The glass transition in high-density amorphous ice

There has been a long controversy regarding the glass transition in low-density amorphous ice (LDA). The central question is whether or not it transforms to an ultraviscous liquid state above 136K at ambient pressure prior to crystallization. Currently, the most widespread interpretation of the experimental findings is in terms of a transformation to a superstrong liquid above 136K. In the last decade some work has also been devoted to the study of the glass transition in high-density amorphous ice (HDA) which is in the focus of the present review. At ambient pressure HDA is metastable against both ice I and LDA, whereas at >0.2GPa HDA is no longer metastable against LDA, but merely against high-pressure forms of crystalline ice. The first experimental observation interpreted as the glass transition of HDA was made using in situ methods by Mishima, who reported a glass transition temperature Tg of 160K at 0.40GPa. Soon thereafter Andersson and Inaba reported a much lower glass transition temperature of 122K at 1.0GPa. Based on the pressure dependence of HDA's Tg measured in Innsbruck, we suggest that they were in fact probing the distinct glass transition of very high-density amorphous ice (VHDA). Very recently the glass transition in HDA was also observed at ambient pressure at 116K. That is, LDA and HDA show two distinct glass transitions, clearly separated by about 20K at ambient pressure. In summary, this suggests that three glass transition lines can be defined in the p–T plane for LDA, HDA, and VHDA. •The recent literature about the glass transition in amorphous ices is reviewed.•LDA, HDA and VHDA show three distinct glass transition temperatures Tg.•HDA's Tg at 1bar is 20K lower than LDA's Tg.•The pressure dependence for HDA's Tg is: Tg(p)=115.9K∗(1+p/0.00779GPa)0.056.•Calorimetry and dielectric data are consistent with liquid nature above Tg.