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

Utilizing the high stability of calcium and rare-earth hydrides, CaFeAsF1-xHx (x = 0.0-1.0) and SmFeAsO1-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 CaFeAsF1-xHx is nonsuperconducting, whereas, SmFeAsO1-xHx is a superconductor, with an optimal T-c = 55 K at x similar to 0.2. It was found that up to 40% of the O2- ions can be replaced by H- ions, with electrons being supplied into the FeAs layer to maintain neutrality (O2- = H- + e(-)). When x exceeded 0.2, T-c was reduced corresponding to an electron overdoped region.