Low-energy neutron vibrational spectra of high pressure phases of ice

An extra intensity was observed in inelastic neutron scattering spectra at low energy transfers for vapour-deposited amorphous ice and high-density amorphous (hda) ice compared to their crystalline analogues, hexagonal ice-Ih and high-pressure phase ice-VI, respectively. These low-energy excitations (LEE) in the deposited amorphous ice can be mainly described as a Boson peak. The origin of the LEE for the hda ice is not clear; it can be either due to the Boson peak or can originate from a softening of sound waves.