Observation of Hexagonal Close-Packed Water Ice at Conditions in Ice Giant Planetary Interiors

Knowing the phase transformations in dense water ice is key to unraveling the peculiar geophysical properties of Uranus and Neptune, whose stratified interior models predict a thick ice layer beneath a convective ionic fluid layer. In the latter, water ice is currently assumed to adopt an fcc superionic structure, a phase that has recently been observed experimentally. Here, we report the observation of an hcp ice phase under such planetary conditions, using synchrotron x-ray diffraction in laser-heated diamond anvil cells. Between 80 and 200 GPa, we observe the coexistence of fcc and hcp ices, arising from stacking disorder developing within the fcc oxygen lattice upon temperature cycling. Above 200 GPa, the hcp phase dominates at high temperature, indicating increased thermodynamic stability upon entering a superionic state suggested by an anomalous thermal expansion. An anisotropic proton conductivity of superionic hcp ice, and the existence of a fcc-hcp martensitic transition may have planetary implications for dynamo models and for the dynamics of the ice mantle.