SPring-8, the large synchrotron radiation facility

Date: 15:00- March 5, 2007

Place: Lecture Hall of Public Relations Center, SPring-8

Speaker: Prof. John Finney

Affiliation: Dept of Physics and Astronomy, University College London, UK

Title: The structures of the amorphous ices

Abstract:
 Neutron diffraction with isotope substitution has been used to determine the structures of the five amorphous ices:
amorphous solid water (ASW), hyperquenched glassy water (HGW), and low density (LDA), high density (HDA) and very high density (VHDA) amorphous ices.
 ASW and HGW have structures that are very similar to each other and to LDA, which itself is a structure which closely resembles a disordered, tetrahedrally coordinated, fully hydrogen bonded network. HDA and VHDA retain this tetrahedral organization at short range, but show significant differences beyond about 3.1 A from a typical water oxygen. Similarities in local order can be seen between LDA and ice Ih on the one hand, and between HDA and liquid water on the other. However, none of the structures can be successfully modelled as microcrystalline ice structures, so we are forced to conclude that, on the basis of these data, that they are all genuinely non-crystalline. The low Q behavior of the structure factors indicates no significant density or concentration fluctuations over the length scale probed.
 Moving from HDA, through liquid water and LDA to ice Ih, the second shell radial order increases at the expense of spatial order. This is linked to a fifth first neighbor interstitial that restricts the orientations of first shell waters. This lynch pin molecule which may keep the HDA structure intact has implications for the nature of the HDA-LDA transition that bear on the metastable water debate. The structure of VHDA is characterised by a doubled occupancy of this interstitial location. As would be expected for a thermally activated unlocking of the stabilising interstitial, the transition from VHDA to LDA is very sharp. Although its higher density makes VHDA a better candidate than HDA for a physical manifestation of the second putative liquid phase of water, as for the HDA case, the VHDA to LDA transition also appears to be kinetically controlled.
 To investigate a possible relationship between HDA on the one hand and LDA and VHDA on the other, a simple two component analysis was performed on the oxygen-oxygen partial radial distribution function. We were unable to reproduce satisfactorily the HDA function by a linear combination of the two other amorphous structures. This suggests further that HDA is structurally distinct and not directly related by a simple linear combination of the two other amorphous states.
 Finally, the x-ray structure factors for all five amorphous systems have been calculated for comparison with other studies.

Organizer: KOHARA (PHS 3910)