Hydrogen in layered iron arsenide: 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 first 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 non-superconducting, whereas SmFeAsO1-xHx is a superconductor, with an optimal Tc = 55 K at x 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, Tc was reduced corresponding to an electron over-doped region. 
Taku Hanna, Yoshinori Muraba, Satoru Matsuishi, Katsuaki Kodama,   Shin-ichi Shamoto and Hideo Hosono
cond-mat.supr-con