When water is cooled below its normal freezing point, it normally freezes to form hexagonal ice, or Ice I. If it is very pure and cooled carefully, it may be supercooled to about −42 °C. If water is cooled very rapidly then it forms an amorphoric glass.
HDA, High-Density Amorphous Ice, may be prepared by submitting Ice I (normal ice), cubic ice, or low density amorphous ice, to high pressure at very low temperatures, like cooled in liquid nitrogen.
HDA has a thermal conductivity less than that of Ice Ih, Ice Ic, or LDA during their pressurized densification; usually the thermal conductivity of materials increases with pressure.
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