# Observation of a mixed close-packed structure in superionic water

## TL;DR

X-ray experiments reveal that superionic water has a mixed structure at high pressures, challenging previous assumptions and improving understanding of planetary interiors.

## Contribution

First experimental evidence of a mixed close-packed superionic phase in water at extreme pressures.

## Key findings

- Diffraction patterns show mixed BCC and FCC structures in superionic water at high pressures.
- Results challenge the pure FCC-SI phase model and align with advanced ab initio predictions.
- Findings help resolve contradictions in previous experimental data on superionic ice.

## Abstract

The study of superionic (SI) water has been a highly active research area since its theoretical prediction. Despite significant experimental and computational efforts, its melting curve and the stability of different oxygen lattices remain debated, impacting our understanding of SI ice’s peculiar transport properties. Experimental results at lower pressures show disagreement, whereas data at higher pressures are scarce due to the extreme challenges of such experiments. In this work, we present ultrafast X-ray diffraction results of water compressed by multiple shocks to pressures up to  ~ 180 GPa. At pressures exceeding 150 GPa and temperatures around 2500 K, our diffraction patterns challenge the pure FCC-SI phase model, providing experimental evidence of the mixed close-packed superionic phase predicted by advanced ab initio calculations. At lower pressures, we observe simultaneous signatures of BCC and FCC structures within a pressure-temperature range consistent with some static-compression experiments, helping to resolve contradictory results in literature. These insights offer new constraints on the stability domains of SI phases and reveal detailed structural features, such as stacking faults. Our results advance the structural understanding of high-pressure SI ice to a level approaching that of ice I polymorphs, with potential implications for water-rich interiors of giant planets.

X-ray study of compressed water shows that superionic ice adopts mixed close-packed structures rather than a single phase - a far more complex behaviour than expected, mirroring solid ice’s rich phases and informing planetary interior models.