Phase diagram of ice polymorphs under negative pressure considering the limits of mechanical stability.

Thermodynamic and mechanical stabilities of various ultralow-density ices are examined using computer simulations to construct the phase diagram of ice under negative pressure. Some ultralow-density ices, which were predicted to be thermodynamically metastable under negative pressures on the basis of the quasi-harmonic approximation, can exist only in a narrow pressure range at very low temperatures because they are mechanically fragile due to the large distortion in the hydrogen bonding network. By contrast, relatively dense ices such as ice Ih and ice XVI withstand large negative pressure. Consequently, various ices appear one after another in the phase diagram. The phase diagram of ice under negative pressure exhibits a different complexity from that of positive pressure because of the mechanical instability.