Using synchrotron x-ray diffraction in laser-heated diamond anvil cells, we report the observation of a hexagonal close-packed (hcp) phase of water ice at high pressure and temperature conditions. Above 200 GPa and 1800 K, the hcp phase becomes dominant upon entering the superionic regime, as evidenced by anomalous thermal expansion. Observations are consistent with the hcp phase becoming thermodynamically more stable than the face-centered cubic (fcc) phase via a martensitic transition extending across the 130–200 GPa pressure range, within the superionic regime. Hcp ice is also observed to emerge from stacking disorder developing within the fcc oxygen lattice upon cooling, during its reversion to the body-centered cubic phase. The presence of an fcc-hcp martensitic transition in the superionic regime of warm dense ice may have implications for planetary models of Uranus and Neptune.

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• Crystal structure
• High-pressure studies
• Phase transitions
• Ice
• Planets & planetary systems
• X-ray diffraction