Incoherent inelastic neutron scattering measurements on ice VII: Are there two kinds of hydrogen bonds in ice?

We report the vibrational spectrum of recovered ice VII measured by inelastic incoherent neutron scattering and compare this to similar data of its fully hydrogen-ordered form, ice VIII, under exactly the same conditions (15 K, 1 bar). The spectra of the two phases have their principal features at similar energies, in both the translational and vibrational bands, with a substantial disorder-related broadening in ice VII. In particular, we find no evidence for a peak at 49 meV in ice VII which earlier was associated with the possible existence of two kinds of hydrogen bonds. Additional Raman measurements in ice VII and ice VIII show that the O-H stretching frequencies in the two phases are almost identical. Therefore, the presence of split molecular-optic bands in ice phases, including ordinary ice Ih, is likely related to an incomplete description of the phonon dispersion rather than to a fundamentally new feature in the nature of the hydrogen bond. (authors)