StrongAnisotropic Dynamics of Ultra-Confined Water.

Dynamicsof water confined in ∼5 Å diameter channels of beryl andcordierite single crystals were studied by using inelastic (INS) andquasielastic (QENS) neutron scattering. The INS spectra for both sampleswere similar and showed that there are no hydrogen bonds acting onwater molecule, which experiences strong anisotropic potential, steepalong the channels and very soft perpendicular to it. The high-resolution(3.4 μeV) QENS data revealed gradual freezing out of the watermolecule dynamics for both minerals at temperatures below about 80K when the scattering momentum transfer was parallel to the channels,but not when it was perpendicular to the channels. The QENS studywith medium energy resolution (0.25 meV) of the beryl with the scatteringmomentum transfer along the channels showed gradual freezing out ofwater molecule dynamics at temperatures below about 200 K, whereasat higher temperatures the data could be described as 2-fold rotationaljumps about the axis coinciding with the direction of the dipole moment(that is, perpendicular to the channels), with a residence time of5.5 ps at 225 K. The energy resolution dependence of the apparentdynamics freezing temperature suggests gradual slowing down of therotational jumps as the temperature is decreased, until the associatedQENS broadening can no longer be detected, rather than actual freezing. [ABSTRACT FROM AUTHOR]