Probing of Structure Factor of Water to 57 GPa and 1500 K
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Probing of Structure Factor of Water to 57 GPa and 1500 K Alexander F. Goncharov1,2, Chrystele Sanloup3, Nir Goldman2, Jonathan C. Crowhurst2, Lawrence E. Fried2, Nicola Guignot4, Mohamed Mezouar4, and Yue Meng5 1 Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, DC, 20015 2 Lawrence Livermore National Laboratory, University of California, Livermore, CA, 94551 3 Universite Pierre et Marie Curie and Institut du Globe de Paris, Paris, France 4 European Synchrotron Radiation Facility, Grenoble, France 5 Argonne National Laboratory, HPCAT, Advanced Photon Source, Argonne, IL, 60439
ABSTRACT The x-ray structure factor of water has been measured along the melting line to 57 GPa and 1500 K using focused monochromatic synchrotron radiation and laser heated diamond anvil cell. The oxygen radial distribution function, g(r) is determined from these data. We have also calculated g(r) using ab initio methods and find a good agreement with the experiment. Based of the similarity of the measured and calculated structure factors determined density of water under extreme conditions unattainable previously. INTRODUCTION The behavior of water under extreme conditions of high pressure and temperature has a fundamental interest for physics, chemistry, geo- and planetary sciences. Knowledge of the structure and properties of water under these conditions has a profound importance for better understanding of the Earth’s and large planets (such as Uranus and Neptune) interior [1], and nature of bonding under high compression. The study of the structure of liquids at moderate pressure and high temperature has recently become feasible with the development of third generation synchrotron sources. A liquidliquid phase transformation has been detected in phosphorus [2], which corresponds to a change of the local atomic coordination. These transformations are intimately related to those that can be seen under elevated pressure in solids. In water, a major change was found at 1 GPa [3,4], in parallel to the break down of open low-coordinated ice structures under pressure. Recent neutron study to 6.5 GPa and 670 K [5] showed that water has a local structure of simple liquid at these conditions. Theoretical calculations and experiments in the laser and externally heated diamond anvil cell (DAC) indicate a dissociation and ionization of water above 20 GPa and a formation a superionic solid phase at higher pressure [6,7,8,9,10]. Here we report x-ray diffraction measurements and first principle molecular dynamics simulations of the structure factor of water to 57 GPa and 1500 K. EXPERIMENT AND THEORY Samples were loaded in a diamond anvil cell (DAC) with tungsten carbide seats of a Boehler-Almax design (with up to 65 degrees total opening) [11] or with conventional seats having a slot-like opening of similar value in one direction. Iridium squares of 40x40x10 µm3
with approximately 12 µm diameter central hole were employed in the latter case to absorb the laser radiation and to transfer
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