Structural characterization of borotellurite and alumino-borotellurite glasses

Borotellurite (xB2O3-(100 − x)TeO2; x = 15, 20, 25, 30 and 35 mol.%) and alumino-borotellurite (yAl2O3-10B2O3-(100 − y)TeO2; y = 5, 10, 20 and 30 mol.%) glasses are prepared and studied by X-ray diffraction, density, differential scanning calorimetry, thermo-gravimetric analysis, UV-visible, infrared and Raman spectroscopy. Borotellurite glasses are hygroscopic and on crushing into powder, they absorb atmospheric water vapors to form crystalline precipitates of TeO2 in an amorphous matrix; the chemical durability of these glasses deteriorates with increasing B2O3 concentration. The refractive index of borotellurite glasses decreases from 2.29 to 2.26 while the glass transition temperature increases from 339 to 366 °C with an increase in B2O3 mol.%. The short-range structure of glasses consists of TeO4, TeO3, BO4 and BO3 structural units. Using the ratio of the areas under TeO4/TeO3 and BO4/BO3 Raman and infrared bands respectively, the TeO coordination is found to decrease from 3.63 to 3.56 and BO co-ordination from 3.34 to 3.17 respectively on increasing B2O3 concentration from 5 to 30-mol.%. Alumino-borotellurite glasses show an amorphous-amorphous phase separation at an alumina concentration of 10-mol.% and above, while alumino-borotellurite sample with 30-mol.% of Al2O3 forms crystalline precipitates of α-Al2O3 in an amorphous matrix.