Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 114, Issue 31, Pages 8193-8198Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1705303114
Keywords
liquid-liquid transition; glass transition; amorphous ice; X-ray photon-correlation spectroscopy; supercooled water
Categories
Funding
- European Research Council [667205]
- Swedish Research Council
- Swiss National Science Foundation [P2ZHP2 148666]
- Deutsche Forschungsgemeinschaft [EXC1074]
- Austrian Science Fund [I1392]
- DOE Office of Science [DE-AC02-06CH11357]
- Swiss National Science Foundation (SNF) [P2ZHP2_148666] Funding Source: Swiss National Science Foundation (SNF)
- European Research Council (ERC) [667205] Funding Source: European Research Council (ERC)
- Austrian Science Fund (FWF) [I1392] Funding Source: Austrian Science Fund (FWF)
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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.
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