Abstract
Direct quantitative measurements of nanoscale dynamical processes associated with structural relaxation and crystallization near the glass transition are a major experimental challenge. These type of processes have been primarily treated as macroscopic phenomena within the framework of phenomenological models and bulk experiments. Here, we report x-ray photon correlation spectroscopy measurements of dynamics at the crystal-melt interface during the radiation induced formation of Se nano-crystallites in pure Se and in binary AsSe4 glass-forming liquids near their glass transition temperature. We observe a heterogeneous dynamical behaviour where the intensity correlation functions g2(q, t) exhibits either a compressed or a stretched exponential decay, depending on the size of the Se nano-crystallites. The corresponding relaxation timescale for the AsSe4 liquid increases as the temperature is raised, which can be attributed to changes in the chemical composition of the melt at the crystal-melt interface with the growth of the Se nano-crystallites.
Highlights
Direct quantitative measurements of nanoscale dynamical processes associated with structural relaxation and crystallization near the glass transition are a major experimental challenge
The intensity correlation function, which is related to the dynamical structure factor, exhibited either compressed or stretched exponential decay behaviour depending upon the size of the nano-crystallites
The shear relaxation timescales are within the range of the calculated mean relaxation time from X-ray photon correlation spectroscopy (XPCS) data for Se sample at all temperatures and AsSe4 sample at 84 °C, indicating that the dynamics at the crystal-liquid interface during the Se crystallization process is mediated via viscous flow of the supercooled liquid phase[7]
Summary
Direct quantitative measurements of nanoscale dynamical processes associated with structural relaxation and crystallization near the glass transition are a major experimental challenge. The standard kinetic models treat crystallization of a liquid as a macroscopic phenomenon and the applicability of these models is often hampered by the lack of a direct microscopic understanding of the atomic scale processes that accompany crystallization[9,10,11] These models use viscosity or diffusivity values for the bulk supercooled liquid, instead of the “local” microscopic transport property at the crystal-liquid interface, which is more relevant, albeit its experimental measurement would be rather difficult, if not impossible. We report the results of x-ray beam induced structural rearrangement leading to crystallization in a supercooled glass-forming liquid of pure Se and AsSe4 using XPCS This technique allows us to capture coherent speckle patterns arising from fluctuations and inhomogeneities associated with the beam induced crystallization of supercooled glass-forming liquids close to their glass transition temperature Tg. The intensity correlation function, which is related to the dynamical structure factor, exhibited either compressed or stretched exponential decay behaviour depending upon the size of the nano-crystallites. The results of the present study highlight the beam induced nanoscale nucleation and growth dynamics and how such dynamics are affected by crystallite size, in highly viscous supercooled glass-forming liquids
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