Isotopic Raman broadening due to Anderson localization of harmonic phonons in partially deuterated ice VII and VIII

Abstract

Raman spectra from molecular systems are typically regarded as comprising a set of sharp peaks. These arise from both single-molecule modes and extended Γ point phonons being, in theory, harmonic oscillators. Broadening is typically assigned to thermal, linewidth, and anharmonic effects. Here we measure significant broadening in the Raman signal from partially deuterated water ices VII and VIII, which is absent in the pure systems. This isotopic broadening is much greater than any anharmonic effects and is reproduced by lattice dynamics calculations assuming only harmonic bonding. Instead, it arises from mass disorder induced phonon localization. This localization leads to many overlapping modes with a range of frequencies dependant on local isotopic environment and Raman activity due to lack of distinct molecular or crystalline symmetry.