  We study hydrogen doping effects in an iron-based superconductor
LaFeAsO_(1-y) by using the first-principles calculation and explore the reason
why the superconducting transition temperature is remarkably enhanced by the
hydrogen doping. The present calculations reveal that a hydrogen cation stably
locating close to an iron atom attracts a negatively-charged FeAs layer and
results in structural distortion favorable for further high temperature
transition. In fact, the lattice constant a averaged over the employed
supercell shrinks and then the averaged As-Fe-As angle approaches 109.74
degrees with increasing the hydrogen doping amount. Moreover, the calculations
clarify electron doping effects of the solute hydrogen and resultant
Fermi-level shift. These insights are useful for design of high
transition-temperature iron-based superconductors.
