College Park, Maryland      June 6 - 10 , 2004

TP49: Structural comparison of supercooled water and intermediate density amorphous ices

J. Urquidi (Dept. of Physics, New Mexico State University; Los Alamos Neutron Science Center, Los Alamos National Laboratory), C. J. Benmore (Intense Pulsed Neutron Source, Argonne National Lab), P. A. Egelstaff (Dept. of Physics, University of Guelph), M. Guthrie (Intense Pulsed Neutron Source, Argonne National Lab), S. E. McLain (Dept. of Chemistry, University of Tennessee), C. A. Tulk (Spallation Neutron Source, Oak Ridge National Laboratory), D. D. Klug (National Research Council, Chalk River, Ontario, Canada), J. F. C. Turner (Dept. of Chemistry, University of Tennessee)

Presented here are new neutron diffraction data on ultrapure bulk supercooled heavy water measured down to 262 K. The data are analyzed in terms of the trends observed in the first sharp diffraction peak (FSDP) parameters. The neutron FSDP position, height and width are compared to literature data for supercooled water, water under pressure and to the same parameters obtained for recently discovered intermediate density amorphous ices. It is found that the FSDP parameters in supercooled water and the amorphous ices generally exhibit a similar behavior, suggesting a new structural regime may occur in deeply supercooled water at about Q0~1.83-1 (T~251K) associated with increased intermediate range ordering. It is argued that this structural regime may be linked to a similar trend in the density, which appears when the density is plotted as a function of FSDP position. A detailed comparison of the neutron and x-ray structure factors for supercooled water and intermediate density amorphous ices having the same FSDP positions is also made. The diffraction data show that although the overall general structures are qualitatively very similar, the amorphous ice correlations are considerably sharper and extend to much higher radial distances.

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