Characterization of Nd 3 +-doped Tellurite Glasses with Low OH Content

Tellurite glasses combine the attributes of reasonable thermal stability, high refractive index, high solubility of rare earth (RE) ions, larger absorption and emission cross-sections, and low phonon energy compared to the silicate, phosphate, and borate glasses1,2, with a wide transmission window in the infrared region2. In particular, tungsten-tellurite glasses have higher phonon energy and a higher glass transition temperature compared to the other tellurite glasses; therefore, they can be used at high optical intensities without exposure to thermal damage3. The RE-doped tungsten-tellurite glasses have been shown to have excellent properties for applications such as planar waveguides4, amplifiers5, and lasers6,7. Several RE ions have been studied for laser application. Among them, the Nd3+ ion is one of the most investigated. This is due to the high quantum efficiency of the F3/2 → I11/2 emission transition near 1060 nm. Lasing has been obtained in different bulk glasses doped with Nd3+ ions6-10. However, tellurite glasses have larger amounts of water when melted in air. The water is incorporated into the glass as hydroxyl (OH) groups, which have a strong and broad vibrational absorption band between 2000 and 3600 cm–1, promoting energy loss. The absorption losses due to the OH groups are disadvantageous because they decrease luminescence quantum efficiency, hindering the practical use of these glasses11-13. In this paper, we investigate the thermal and structural properties of the Nd3+-doped tungsten-tellurite glasses according to neodymium concentration, with samples prepared in ambient and oxygen atmospheres.


Introduction
Tellurite glasses combine the attributes of reasonable thermal stability, high refractive index, high solubility of rare earth (RE) ions, larger absorption and emission cross-sections, and low phonon energy compared to the silicate, phosphate, and borate glasses 1,2 , with a wide transmission window in the infrared region 2 .In particular, tungsten-tellurite glasses have higher phonon energy and a higher glass transition temperature compared to the other tellurite glasses; therefore, they can be used at high optical intensities without exposure to thermal damage 3 .The RE-doped tungsten-tellurite glasses have been shown to have excellent properties for applications such as planar waveguides 4 , amplifiers 5 , and lasers 6,7 .Several RE ions have been studied for laser application.Among them, the Nd 3+ ion is one of the most investigated.This is due to the high quantum efficiency of the 4 F 3/2 → 4 I 11/2 emission transition near 1060 nm.Lasing has been obtained in different bulk glasses doped with Nd 3+ ions [6][7][8][9][10] .However, tellurite glasses have larger amounts of water when melted in air.The water is incorporated into the glass as hydroxyl (OH) groups, which have a strong and broad vibrational absorption band between 2000 and 3600 cm -1 , promoting energy loss.The absorption losses due to the OH groups are disadvantageous because they decrease luminescence quantum efficiency, hindering the practical use of these glasses [11][12][13] .
In this paper, we investigate the thermal and structural properties of the Nd 3+ -doped tungsten-tellurite glasses according to neodymium concentration, with samples prepared in ambient and oxygen atmospheres.
For the set prepared in the Amb atmosphere, the mixture of reagents was melted in a platinum crucible at 880 °C for 30 minutes.The melt was poured into a stainless-steel mold, pre-heated near the glass transition temperature (~350 °C).For the set prepared in the O 2 atmosphere, the same procedures were carried out, except that the furnace was placed inside a sealed camera, where a vacuum was created.This was followed by an injection of O 2 (until the inner pressure reached 1 atm).After their contents were poured, the glasses were returned to the other furnace for annealing.The annealing lasted for 2 hours at 350 °C and then the sample was slowly cooled to room temperature.The produced glasses were cut and polished, reaching thicknesses between 0.71 and 0.95 mm.
The glassy state in these samples was confirmed by X-ray diffraction (XRD) analysis using Cu K α radiation.Glass transition (T g ) and crystallization onset (T x ) temperatures were measured using the differential thermal analysis (DTA) technique at a heating rate of 10 °C/min.The absorption spectra in the range of 400 to 700 nm were obtained using a Perkin Elmer Lambda 900 UV/VIS/NIR spectrophotometer.The densities of the samples were determined by Archimedes' method, using distilled water as the immersion liquid.The absorption spectra were recorded in the range of 400 to 1000 cm -1 for the powder samples and in the range of 2000 to 4000 cm -1 for the bulk samples using an N 2 purge, and were obtained by Fourier transform infrared (FTIR) spectroscopy using a Nicolet Nexus 670 FTIR spectrometer.

