  Many sub-Neptune exoplanets have been believed to be composed of a thick
hydrogen-dominated atmosphere and a high-temperature heavier-element-dominant
core. From an assumption that there is no chemical reaction between hydrogen
and silicates/metals at the atmosphere-interior boundary, the cores of
sub-Neptunes have been modeled with molten silicates and metals (magma) in
previous studies. In large sub-Neptunes, pressure at the atmosphere-magma
boundary can reach tens of gigapascals where hydrogen is a dense liquid. A
recent experiment showed that hydrogen can induce the reduction of Fe$^{2+}$ in
(Mg,Fe)O to Fe$^0$ metal at the pressure-temperature conditions relevant to the
atmosphere-interior boundary. However, it is unclear if Mg, one of the abundant
heavy elements in the planetary interiors, remains oxidized or can be reduced
by H. Our experiments in the laser-heated diamond-anvil cell found that heating
of MgO + Fe to 3500-4900 K (close to or above their melting temperatures) in a
H medium leads to the formation of Mg$_2$FeH$_6$ and H$_2$O at 8-13 GPa. At
26-29 GPa, the behavior of the system changes, and Mg-H in an H fluid and
H$_2$O were detected with separate FeH$_x$. The observations indicate the
dissociation of the Mg-O bond by H and subsequent production of hydride and
water. Therefore, the atmosphere-magma interaction can lead to a fundamentally
different mineralogy for sub-Neptune exoplanets compared with rocky planets.
The change in the chemical reaction at the higher pressures can also affect the
size demographics (i.e., "radius cliff") and the atmosphere chemistry of
sub-Neptune exoplanets.
