Fig. 1.

Pressure and temperature conditions (circles) of the experimental runs (SI Appendix, Table S1). Black and white circles represent the presence and absence of Mg${}_2$FeH${}_6$, respectively. A color gradient in the background indicates a possible change in the chemical behavior of the system at 15 to 25 GPa. The arrow at the top shows the estimated range for the pressure at the atmosphere–magma boundary $P(\mathrm{AMB})$ in sub-Neptunes with masses of 9.2 to 13.6$M(E)$ for a radius of $3R(E)$ and 2% hydrogen (SI Appendix, Text 5). $R(E)$: Earth radius. $M(E)$: Earth mass. The MgO melting curve is from ref. 17. H-2023: a data point for MgO + Fe in a pure H medium from ref. 11.