Short-range structure and thermal properties of lead tellurite glasses

PbO-TeO2 glasses having composition: xPbO–(100 x)TeO2 (x = 10, 15 and 20 mol%) were prepared by melt quenching and characterized by X-ray diffraction, density measurements, differential scanning calorimetry and Raman spectroscopy. Glass density increases from 5.89 to 6.22 g cm with increase in PbO concentration from 10 to 20 mol%, due to the replacement of TeO2 by heavier PbO. DSC studies found that glass transition temperature (Tg) decreases from a value of 295C to 281C. Raman studies found that glass short-range structure consists of TeO4 and TeO3 structural units and that PbO modifies the network by the structural transformation: TeO4 to TeO3.


INTRODUCTION
Tellurite glasses are considered as good candidates for efficient photonic, memory switching devices and gas sensing devices due to their a wide optical transmission windows, extraordinary nonlinear-optical properties, relatively low phonon energies, lack of toxicity, good durability and glass stability.The structural variability of tellurite glasses can be used to tailor the their thermal, optical and chemical durability parameters [1].This possibility has generated lot of interest in structural investigations of tellurite glasses.TeO 2 is a conditional glass former which requires very high quenching rates~10 5 to 10 6 K s -1 to form glass and modifiers such as metal oxides are added to enhance its glass forming ability and produce glasses at lower quenching rates of ~ 10 2 K s -1 [1,2].The basic structural units of tellurite glasses are TeO 4 trigonal bipyramids (tbp) and TeO 3 trigonal pyramid (tp).The addition of modifier BaO to the tellurite glasses breaks the random network and produces non-bridging oxygens (NBO) and substantial change in the glass network structure [3].Barium tellurite glasses have ability to form a zero stress-optic and negative stress-optic response glasses [4].Er 3+ -doped barium tellurite glass with high upconversion and near-infrared (NIR) luminescence efficiency show great potential for infrared concentrator and NIR sensor and is also, a promising candidate for energetic mid IR lasers [5].The present work is devoted to the synthesis and characterization of glasses from the system: (BaO) x (TeO 2 ) (100-x) .

EXPERIMENTAL
The binary (BaO) x (TeO 2 ) (100-x) tellurite glasses with x=10,15,20 mol% were prepared.The Analytical Reagent grade chemicals of TeO 2 (Aldrich, India 99%) and BaCO 3 (CDH, India 99.9%) were used as starting materials.The chemicals were mixed and ground in an agate mortar pestle for about 30 min and then transferred to a platinum crucible.A homogeneous mixture of chemicals was melted in an electric furnace at 850°C for 20 min.Each glass sample was prepared by normal quenching in which the melt was poured on a brass block and a button shaped sample was prepared and immediately transferred to another furnace where it was annealed at 320°C for 30 min to avoid cracking by relieving internal stresses generated by quenching. All

Structure
Figure 1 shows the XRD patterns of three barium tellurite glasses, all samples exhibit broad halos o and 50 o due to the short-range order in the samples.

Density measurements:
It can be seen from Table 1 and Figure 2 that as the concentration of BaO increases from 10 to 20 mol%, density, d decreases from 5.58 to 5.52 g cm -3 while the molar volume, V M increases slightly from 28.48 to 28.68 cm 3 mol -1 .

DSC Results:
The thermal characteristics i.e. the glass transition temperature (T g ), crystallization temperature (T c ) and liquidus temperature (T m ) were measured by DSC and the values are given in Table 1. Figure 3 shows that T g increases with BaO molar concentration (glass modifier).Glass transition temperature characterizes the strength of the glass network.The parameter c -T g is commonly used to evaluate the glass     070015-2 cm -1 (stretching vibrations of TeO 4 tbp and TeO 3 tp structural units).The band at ~54 cm -1 is the boson peak, which is a universal feature of glassy phase.The Raman spectra were baseline corrected and deconvoluted with peaks centered at 609, 662, 719, 778 cm -1 (Figure 5).The co-ordination number of Te ions with oxygens was calculated from the area ratios of the deconvoluted bands [6]:

Raman Study:
The Te-O coordination number decreases from a value of 3.54 to 3.43 with increase in BaO concentration from 10 to 20 mol%.

o-
70 o .Density of glasses was measured by Archimedes method on an electronic balance of sensitivity 10 -4 g using dibutylpthalate (DBP) as an immersion fluid.DSC studies were performed on a SETARAM SETYS 16 TG-DSC system in temperature range of 200-800°C at heating rate of 10°Cmin -1 .Raman studies were performed on Renishaw inVia Reflex Micro-Raman Spectrometer having 514.5 nm Argon Laser (50 mW), 2400 line/mm diffraction grating and an edge filter for recording the Stokes spectra and a Peltier cooled CCD detector.
decreases from 143 o C to 102 o C on increasing BaO concentration.Hence, the thermal stability of glasses against devitrification decreases.

FIGURE 2 .
FIGURE 2. Variation of density and molar volume in BaO-TeO 2 glasses with BaO mol%.

Figure 4 TABLE 1 .
Figure 4 is the Raman spectra of three glasses which contains two broad bands in the ranges: 375-550 cm -1 (bending vibration of Te-O-Te linkages) and 550-840

FIGURE 5 .
FIGURE 5. Deconvoluted Raman spectra of 10-mol.%BaO of Barium Tellurite glass.CONCLUSIONSBarium tellurite glasses were prepared and characterized by XRD, density, DSC and Raman studies.Glass density correlates directly with molecular mass of the