Raman Investigation of the Ice Ic-Ice Ih Transformation

Among the many ice polymorphs, ice I, that is present in nature at ambient pressure, occurs with two different structures, i.e., stable hexagonal (Ih) or metastable cubic (Ic). 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 structures with remarkable accuracy. Moreover, the comparison of two determinations of the transformation kinetics also provides a measurement of the activation energy for this transformation process.