ISOTOPE AND IMPURITY EFFECTS ON THE GLASS-TRANSITION AND CRYSTALLIZATION OF PRESSURE-AMORPHIZED HEXAGONAL AND CUBIC ICE

Amorphous solid D2O from hexagonal and cubic ices and H2O from (HF, NH3, and NH4F) doped hexagonal ice have been prepared by Whalley's method, and their thermal behavior has been investigated by differential scanning calorimetry from 103 to 250 K. For a heating rate of 30 K min-1, the temperature of the onset of the reversible glass half-arrow-right over half-arrow-left liquid transition, T(g), is raised by about 4 K when H2O is substituted by D2O. Doping with NH3 and NH4F causes little change in T(g) but with HF T(g) becomes undetectable. The increase in heat capacity at T(g) is reduced by about a factor of 3 on deuteration but marginal changes occur on doping with NH3 or NH4F. The increase in T(g) on deuteration is almost-equal-to 4 times of that observed for the amorphous forms made from the vapor and the liquid, and more than twice of that expected from isoviscous or isorelaxational states, assuming that the functional form of the molecular reorientation rates with temperature is unaffected by isotopic substitution. NH3 and NH4F as dopants, which generally increase the reorientation rate of water molecules in hexagonal and cubic ices, have no detectable effect on the calorimetric relaxation rate of the pressure-amorphized forms, thus indicating that the increase in the concentration of Bjerrum defects for these two dopants does not alter T(g). All these observations suggest that the structure of pressure-amorphized solids is topologically different from that of the amorphous forms made from the vapor and the liquid.