Effect of heating rate and pressure on the crystallization kinetics of high-density amorphous ice on isobaric heating between 0.2 and 1.9 GPa

The crystallization kinetics of high-density amorphous ice (HDA), made by pressure-amorphizing hexagonal ice at 77 K, and its dependence on heating rate and pressure was studied on isobaric heating between 0.2 and 1.9 GPa by displacement-temperature curves. The crystalline phases, recovered at 77 K and 1 bar, were characterized by X-ray diffraction. The general pattern on isobaric crystallization of HDA is that of a parallel reaction, with one ice phase crystallizing slowly at low temperatures, and the other rapidly at higher temperatures. Their relative yields can be varied by varying the heating rate. The pairs of ice phases formed at constant pressure are: ice Ih and ice IX at 0.21 GPa, ice IX and ice V at 0.51 GPa, ice IV and ice XII at 0.81 and 1.21 GPa, and ice XII and ice VI at 1.41 GPa. Crystallization kinetics becomes more complicated at 0.71 GPa and up to 4 ice phases form. Our observation of parallel reactions requires that at a given pressure the rate constant for formation of one ice phase increases much more with temperature than that of the other. It follows that one ice phase forms predominantly on slow heating at low temperatures (called type 1), whereas the other forms mainly on rapid heating at higher temperatures (called type 2). The effect of pressure increase for a constant rate of heating is for type 1 ice phases to slow down further and suppress their formation, whereas for type 2 ice phases slowing down of kinetics leads to crystallization as type 1 phases. The ice phases crystallize with increasing pressure in the order ice Ih, ice IX, ice V, ice IV, ice XII, and ice VI as expected for their increasing density. We further speculate that parallel reactions may also be dominant in the crystallization kinetics of pressure-amorphized silica, and that the heating rate at a given pressure may have a strong effect on percentage of the crystallized phase.