Results and Discussion
The XRD curves of the undoped and Nd 3+ -doped TW glasses prepared in both atmospheres presented two halos that confirm the amorphous nature of the glasses.
The DTA analyses were performed to determine the thermal behavior of the prepared glasses.The DTA curves of the glasses prepared in Amb and O 2 atmospheres are shown in Figures 1a and 1b, respectively.The T g and T x increase as a function of the Nd 2 O 3 concentration.The increase of the T g can be explained by the increase in the molar mass (Table 1).It could also be due to the high bond energy of the Nd-O (703 kJ/mol) compared to Te-O (376 kJ/mol) and W-O (672 kJ/mol), and due to the increase in the average crosslink density (Table 1), explained by replacement of the TeO 2 (coordination numbers 3 and 4) and WO 3 (coordination number 4 or 6) for Nd 2 O 3 (in general, coordination number 8), which may be assigned to the creation of a more compact structure 14 .
The average crosslink density was calculated according to Equation 1, where x i is the mol fraction of component oxide; n c is the crosslink density per cation, which is equal to n f -2, with n f being the coordination number of cation; and N c is the number of cations per glass formula unit.
Comparing atmospheres, the T g in the Amb atmosphere is slightly greater than in O 2 atmosphere, because the glasses prepared in Amb present a higher number of OH groups, leading to significant intermolecular bond strength 15 .The temperature difference between the T x and T g (ΔT), indicates the thermal stability of the glass.The ΔT increased from 92 to 111 °C and from 89 to 106 °C for the glasses in Amb and O 2 atmospheres, respectively, depending on the addition of Nd 2 O 3 , indicating higher resistance to temperature variations.
Figures 2a and 2b show the IR absorption spectra of the glasses prepared in Amb and O 2 atmospheres, respectively, in the range of 400 to 1000 cm -1 .It was observed that the introduction of Nd 2 O 3 to the glass led to variations in the glass network.The intensity of the absorption band centered at 495 cm -1 , which is associated with Te-O-Te/Te-O-W linkages of TeO 4 units, decreases and shifts to lower wavenumbers with the addition of Nd 2 O 3 .This suggests that the Nd 2 O 3 addition favors the cleavage of such linkages and may form Te-O-Nd bonds 14 .Furthermore, the intensity of the absorption band centered at 649 cm -1 , which is associated with Te-O bonds of TeO 4 units, decreases compared with that associated with Te-O bonds of TeO 3+1/3 units located at 778 cm -1 [16][17][18] .This indicates that the conversion of TeO 4 to TeO 3 increases the TeO 3 isolated units with the non-bridging oxygen (NBO) 19 , contributing to the formation of the Te-OH groups.The shoulder observed at 870 cm -1 is associated with W-O-W linkages, and the band at 942 cm -1 corresponds to W=O and W-O -bonds of WO 4 or WO 6 units 20 .The phonon energy of the TW glasses is 942 cm -1 and decreases slightly with Nd 2 O 3 content.
Figure 3a shows the absorption spectra of the TW-0.05Nd and TW-4Nd glasses prepared in both atmospheres in the spectral range from 400 to 700 nm.The Nd 2 O 3 addition causes a blue shift of the fundamental absorption edge tail (inset of Figure 3a).This shift has been observed in previous studies 14,21,22 .It has been related to the structural conversion from TeO 4 (tbp = trigonal bipyramid) to TeO 3+1 and TeO 3 (tp = trigonal pyramid) units 21 and can be responsible for the color change of glasses from yellow to green (Figure 3b), since the luminescent emissions are outside of the visible region.
