Data for publication “Ultrahigh Pressure Phase Transition in Ice” by I.Osmond et al

X-ray powder diffraction data for H2O in cell UHP8, collected upon compression from 103 to 346 GPa at the P02.2 beamline at the PETRA III synchrotron facility during beamtime I-20241017. One of the 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. 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, 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.