Short-range structure and thermal properties of alumino-tellurite glasses

Alumino-tellurite glasses with wide range of Al2O3 concentration (1 to 20-mol%) are prepared and their short-range structural and thermal properties are studied by Raman spectroscopy, Al-27 Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) and Differential Scanning Calorimetry (DSC). Glass sample with 1-mol% of Al2O3 has Te-O co-ordination of 3.66, which is close to the value of 3.68 reported in pure TeO2 glass. Te-O coordination decreases steadily to 3.33 on increasing alumina content to 20-mol%. Al-27 MAS-NMR studies found that AI ions exist in hexa, penta and tetrahedral co-ordination in the alumino-tellurite network. On increasing alumina content from 3 to 20-mol%, the concentration of hexa-co-ordinated Al-O units decreases from 54% to 35% with a simultaneous increase in the concentration of penta and tetra co-ordinated Al-O units. Glass transition temperature increases with increase in Al2O3 concentration and correlates directly with the average single bond enthalpy of the glass network.