Possible hydrogen doping and enhancement of Tc (=35 K) in a LaFeAsO-based superconductor

We report that the incorporation of hydroxide ions (OH)(-) significantly enhances the superconducting transition temperature (T-c) in the LnFeAsO-based superconductors (Ln1111: Ln=La, Ce, and Pr). For La1111, T-c of the (OH)(-) incorporated sample synthesized using high-pressure technique becomes 35 K, which is higher by 7 K than the typical optimally doped La1111 superconductors. Similar enhancement in T-c is also observed for Ce1111 and Pr1111. H-1-NMR measurement have confirmed the existence of hydrogen atoms in the samples. Accompanying the (OH)(-) incorporation, the lattice parameters are largely contracted, down to the values which have never been attained by any other dopings/substitutions.