Thermal study and linear optical properties of (1 - x)TeO₂-(x)PbF₂ (x = 0.10, 0.15 and 0.25 mol) glasses

The (1 - x)TeO₂-xPbF₂ binary glass system was investigated by differential thermal analysis (DTA) and ultraviolet-visible-near-infrared (UV-VIS-NIR) absorption spectrophotometer. Samples were prepared by melting the mixture of TeO₂ and PbF₂ in a platinum crucible at 800 °C in air. Glass transition and the crystallization temperatures as function of the glass composition were measured by DTA. An exothermic peak of crystallization temperature was observed at about 340 ± 1 °C for all three samples. A second peak of peak crystallization temperature was observed at about 400 °C for only the sample with 0.15 mol PbF₂. The mechanism and the activation energy for each crystallization peak were determined from the DTA curves measured with different heating rates between 5 and 20 °C/min. The mechanism of the crystallization was found to be surface crystallization for the first exothermic peak for only 0.15 mol PbF₂ sample; on the other hand, bulk crystallization was found for all samples. Corresponding activation energies are 814, 748, and 387 kJ/mol for the samples with 0.10, 0.15 and, 0.25 mol PbF₂, respectively. The mechanism and the activation energy for the second exothermic peak observed in the sample with 0.15 mol PbF₂ were found to be bulk crystallization and 415 kJ/mol, respectively. The optical band gap, and Urbach energies were calculated from the absorption spectra measured between 300 and 800 nm at room temperature. The optical bandgap varies from 2.02 to 1.90 eV when the PbF₂ content increases from 0.10 to 0.25 mol in the glass matrix.