Hydrogen in layered iron arsenides: Indirect electron doping to induce superconductivity

Utilizing the high stability of calcium and rare-earth hydrides, CaFeAsF_1-xHₓ (x $=$ 0.0--1.0) and SmFeAsO_1-xHₓ (x $=$ 0.0--0.47) have been synthesized using high pressure to form hydrogen-substituted 1111-type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H^- ions occupying F^- sites in the blocking layer of CaFeAsF. The resulting CaFeAsF_1-xHₓ is nonsuperconducting, whereas, SmFeAsO_1-xHₓ is a superconductor, with an optimal Tc $=$ 55 K at x {~} 0.2. It was found that up to 40% of the O^2- ions can be replaced by H^- ions, with electrons being supplied into the FeAs layer to maintain neutrality (O^2- $=$ H^- $+$ e^-). When x exceeded 0.2, Tc was reduced corresponding to an electron overdoped region.