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.
VHDA, Very-high density amorphous ice, discovered in 2001, may be prepared by submitting High density amorphous ice (HDA) to temperatures of around 77 K, to isobaric heating, then to tempertures of around 160 K at 1.15 gigapascals (GPa). The molecular structure is unknown but it has been assumed that the hydrogen-bond-topology is mostly similar to low density amorphous ice (LDA) or HDA. If higher pressure is used (2–4 GPa) on HDA then high pressure ices (Ice VII or Ice IIX) are formed.
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