Fig. 5: Experimental P-T diagram with the observed H₂O phases.

Square symbols represent data points from this study: black squares indicate the BCC phase, green squares denote regions where BCC and FCC phases coexist, while violet squares indicate areas with observed FCC/HCP. Error bars on P-T conditions represent the variation among simulations consistent with the VISAR uncertainty. Yellow, blue, green, and purple shaded areas are indicative regions for BCC, fluid, BCC+FCC, and Mixed stacking phases, respectively. Triangles show data from dynamic compression experiments: blue and orange triangles correspond to BCC and FCC-SI phases, respectively, as reported in ref. ¹⁴. Circles represent results from ref. ¹²: filled circles indicate coexistence of FCC and BCC phases, while open circles correspond to the BCC phase only. The solid blue and black lines indicate the experimentally determined melting curve and the BCC-FCC transition line, respectively, based on refs. ^13,23. Dashed lines represent predicted melting curves from refs. ^7,35, and the dash-dotted line shows the theoretical phase boundary proposed by Ref. ⁶. The hatched area indicates the BCC-FCC coexistence region from ref. ⁷ (chemical potentials μ_bcc = μ_fcc). For additional comparisons with previous experimental and theoretical results, as well as the Pressure-density diagram, see Supplementary Information Figs. S17 and S18.