Investigation on chalcogenide glasses for its threshold phenomena using photothermal methods

Photoacoustic spectroscopy (PAS) and Photothermal beam deflection spectroscopy (PBS) was used to study the bulk $Ge_{7.5}As_xTe_{92.5-x}$ alloy glasses in the composition range, 17.5 ≤ x ≤ 60 [co-ordination number (< r >), 2.35-2.75] for its thermal diffusivity (\alpha). \alpha was found to be maximum for < r > = 2.4 $(0.85 \times 10^{-2} cm^2/s)$, with increase in < r > there was a steep decrease except for a small hump observed at < r > = 2.67 $(0.44 \times 10^{-2} cm^2/s)$. This observed behaviour is explained on the basis of two transitions points at < r > = 2.4 and < r > = 2.67, that correspond to mechanical and chemical thresholds respectively. The advantage of PAS and PBS is highlighted in comparison with the conventional absorption spectrophotometric technique. These methods are independent of sample thickness and are not affected by light scattering which is ideally suited for studying chalcogenide glassy systems. The experimental data was curve-fitted using the one-dimensional heat diffusion equation to evaluate the accuracy of the thermal diffusivity values obtained