433 cm−1 reveals that [GeS4/2] Td are mainly connected by their corners. The presence of a weak band at 476 cm−1 , attributed to the presence of S–S bonds, may explain the presence of a weak band at 269 cm−1 associated with the presence of homopolar bonds (S3Ga–GaS3) [23]. In the case of selenide glasses, structural entities are similar according to Raman spectroscopy although the homopolar (Se–Se and Ge–Ge) bonds proportion and the presence of Td linked by edges appear to be greater [10]. The EXAFS studies on pure and RE doped selenide glasses will be carried out to support this hypothesis. The [GaS(Se)4] Td in stoichiometric glass are proposed to be distributed in the glass network to enable a good dispersion of REs forming bridges Ga–S (Se)–RE to offset the partial negative charge of [GaS(Se)4] Td [24,25] to the extent that the Ga concentration is greater at least by an order of magnitude with respect to RE ions. In sulfide glasses, our EXAFS results suggest that Dy3þ is surrounded by 7.970.5 sulfur atoms for a concentration of 1 wt % of Dy3þ (Fig. 1) whereas for a lower concentration (0.1 wt%), more representative for the doping of optical fibers, the coordina￾tion number appears to be lower, 6.370.5. It is reasonable to 125 150 175 200 225 250 275 300 325 200 250 300 350 400 450 500 Ga5 Ge20 Sb10 Se65 Ga5 Ge20 Sb10 S65 Reduced Intensity [a.u.] Wavenumber [cm-1] Fig. 1. (a) Radial distribution function curves of Dy3þ ions in Ga5Ge20Sb10S65 (phase shifts not corrected) and results of the fitting of the first coordination shell around Dy3þ ions and (b) Raman spectra of Ga5Ge20Sb10S(Se)65 glasses. Fig. 2. Fluorescence spectra of Dy3þ:GaGeSbS fiber (0.1 wt%). 22 F. Charpentier et al. / Materials Letters 101 (2013) 21–24