Israel Osmond, Mikhail A Kuzovnikov, David A Lewis, Hannah A Shuttleworth, Tomás Marqueño, Milo J Dixon, Cerian E A Robertson, Vitan Cherepnalkoski, Bihan Wang, Andreas Hermann, Miriam Peña-Alvarez, Ross T Howie
One of the most enduring problems in dense H2O ice has been the experimental realization of the pressure-induced symmetry breaking of cubic ice X to an orthorhombic crystal structure. Here, through diamond anvil cell experiments combined with synchrotron X-ray diffraction and Raman spectroscopy techniques, we demonstrate that post-symmetrization, the crystal structure of ice X continuously distorts with pressure. Subjecting ice to pressures in excess of 308(5) GPa, we observe a first-order phase transition to ice XXII, consistent with the long-predicted phase possessing an orthorhombic crystal structure with Pbcm symmetry. We find that ice XXII is stable to pressures of at least 340(5) GPa.