Anomalous transformations in ice VIII

Synchrotron far-infrared spectroscopy and linear response density-functional theory provide a new interpretation of the unique low-temperature isostructural transition in ice VIII. This study shows that it is a first-order thermodynamic transformation. Previously unexplained discrepancies have been resolved: the transformation between two forms of ice VIII arises from subtle phonon instabilities with an isostructural movement of molecules in the ice VIII structure.