Molecular Reorientation Dynamics Govern the Glass Transitions of the Amorphous Ices.

The glass transitions of low-density amorphous ice (LDA) and high-density amorphous ice (HDA) are the topic of controversial discussions. Understanding their exact nature may be the key to explaining the anomalies of liquid water but has also got implications in the general context of polyamorphism, the occurrence of multiple amorphous forms of the same material. We first show that the glass transition of hydrogen-disordered ice VI is associated with the kinetic unfreezing of molecular reorientation dynamics by measuring the calorimetric responses of the corresponding H2O, H2(18)O, and D2O materials in combination with X-ray diffraction. Well-relaxed LDA and HDA show identical isotopic-response patterns in calorimetry as ice VI, and we conclude that the glass transitions of the amorphous ices are also governed by molecular reorientation processes. This "reorientation scenario" seems to resolve the previously conflicting viewpoints and is consistent with the fragile-to-strong transition from water to the amorphous ices.