  The structural flexibility at three substitution sites in LaFeAsO enabled
investigation of the relation between superconductivity and structural
parameters over a wide range of crystal compositions. Substitutions of Nd for
La, Sb or P for As, and F or H for O were performed. All these substitutions
modify the local structural parameters, while the F/H-substitution also changes
band filling. It was found that the superconducting transition temperature
$T_{c}$ is strongly affected by the pnictogen height $h_{Pn}$ from the Fe-plane
that controls the electron correlation strength and the size of the $d_{xy}$
hole Fermi surface (FS). With increasing $h_{Pn}$, weak coupling
superconductivity switches to the strong coupling one where the $d_{xy}$ hole
FS is crucially important.
