Crystal Structure of Symmetric Ice X in H₂O-H₂ and H₂O-He under Pressure

Ice VII and ice X are the two most dominant phases, stable over a large pressure range between 2 and 150 GPa and made of fundamentally different chemical bonding. Yet, the two ice phases share a similar bcc-based crystal structure and lattice constants, resulting in a challenge to discern the crystal structure of ice VII and ice X. Here, we present well-resolved X-ray diffraction data of H₂O in quasi-hydrostatic H₂ and He pressure media, clearly resolving the two ice phases to 130 GPa and the dissociative nature of ice VII to X transition occurring at 20-50 GPa in H₂O-H₂ and 60-70 GPa in H₂O-He. The present diffraction data permits, for the first time, the accurate determination of the bulk moduli B₀ of 225 (or 228) GPa for ice X and 6.2 (or 4.5) GPa for ice VII, in H₂O-H₂ (or H₂O-He), which can provide new constraints for Giant planetary models.