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 %such as the complex hP4 phase of sodium exhibit unexpected structural complexity at extreme pressures. Here we report experimental evidence that water ice undergoes a transformation from ice-X to a less symmetric structure above 222(12) GPa. Synchrotron X-ray diffraction reveals a distortion of the cubic lattice consistent with either \textit{Pbcm} or \textit{P}4$_2$\textit{/nnm} symmetry, the former being strongly favorable by \textit{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.