Superconductivity at 52 K in hydrogen-substituted LaFeAsO1-xHx under high pressure

The 1111-type iron-based superconductorLnFeAsO1-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 Kat 6 GPa for theTc-valley composition between the twoTc domes. This is the first instance of theTc 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.