Reversibility and isotope effect of the calorimetric glass --> liquid transition of low-density amorphous ice.

We here report differential scanning calorimetry (DSC) scans recorded by repeatedly heating the H(2)O (D(2)O) low density amorph (LDA) which was made by isothermal decompression of very high-density amorphous ice (VHDA) at 140 K from 1.1 to 0.006 GPa. These DSC scans show a glass --> liquid transition endotherm with an onset temperature (T(g)) of approximately 137 (140) K at a heating rate of 30 K min(-1) accompanied by an increase in heat capacity of approximately 1.7 (1.5) J K(-1) mol(-1). We establish the reversibility of this effect by thermally cycling between its glassy state below 137 K and its highly viscous liquid state at 149 K. All calorimetric signatures, including H/D isotope effect, are highly similar to the signatures in hyperquenched glassy water (HGW). We argue that the observation of almost identical calorimetric traces for HGW and LDA implies that there is no need to reassign HGWs T(g) to higher temperatures provided that the viscous liquid state connected to both LDA and HGW behaves as an ideally "strong" liquid in the Angell classification. We furthermore show that LDA prepared by isothermal decompression of VHDA is more crystallization-resistant than LDA made from high-density amorphous ice (HDA) by isobaric warming. We suggest that the former route via VHDA removes "nanocrystalline remnants" in LDA which are still present in the latter after pressure-amorphization of hexagonal ice to HDA at 77 K.