Theoretical Studies of Structure and Dynamics of Chalcogenide Glasses

In this work, the structural and dynamical properties of Ge-Se and Ag-Ge-Se glasses, using first principle molecular dynamics simulations, is presented. Raman and calorimetric studies on Ge_XSe_1-x glasses have provided evidence for the existence of the intermediate phase (IP) in chalcogenides and other glasses. Here we thoroughly discuss the ab-initio models of Ge_xSe_1-x glasses over a wide composition range and provide an atomistic picture of the IP. A thorough analysis of our models reveals that the IP in these glasses may arise from the competition between amorphous GeSe₂ and a polymeric a-Se phases, which gives rise to the non-monotonic evolution (a topological feature of the IP) of the network parameters through the IP window. A qualitative comparison is made between the topological features extracted from the models and the experiments. A possible electronic signature of the IP in terms of the shift in the conduction edge energy in the IP range is predicted. The results agree with the shift in the white line position for the Se atoms in the K-edge X-Ray Absorption Near Edge Structure (XANES) spectra.

A microscopic picture of the silver dynamics in GeSe₃:Ag glass is presented. The dynamics of Ag is explored at two temperatures: 300K and 700K. In the relaxed GeSe₃ glass network, it is shown that the Ag occupies trapping centers (TC) that exist between pairs of suitably separated host sites. The charge state of the Ag in the glass network is computed and it is shown that Ag is neutral if weakly bonded and is Ag⁺ if in a trapping center. The dynamics of the Ag is shown to be largely effected by the neighboring host sties. At room temperature, Ag is mainly trapped at the TC with small hopping probability between neighboring TCs. At higher temperature (700 K), the Ag motion is diffusive and is proceeded via a trapping-release dynamics between supertraps or cages consisting of multiple trapping center sites in a small volume. Our work offers a first-principles identification of trapping centers invoked in current theories, with a description of their properties and associated Ag dynamics. The understanding of the Ag dynamics in bulk glass is exploited to model the surface of a solid electrolyte (GeSe₃:Ag).

Finally, the topological-electronic correlations, previously reported in the realistic models of a-Si, are explored further. It is demonstrated that analogous correlations exist in amorphous SiO₂ and in the organic molecule beta-carotene, which hints towards the universal character of these correlations.