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sábado, junho 02, 2018

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

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

PNAS August 1, 2017. 114 (31) 8193-8198; published ahead of print June 26, 2017. https://doi.org/10.1073/pnas.1705303114

Edited by Pablo G. Debenedetti, Princeton University, Princeton, NJ, and approved May 31, 2017 (received for review March 31, 2017)

Source/Fonte: New Scientist


The importance of a molecular-level understanding of the properties, structure, and dynamics of liquid water is recognized in many scientific fields. It has been debated whether the observed high- and low-density amorphous ice forms are related to two distinct liquid forms. Here, we study experimentally the structure and dynamics of high-density amorphous ice as it relaxes into the low-density form. The unique aspect of this work is the combination of two X-ray methods, where wide-angle X-ray scattering provides the evidence for the structure at the atomic level and X-ray photon-correlation spectroscopy provides insight about the motion at the nanoscale, respectively. The observed motion appears diffusive, indicating liquid-like dynamics during the relaxation from the high-to low-density form.


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.

liquid–liquid transitionglass transitionamorphous iceX-ray photon-correlation spectroscopysupercooled water