Pure ices IV and XII from high-density amorphous ice

High-density amorphous ice (HDA), made by compression of hexagonal ice at 77 K, was heated at a constant pressure of 0.81 GPa up to 183 K and its phase transition followed by displacement-temperature curves. The crystalline phases, recovered at 77 K and 1 bar, were characterized by X-ray diffraction. Pure D2O ice IV and nearly pure H2O ice IV were formed on slow heating at a rate of 0.4 K min–1, whereas pure H2O ice XII and D2O ice XII were formed on fast heating at [Formula: see text] 15 K min–1. On heating HDA at rates in between these two values a mixture of ice IV and ice XII was obtained, where their relative yields depended in a systematic manner on the heating rate. Conversion of HDA into either ice IV or ice XII is an example of a "parallel reaction" where the relative yields of ice IV and ice XII can be controlled by temperature, i.e., by the rate of heating, in our approach. It is conceivable that a similar behaviour occurs on crystallization of the related pressure-amorphized silica. PACS Nos.: 61.10-i, 64.60-i, 64.70-p