The low-frequency density of states for amorphous and crystalline ices

Incoherent inelastic neutron scattering studies have been performed on two amorphous polymorphs and two crystalline polymorphs of ice, including high-density amorphous (hda) ice. Subsequent annealing cycles allowed data to be collected from low-density amorphous ice (lda), and crystalline ices Ic and Ih. Data were collected between 5 and 80 K for hda, and between 5 and 120 K for the other phases. The low-frequency (v < 1 THz) densities of states of the four phases are compared. The data illustrate clearly an excess number of modes in the hda density of states at 5 K centered at 0.65 THz. The magnitude of the excess g(v) is considerably reduced upon heating the sample from 5 to 20K. Above 60K the density of states of hda was found to exhibit predominantly a Debye behavior, g(v) ∝ Av², as v → 0 THz. No such excess modes were observed in lda or the two crystalline phases studied.