Vibrational dynamics of amorphous ice structures studied by high-resolution neutron spectroscopy

Abstract

The dynamics of amorphous water ice structures of different densities have been studied by high-resolution neutron time-of-flight and backscattering spectroscopy. An accurate determination of the vibrational density of states $G(\ensuremath{\omega})$ in the energy range of phonons $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\lesssim}40\text{ }\text{meV}$ of a many fold of structures comprising the low-density amorphous (LDA, $\ensuremath{\rho}\ensuremath{\approx}31\text{ }\text{molecules}/{\text{nm}}^{3}$), high-density amorphous (HDA, $\ensuremath{\rho}\ensuremath{\approx}39\text{ }\text{molecules}/{\text{nm}}^{3}$), very-high-density amorphous (vHDA, $\ensuremath{\rho}\ensuremath{\approx}41\text{ }\text{molecules}/{\text{nm}}^{3}$) and modifications of intermediate density in respect to HDA and vHDA has been achieved. Unlike the $G(\ensuremath{\omega})$ of high-density crystalline phases IX, V, and XII, which have been measured as reference systems, the $G(\ensuremath{\omega})$ of all high-density amorphous counterparts proves to be textureless except for a predominant peak at low energies. In vHDA this peak is centered at about 10 meV and redshifted upon density decrease to 7 meV in LDA. A concomitant upshift of the low-energy librational band edge from 34 to 45 meV is detected in deuterated samples. Mean-square displacement and Debye temperatures ${T}_{D}$ for vHDA, ${\mathrm{HDA}}^{\ensuremath{'}}$---a structure obtained as a transient product of the temperature induced vHDA to LDA transformation---and LDA are extracted from the highest resolution backscattering experiments. ${T}_{D}$ values indicate the absence of a dominant excess of low-energy modes in $G(\ensuremath{\omega})$, referred to as boson peak in the literature, being in agreement with the $G(\ensuremath{\omega})$ properties directly monitored by the time-of-flight technique. Having applied deuterated sample material we are able to display phase coherence effects within the phonon system in the second and third pseudo-Brillouin zone $(1\text{ }{\text{\AA{}}}^{\ensuremath{-}1}\ensuremath{\le}Q\ensuremath{\le}5\text{ }{\text{\AA{}}}^{\ensuremath{-}1})$ of the amorphous samples. A phase coherent signal from acoustic phonons can be followed up to energies of at least 15 meV.