Journal Article

Diffusive dynamics during the high-to-low density transition in amorphous ice

Pathak, Harshad, Sprung, Michael, Mariedahl, Daniel, Aubree, Flora, Ricci, Alessandro, Massani, Bernhard, Jain, Avni, Sellberg, Jonas A., Cavalca, Filippo, Amann-Winkel, Katrin, Pettersson, Lars G. M., Nilsson, Anders, Grübel, Gerhard, Lehmkühler, Felix, Späh, Alexander, Perakis, Fivos, Loerting, Thomas, Benmore, Chris J., Schlesinger, Daniel

2017

Overview

Water exists in high- and low-density amorphous ice forms (HDA and LDA), which could correspond to the glassy states of high-(HDL) and low-density liquid (LDL) in the metastable part of the phase diagram. However, the nature of both the glass transition and the high-to-low-density transition are debated and new experimental evidence is needed. Here we combine wide-angle X-ray scattering (WAXS) with X-ray photon-correlation spectroscopy (XPCS) in the small-angle X-ray scattering (SAXS) geometry to probe both the structural and dynamical properties during the high-to-low-density transition in amorphous ice at 1 bar. By analyzing the structure factor and the radial distribution function, the coexistence of two structurally distinct domains is observed at T = 125 K. XPCS probes the dynamics in momentum space, which in the SAXS geometry reflects structural relaxation on the nanometer length scale. The dynamics of HDA are characterized by a slow component with a large time constant, arising from viscoelastic relaxation and stress release from nanometer-sized heterogeneities. Above 110 K a faster, strongly temperature-dependent component appears, with momentum transfer dependence pointing toward nanoscale diffusion. This dynamical component slows down after transition into the low-density form at 130 K, but remains diffusive. The diffusive character of both the high- and low-density forms is discussed among different interpretations and the results are most consistent with the hypothesis of a liquid–liquid transition in the ultraviscous regime.

• Publisher: National Academy of Sciences,National Academy of Sciences, Washington, DC (United States)
• Subject: amorphous ice / CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS / Coexistence / Density / Diffusion / Diffusion rate / Distribution functions / Geometry / glass transition / High density lipoprotein / Ice / Ice formation / liquid-liquid transition / Low density lipoprotein / Momentum transfer / Physical Sciences / Physics / Probes / Radial distribution / Scattering / SEE COMMENTARY / Small angle X ray scattering / Spectroscopy / Spectrum analysis / Stress relaxation / Structure factor / supercooled water / Temperature / Time constant / Viscoelasticity / X-ray photon-correlation spectroscopy / X-ray scattering