  Among the many ice polymorphs, ice I, that is present in nature at ambient
pressure, occurs with two different structures, i.e. the stable hexagonal (Ih)
or the metastable cubic (Ic) one. An accurate analysis of cubic ice Ic was
missing until the recent discovery of an easy route to obtain it in a
structurally pure form. Here we report a Raman spectroscopy study of the
transformation of metastable ice Ic into the stable form Ih, by applying a very
slow temperature ramp to the ice Ic samples initially at 150 K. The thermal
behavior of the spectroscopic features originating from the lattice and
OH-stretching vibrational modes was carefully measured, identifying the
thermodynamic conditions of the Ic-Ih transition and the dynamics of both
structure with remarkable accuracy. Moreover, the comparison of two
determinations of the transformation kinetics allowed also to provide an
estimation of the activation energy for this transformation process.
