Metastable ice VII at low temperature and ambient pressure

Ice exhibits many solid-state transformations under pressure, and also displays a variety of metastable phases(1). Most of the high-pressure phases of ice can be recovered at ambient pressure provided that they are first cooled below about 100 K. These ice polymorphs might exist on the surfaces of several satellites of the outer planets(2), One of the few exceptions to this (meta) stability on quenching has been ice VII, the dominant high-pressure phase. Here we show that isothermal compression of D2O ice VI below 95 K produces pure ice VII, and that this phase can remain stable at atmospheric pressure. It remains metastable indefinitely at 77 K. Like the other recoverable ice phases, it transforms to low-density amorphous ice between about 120 and 150 K at 1 bar. The temperature range over which ice VII remains metastable increases markedly on compression to 6 GPa, indicating that ice VII is in fact the most robust of all the metastable ice phases.