Volumetric study consistent with a glass-to-liquid transition in amorphous ices under pressure

Dilatometry experiments on low- and high-density amorphous ices up to 0. 30 GPa are presented together with powder x-ray diffraction data. Repeated isobaric heating and cooling cycles reveal three competing processes: irreversible (micro) structural relaxation, reversible relaxation, and (irreversible) crystallization. The third and subsequent heating runs produce identical curves, i. e. , irreversible relaxation is absent. We interpret the deviation from linear expansivity in these curves as the onset temperature of the volumetric glass-to-liquid transition (T₆, ₎₍ₒ₄ₓ) and report its dependence on pressure.