Evidence for symmetry-reduction in solid H2O above 222 GPa

Water (H 2 O), one of the most consequential compounds in the Solar System, governs the structure and dynamics of icy planets, the terrestrial water cycle, and through the pressure-induced transition from hydrogen to ionic bonding, it exhibits marked changes over the pressure range of planetary interiors. While many materials are expected to favour simple, densely packed lattices with increasing compression, as observed in water ice-X, others exhibit unexpected structural complexity at extreme pressures. Here we report experimental evidence that water ice undergoes a transformation from ice-X to a loss of cubic symmetry above 222(12) GPa. Synchrotron X-ray diffraction reveals a distortion of the cubic lattice consistent with either Pbcm or P 4 2 /nnm symmetry, the former being strongly favorable by ab initio calculations. This transition marks the onset of a new regime of structural complexity in high-pressure H 2 O, redefining the upper stability limit of ice-X.