  The 1111-type iron-based superconductor LnFeAsO1-xFx (Ln stands for
lanthanide) is the first material with a Tc above 50 K, other than cuprate
superconductors. Electron doping into LaFeAsO by H, rather than F, revealed a
double-dome-shaped Tc-x diagram, with a first dome (SC1, 0.05<x<0.20) and a
second dome (SC2, 0.2<x<0.5). Here, we report the Tc for the whole
hydrogen-doping range in LaFeAsO1-xHx under pressures of up to 19 GPa. Tc rises
to 52 K at 6 GPa for the Tc-valley composition between the two Tc domes. This
is the first instance of the Tc exceeding 50 K in La-1111-type iron-based
superconductors. On the other hand, the Tc of SmFeAsO1-xHx decreased
continually, keeping its single-dome structure up to 15 GPa. The present
findings strongly suggest that the main reason for realization of the Tc >50 K
observed in RE-1111 compounds (RE: Pr, Sm, and Gd) at ambient pressure is the
merging of SC1 and SC2.
