An Investigation of Dynamic Processes in Selenium Based Chalcogenide Glasses

Abstract

Owing to their excellent infrared transmittance and good rheological properties, selenium based chalcogenide glasses have been materials of choice for a number of technological applications. However, chalcogenide glasses can undergo substantial structural relaxation even at room temperature due to their low glass transition temperatures. The origins of these dynamic processes and their correlation to the glass structure is therefore of fundamental and practical interest. In particular, a deep understanding of the dynamic response near the glass transition region could help elucidate the mechanism of these structural relaxation processes. The correlation between structure and dynamic properties of selenium based glass systems were therefore investigated. NMR and Raman spectroscopy measurements reveal that the structure of AsₓSe₁₋ₓ glass follow the chain crossing model in selenium-rich glasses but contain increasing amounts of cage molecules in arsenic-rich compositions. This structural pattern leads to systematic extrema in physical properties at the stoichiometric composition As₄₀Se₆₀.The dynamic response of AsₓSe₁₋ₓ glasses investigated by heat capacity spectroscopy shows two minima in melt fragility as a function of composition which correlate well with the dimensionality of the glassy network. The structure evolves from 2D to 3D during crosslinking of selenium chains by arsenic but reduces into a 2D layer-like structure at the stoichiometric composition. Upon precipitation of arsenic-rich cages the network first reverts back to 3D and eventually becomes a mix of 2D and 0D structural units. The presence of molecular clusters in the network is evidenced by a strong bimodal dynamic response at high arsenic contents. NMR and Raman spectroscopy measurements of GeₓSe₁₋ₓ glasses suggest a structure composed of aggregated tetrahedral units and long selenium chains with little or no connectivity. Distinct dynamic responses of these two separated structural motifs are revealed by heat capacity spectroscopy. A non-Gaussian distribution of the imaginary heat capacity peak provided further evidence for the structural heterogeneity. This behavior is consistent with high temperature NMR measurements which show that the dynamic response of floppy selenium chains is distinct from that of rigid tetrahedral units. Finally, heat capacity spectroscopy applied to pure selenium provides strong evidence for the microscopic origin of the non-exponential structural relaxation, a universal feature of fragile glasses. The evolution of the imaginary heat capacity peak shape during annealing shows a non-monotonic trend which remarkably matches model predictions based on the enthalpy landscape. These results indicate that the non-exponential character of the relaxation process is linked to density fluctuations in the glass.