AbstractThe 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 $$T_{\text{c}}$$
T
c
is strongly affected by the pnictogen height $$h_{Pn}$$
h
Pn
from the Fe-plane that controls the electron correlation strength and the size of the $$d_{xy}$$
d
xy
hole Fermi surface (FS). With increasing $$h_{Pn}$$
h
Pn
, weak coupling BCS superconductivity switches to the strong coupling non-BCS one where electron correlations and the $$d_{xy}$$
d
xy
hole FS may be important.
Universal behavior of the upper critical field in iron-based superconductors
