Preparation and characterization of lead and zinc tellurite glasses

Glasses of two systems: xPbO–(100 − x)TeO2 (x = 13, 15, 17, 19 and 21 mol%) and yZnO–(100 − y)TeO2 (y = 18, 20, 22, 25, 30, 33 and 35 mol%) were prepared at two melt-cooling rates and characterized by density, UV–visible and Raman spectroscopy, DSC and XRD measurements. ZnO produces a larger compaction in the tellurite network than PbO. Density decreases with melt-cooling rate in both glass series. DSC studies found that the glass transition temperature decreases with increase in PbO concentration in lead tellurite glasses, but increases with increase in ZnO concentration in zinc tellurite glasses. Lead tellurites prepared at higher cooling rates were mostly amorphous while samples containing 19 and 21 mol% of PbO, prepared at slower cooling rates were translucent glass–ceramics containing crystals of PbTeO3, Pb2Te3O8 and TeO2. These slowly cooled samples exhibit an absorption shoulder in the UV–visible absorption spectra, just below the absorption edge. This is attributed to excitonic transitions in nanocrystals that coexist with glassy phase in slowly cooled samples. Raman scattering studies found that the addition of PbO and ZnO produces an increase in the intensity of the absorption band between 720 and 745 cm−1 relative to the intensity of band between 645 and 670 cm−1. This is due to the continuous distortion of symmetry of TeO4 units, leading to the creation of TeO3+1 polyhedron and/or TeO3 trigonal units. Raman studies found a very sharp peak at 76 cm−1 in both lead and zinc tellurite glasses. The position of this peak was independent of the glass composition.