Abstract
We have collected neutron-diffraction data of sufficient quality for structural refinement from deuterated ice VII to over 60 GPa. At these pressures, we are approaching the onset of molecular dissociation where the water molecule should be significantly distorted. We fitted our data with both the conventional structural model for ice VII and a recently proposed "interstitial" model. We found these to be statistically indistinguishable and, therefore, adopted the conventional model in this work. Our refinements yield the first direct experimental determination of the molecular structure in this pressure regime. They show a clear elongation of the covalent bond, the rate of which rapidly increases with pressure, indicating we have entered a distinct regime on the approach to ice X. In addition, we have directly measured the pressure dependence of the separation of the two equivalent deuteron sites along the hydrogen bonds, which provides an upper bound on the hydrogen-bond centering pressure.
| Original language | English |
|---|---|
| Article number | 184112 |
| Journal | Physical Review B |
| Volume | 99 |
| Issue number | 18 |
| DOIs | |
| State | Published - May 23 2019 |
Funding
The authors thank Dr. J. S. Loveday for useful discussions and assistance in retrieving the ISIS datasets. Neutron diffraction was conducted at the SNAP beamline of the Spallation Neutron Source, a Department of Energy (DOE) Office of Science User Facility operated by Oak Ridge National Laboratory, managed by UT-Batelle LLC, under Contract No. DE-AC95-00OR22725 for the US Department of Energy. X-ray diffraction was performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974, with partial instrumentation funding by the National Science Foundation (NSF). The Advanced Photon Source is a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.