Structural analysis and superconductivity of CeFeAsO _1-xH _x

We performed the neutron powder diffraction (NPD) and synchrotron x-ray diffraction measurements on CeFeAsO _1-x(D,H) _x (x = 0.0 - 0.48) as a representative of 1111-type family of iron-based superconductors LnFeAsO _1-xH _x (Ln = lanthanoid). Deuterated and hydrogenated samples (CeFeAsO _1-xD _x and CeFeAsO _1-xH _x) were synthesized by the solid-state reaction of a metal oxide, arsenides, and a hydride and a deuteride source under an applied pressure of 2 GPa. No distinct differences were found between the structural and superconducting properties of the hydride and deuteride samples. Rietveld analyses of the NPD patterns demonstrated that deuterium exclusively substitutes on the oxygen sites in the 1111-type structures according to the nominal composition. Bulk superconductivity was observed over a wide x region (0.1 < x < 0.4) and the superconducting dome had a rather flat shape with a maximum T _c = 47 K at around x = 0.25. It was concluded from density functional theory calculations and comparison with the superconducting dome of the fluorine-substituted system that the charge state of the hydrogen substituting the oxygen sites was -1. The relationship between the lattice parameter a and T _c in our samples prepared from metal hydrides is almost the same as that reported previously for samples prepared from cerium hydroxide. These results strongly suggest that H ^- ions exclusively occupy the oxygen sites in both samples, regardless of the hydrogen species in the starting material.