Abstract
Utilizing the high stability of calcium and rare-earth hydrides, CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 0.0--1.0) and SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 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${}^{\ensuremath{-}}$ ions occupying F${}^{\ensuremath{-}}$ sites in the blocking layer of CaFeAsF. The resulting CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ is nonsuperconducting, whereas, SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ is a superconductor, with an optimal ${T}_{\mathrm{c}}$ $=$ 55 K at $x$ \ensuremath{\sim} 0.2. It was found that up to 40% of the O${}^{2\ensuremath{-}}$ ions can be replaced by H${}^{\ensuremath{-}}$ ions, with electrons being supplied into the FeAs layer to maintain neutrality (O${}^{2\ensuremath{-}}$ $=$ H${}^{\ensuremath{-}}$ $+$ ${e}^{\ensuremath{-}}$). When $x$ exceeded 0.2, ${T}_{\mathrm{c}}$ was reduced corresponding to an electron overdoped region.
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