Distinguish ice XX from plastic ice VII

Determine whether the dynamically disordered body‑centered‑cubic ice phase referred to as ice XX is thermodynamically distinct from the plastic ice VII phase, by establishing if they constitute separate phases rather than manifestations of the same bcc dynamical state.

Background

The paper discusses ongoing controversy over the designation of high‑temperature bcc phases of ice, including a superionic phase labeled ice XX and a recently reported plastic ice VII. Both share a body‑centered‑cubic oxygen sublattice and exhibit dynamic disorder, leading to ambiguity about whether they are distinct phases.

The authors express concern that current experimental evidence may be insufficient to resolve whether ice XX and plastic ice VII are different thermodynamic phases. They suggest that direct evidence of superionic proton dynamics and clear thermodynamic discontinuities marking phase transitions are needed for firm identification and differentiation.