The glassy water-cubic ice system: a comparative study by X-ray diffraction and differential scanning calorimetry

Mixtures of various ratios of cubic ice and glassy water were obtained by s o-called hyperquenching of micrometer-sized water droplets at cooling rates of approximate to 10(6)-10(7) K s(-1) on a substrate held at selected temp eratures between 130 and 190 K. These samples were characterized by differe ntial scanning calorimetry (DSC) and X-ray diffraction. The minimum deposit ion temperature to obtain almost entirely vitrified samples is approximate to 140 K. Glassy water prepared at this temperature exhibits on heating an endothermic step assignable to a glass --> liquid transition, without the r equirement for previous annealing. Cubic ice samples obtained by deposition at 160 and 170 K undergo on heating two distinct exothermic processes of c omparable intensity. One centered at approximate to 230 K is caused by the phase transition to hexagonal ice. The other is centered at approximate to 201 K in a sample deposited at 170 K, and it shifts to approximate to 193 K on deposition at 160 K. The latter process is attributed to the increase i n particle size, relief of non-uniform strain and/or healing of different k inds of defects. Since the temperature of this second exotherm depends on t he deposition temperature of the sample, it merges on sample deposition at 190 K with the exotherm from the cubic --> hexagonal ice phase transition. Therefore, this can lead to an overestimation of the heat of the cubic --> hexagonal phase transition. For samples deposited at less than or equal to 150 K, the low temperature exotherm merges with the intense exotherm due to glassy water --> cubic ice phase transition. X-ray diffractograms and DSC scans of cubic ice samples of different thermal history show, after anneali ng at the same temperature of 183 K for 5 min, essentially identical patter ns. Likewise, X-ray diffractograms of cubic ice made on heating hyperquench ed glassy water or vapor-deposited amorphous solid water up to 183 K are in distinguishable. Cubic ice deposited at 190 K, or annealed at 183 K, contai ns at most 20% amorphous component which persists up to the cubic to hexago nal ice phase transition. This is in contrast to recent claims of Jennisken s et al. (J. Chem. Phys. 1997, 107, 1232) that cubic ice obtained by heatin g thin films of vapor-deposited amorphous water contains more than 50% of a morphous, or even liquid, water.