Density and molar volume increase with Nd 2 O 3 addition (Table 2).This increase in density is because the molar mass of Nd 2 O 3 (336.48g/mol) is greater than that of TeO 2 and WO 3 (159.60and 231.84 g/mol, respectively), increasing the molar mass of the doped glasses (Table 1).This increase in molar volume is probably due to an increase in the bond length between Te and O atoms resulting from the observed structural conversion (TeO 4 → TeO 3 ) 23 .
The inset of the Figure 4 gives the IR absorption spectra of the TW and TW-4Nd glasses, prepared in both atmospheres, in the range of 2000 to 3600 cm -1 , which are associated with OH -absorption.The spectra of the glasses exhibit a broad absorption band centered at 3160 cm -1 (with a shoulder at ~3300 cm -1 ) and a weaker and sharper band between 2000 and 2400 cm -1 which are ascribed to the stretching mode of the weakly and strongly hydrogen-bonded Te-OH…O-Te groups, respectively 24,25 .The band around 3300 cm -1 is assigned to a combination of the free Te-OH groups and molecular water 25 .The broad absorption band at 3160 cm -1 has been used to assess the reduction of the OH content in the glasses 12,26 .The amplitude this absorption band of the samples prepared in O 2 atmosphere decreased by an average of 48%.
The amount of weakly hydrogen-bonded Te-OH…O-Te and free Te-OH groups/molecular water groups decreased with the addition of Nd 2 O 3 , whereas the number of strongly hydrogen-bonded Te-OH…O-Te groups increased.The same behavior is observed in the samples prepared in O 2 atmosphere.This is explained by an increase in the intensity of the absorption band at 2000-2400 cm -1 .The shift from higher to lower wavenumbers (74 cm -1 ) of this band according to the Nd 2 O 3 addition can be explained by the decrease of the O...O distances, since this addition causes an increase in the molar volume of the TW glass (Table 2) and, consequently, a   decrease of the free space in which the water impurities could reside 26 .The appearance of a shoulder is also observed in this band for TW-1Nd, TW-2Nd and TW-4Nd samples.The shift to lower wavenumbers led to a superposition of the strongly hydrogen-bonded band with weak bands of neodymium oxide, with a maximum at 2075 cm -1 (Figure 4).The deconvolution indicates only one absorption peak for TW, TW-0.05Nd, and TW-0.5Nd samples and two absorption peaks for the other doped samples, centered at ~2100 and ~2220 cm -1 .Thus, the shoulder observed at ~2100 cm -1 is assigned here to the neodymium.This shoulder also undergoes a shift of 24 cm -1 when the concentration of neodymium increases from 1 to 4 mol%, which is probably due to decreases of Nd-O bond length.

Conclusions
Measurements of DRX, DTA, UV/VIS/NIR, density, and FTIR, were performed for thermal and structural characterization of the undoped and Nd 3+ -doped TW glasses prepared in Amb and O 2 atmospheres.We conclude that in each sample set, the increase of Nd in the TW matrix caused structural changes (TeO 4 → TeO 3 ) and consequently increased the density, molar volume, T g and ΔT.In addition, it caused a blue shift of the glass fundamental absorption edge tail.

Figure 2 .
Figure 2. Absorption spectra in the range from 400 to 1000 cm -1 of the Nd 3+ -doped TW glasses prepared in (a) Amb and (b) O 2 atmospheres.

Figure 4 .
Figure 4. Deconvolution of the absorption band centered at 2200 cm -1 of the TW-4Nd glass prepared in Amb atmosphere and absorption spectra of the pure Nd 2 O 3 .The inset shows the OH absorption spectra of the samples prepared in both atmospheres.
*The density error is 0.02.