An advanced hydride-ion substitution method on LaFeAsO uncovered the second superconducting (SC) phase with a higher Tc of 36 K (x～0.35) in addition to the first SC phase with the maximum Tc of 26 K (x～0.1). To investigate the origin of the two SC domes and whether a certain hidden phase exists beyond the second SC phase, we have performed a multi-probe study in the range 0.40 ≤ x ≤ 0.51 using X-ray, neutron and muon beams. We discover an antiferromagnetic phase and a unique structural transition in heavily electron-doped LaFeAsO1−xHx (x～0.5), which is regarded as a second parent phase.
