Observation of Collective Molecular Dynamics in a Chalcogenide Glass: Results from X-ray Photon Correlation...

The structural relaxation processes in a Ge₃As₅₂S₄₅ molecular chalcogenide glass sample were directly studied by X-ray photon correlation spectroscopy (XPCS). XPCS was conducted at the first sharp diffraction peak at q = 1.16 Å^-1 at temperatures ranging from 123 K to above the glass transition at 328 K, and the results showed two different dynamical regimes. At a low temperature, the observed glass dynamics are slow and dominated by X-ray-photon-induced effects, which are temperature independent. At a higher temperature, we observed a dramatic decrease in the fluctuation timescales, indicating that the dynamics were mainly due to the intermolecular correlation of the As₄S₃ molecule in the glass. The timescales in this high-temperature range agree well with those determined from measurements of the Newtonian viscosity. Our XPCS studies suggest an extended length scale of the relaxation process in glassy Ge₃As₅₂S₄₅ from the single molecule to the intermolecular range across the glass transition, providing a unique direct probe of the dynamics beyond the length scales of the individual molecule.