  We report that the incorporation of hydroxide ions (OH)- significantly
enhances the superconducting transition temperature (Tc) in the LnFeAsO-based
superconductors (Ln1111: Ln = La, Ce and Pr). For La1111, Tc 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 Tc is also observed for Ce1111 and Pr1111. 1H-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.
