Composition and temperature-induced structural changes in lead-tellurite glasses on different length scales

Processes occurring at macroscopic and microscopic length scales across the glass transition (Tg) in lead-tellurite glass (PbO)x(TeO2)1-x (x = 0.1-0.3) are investigated using Brillouin and Raman spectroscopy, respectively. For all the samples, the temperature dependence of the longitudinal acoustic (LA) mode is found to exhibit a universal scaling below Tg and a rapid softening above Tg. The lower value of elastic modulus at a higher concentration of network modifier PbO, estimated from Brillouin data, arises due to loss of network rigidity. From quantitative analysis of the reduced Raman spectra, several modes are found to exhibit anomalous changes across Tg. Instead of the expected anharmonic behaviour, several modes exhibit hardening, suggesting stiffening of the stretching force constants with temperature, the effect being more pronounced in glasses with higher x. In addition, incorporation of PbO in the glass is also found to narrow down the bond-length distribution, as evident from the sharpening of the Raman bands. The stiffening of the force constants of molecular units at a microscopic length scale and the decrease of elastic constant attributed to loss of network rigidity on a macroscopic length scale appear to be opposite. These different behaviours at two length scales are understood on the basis of a microscopic model involving TeOn and PbO units in the structure.