Characterization of Nd3+-doped Tellurite Glasses with Low OH Content

Abstract

This work presents the results of the investigation of structural and thermal properties of Nd³⁺-doped tellurite glasses with low OH content. The samples were characterized by XRD, FTIR, DTA, UV/VIS/NIR and Archimedes’ method. Tellurite glasses of composition (100 – x)(0.8TeO₂ + 0.2WO₃) + xNd₂O₃ (x = 0, 0.05, 0.5, 1, 2, and 4 mol%) were prepared in both ambient and oxygen atmospheres. All samples showed an increase of the values of T_g, T_x, and T_x-T_g with Nd₂O₃ addition. The reduction of OH content implies a slight decrease of T_g. The density and the molar volume of the glasses increased with Nd₂O₃. The intensity of the absorption bands associated with Te-O bonds of TeO₄ units decreased compared with the bands associated with Te-O bonds of TeO_3+1/3 units. This indicates that Nd₂O₃ favors the transformation of the TeO₄ groups in TeO₃ groups via TeO₃₊₁, increasing the NBOs and contributing to the formation of strongly hydrogen-bonded OH groups. The samples made in O₂ showed a reduction of 48% of “free” OH ions compared with the Amb ones.

Keywords:
tungsten-tellurite glasses; rare-earth; low OH content

1 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 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 lasers^6,7. 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 ⁴F_3/2 → ⁴I_11/2 emission transition near 1060 nm. Lasing has been obtained in different bulk glasses doped with Nd³⁺ ions^6-10. 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^11-13.

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.

2 Experimental

Tungsten-tellurite glasses with nominal composition (100 – x)(0.8TeO₂ + 0.2WO₃) + xNd₂O₃, where x = 0, 0.05, 0.5, 1, 2, and 4 mol%, were prepared by conventional melt-quenching method in two different atmospheres: ambient (Amb) and oxygen (O₂). Table 1 shows the nominal composition (mol%) and molar mass (g/mol) of the glasses. The reagents (with > 99% or higher purity) were weighed and mixed into an agate mortar.

Table 1
Nominal composition (mol%), mass molar (g/mol), and average crosslink density of the Nd³⁺-doped TW glasses.

Sample	TeO2 (mol%)	WO3 (mol%)	Nd2O3 (mol%)	MM (g/mol)
TW	80	20	0	174.05	2.40
TW-0.05Nd	79.96	19.99	0.05	174.13	2.40
TW-0.5Nd	79.6	19.9	0.5	174.86	2.44
TW-1Nd	79.2	19.8	1.0	175.67	2.47
TW-2Nd	78.4	19.6	2.0	177.30	2.54
TW-4Nd	76.8	19.2	4.0	180.54	2.68

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 O₂ 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 O₂ (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 N₂ purge, and were obtained by Fourier transform infrared (FTIR) spectroscopy using a Nicolet Nexus 670 FTIR spectrometer.

3 Results and Discussion

The XRD curves of the undoped and Nd³⁺-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 O₂ atmospheres are shown in Figures 1a and 1b, respectively. The T_g and T_x increase as a function of the Nd₂O₃ 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 TeO₂ (coordination numbers 3 and 4) and WO₃ (coordination number 4 or 6) for Nd₂O₃ (in general, coordination number 8), which may be assigned to the creation of a more compact structure¹⁴.

Figure 1
DTA curves of the undoped and Nd³⁺- doped TW glasses prepared in: (a) Amb and (b) O₂ atmospheres.

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.

$$\stackrel{}{\overline{n_c}}=\frac{{{\displaystyle \sum }}_i{x}_i{\left(n_c\right)}_i{\left(N_c\right)}_i}{{{\displaystyle \sum }}_i{x}_i{\left(N_c\right)}_i}$$ (1)

Comparing atmospheres, the T_g in the Amb atmosphere is slightly greater than in O₂ atmosphere, because the glasses prepared in Amb present a higher number of OH groups, leading to significant intermolecular bond strength¹⁵. 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 O₂ atmospheres, respectively, depending on the addition of Nd₂O₃, indicating higher resistance to temperature variations.

Figures 2a and 2b show the IR absorption spectra of the glasses prepared in Amb and O₂ atmospheres, respectively, in the range of 400 to 1000 cm^–1. It was observed that the introduction of Nd₂O₃ 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 TeO₄ units, decreases and shifts to lower wavenumbers with the addition of Nd₂O₃. This suggests that the Nd₂O₃ addition favors the cleavage of such linkages and may form Te-O-Nd bonds¹⁴. Furthermore, the intensity of the absorption band centered at 649 cm^–1, which is associated with Te-O bonds of TeO₄ units, decreases compared with that associated with Te-O bonds of TeO_3+1/3 units located at 778 cm^{–1[16–18]}. This indicates that the conversion of TeO₄ to TeO₃ increases the TeO₃ 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 WO₄ or WO₆ units²⁰. The phonon energy of the TW glasses is 942 cm^–1 and decreases slightly with Nd₂O₃ content.

Figure 2
Absorption spectra in the range from 400 to 1000 cm^–1 of the Nd³⁺-doped TW glasses prepared in (a) Amb and (b) O₂ atmospheres.

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 Nd₂O₃ 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 TeO₄ (tbp = trigonal bipyramid) to TeO₃₊₁ and TeO₃ (tp = trigonal pyramid) units²¹ 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.

Figure 3
(a) Absorption spectrum of the TW-0.05Nd and TW-4Nd glasses and (b) Undoped and Nd³⁺-doped TW glasses, in Amb and O₂ atmospheres.

Density and molar volume increase with Nd₂O₃ addition (Table 2). This increase in density is because the molar mass of Nd₂O₃ (336.48 g/mol) is greater than that of TeO₂ and WO₃ (159.60 and 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 (TeO₄ → TeO₃)²³.

Table 2
Density (g/cm³) and molar volume (cm³/mol) of the undoped and Nd³⁺-doped TW glasses prepared in Amb and O₂ atmospheres.

Sample	ρ (g/cm3) *		MV (cm3/mol)
	Amb	O2	Amb	O2
TW	5.89	5.88	29.55	29.60
TW-0.05Nd	5.89	5.88	29.56	29.61
TW-0.5Nd	5.89	5.89	29.69	29.69
TW-1Nd	5.89	5.89	29.83	29.83
TW-2Nd	5.93	5.92	29.90	29.95
TW-4Nd	5.94	5.93	30.39	30.45

• ^*
  The density error is 0.02.

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 water²⁵. 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 O₂ atmosphere decreased by an average of 48%.

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 Nd₂O₃. The inset shows the OH absorption spectra of the samples prepared in both atmospheres.

The amount of weakly hydrogen-bonded Te-OH…O-Te and free Te-OH groups/molecular water groups decreased with the addition of Nd₂O₃, whereas the number of strongly hydrogen-bonded Te-OH…O-Te groups increased. The same behavior is observed in the samples prepared in O₂ 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 Nd₂O₃ 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 reside²⁶. 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.

4 Conclusions

Measurements of DRX, DTA, UV/VIS/NIR, density, and FTIR, were performed for thermal and structural characterization of the undoped and Nd³⁺-doped TW glasses prepared in Amb and O₂ atmospheres. We conclude that in each sample set, the increase of Nd in the TW matrix caused structural changes (TeO₄ → TeO₃) 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.

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Publication Dates

• Publication in this collection
  23 Oct 2015
• Date of issue
  Dec 2015

History

• Received
  29 Aug 2014
• Reviewed
  16 June 2015