The changes of enthalpy at the irreversible transformation of high-density amorphous ice to low-density amorphous ice and of low-density amorphous ice to ice Ih have been measured in a Tian–Calvet heat-flow calorimeter. The equilibrium pressures of the two transformations have been estimated from the measured enthalpy and volume changes, neglecting the unknown, but small, entropy changes, as about 5 and 2 kbar, respectively. The course of the transformations has been followed in a piston-cylinder apparatus having an internal diameter of either 50 or 12 mm. The transformations are extraordinarily sharp when they are done slowly, the transformation pressure being constant to ±50 bar in 5 or 10 kbar in several transformations, although sometimes a sample transformed in two steps that were a few hundred bars apart. It seems that the activation volumes of the transformations are large and negative, and so the mechanism of the transformations is highly cooperative.

Floriano, M. A., Handa, Y. P., Klug, D. D., & Whalley, E. (1989). Nature of the transformations of ice I and low-density amorphous ice to high-density amorphous ice. The Journal of Chemical Physics, 91(11), 7187–7192.