Superconductivity at 52 K in hydrogen-substituted LaFeAsO(1-x)H(x) under high pressure

The 1111-type iron-based superconductor LnFeAsO(1-x)F(x) (Ln stands for lanthanide) is the first material with a T(c) above 50 K, other than cuprate superconductors. Electron doping into LaFeAsO by H, rather than F, revealed a double-dome-shaped T(c)-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 T(c) for the whole hydrogen-doping range in LaFeAsO(1-x)H(x) under pressures of up to 19 GPa. T(c) rises to 52 K at 6 GPa for the T(c)-valley composition between the two T(c) domes. This is the first instance of the T(c) exceeding 50 K in La-1111-type iron-based superconductors. On the other hand, the T(c) of SmFeAsO(1-x)H(x) decreased continually, keeping its single-dome structure up to 15 GPa. The present findings strongly suggest that the main reason for realization of the T(c) >50 K observed in RE-1111 compounds (RE: Pr, Sm, and Gd) at ambient pressure is the merging of SC1 and SC2.