A gradual increase of the hydrogen content in LaFeAsO${}_{0.85}$H${}_x$ ($x$ = 0–0.85) resulted in a continuous $T_{\mathrm{c}}$ enhancement behavior. $T_{\mathrm{c}}$ increased significantly from ∼26 K ($x$ = 0) to a value of ∼35.5 K ($x$ = 0.85). As the $T_{\mathrm{c}}$ was enhanced, structure analysis revealed a coupled gradual contraction of the lattice constants and a reduction of the As-Fe-As bond angle towards the ideal value of 109.5° of the regular tetrahedron. Since the Hall coefficient measurement detected no substantial change in the carrier number, optimization of structural parameters rather than carrier doping is likely to have played the leading role in the $T_{\mathrm{c}}$ enhancement, which could provide an important clue to the strong correlation between superconductivity and the structural parameters of iron-based superconductors. Some key phenomenological parameters, including the lower and the upper critical magnetic fields, critical current density, the density of states at the Fermi level of hydrogen-free and substituted samples, were also compared based on the magnetic and electrical characterizations, but no substantial changes were found to be associated with the hydrogen addition.

• Received 9 March 2012

DOI:https://doi.org/10.1103/PhysRevB.86.054523