Structural Analysis on Iron-Based Superconductor Pr1111 System with   Oxygen Deficiency and Flourine Substitution
  We have performed structural analyses on iron-based superconductors, PrFeAsO$_{1-y}$ and PrFeAsO$_{1-x}$F$_x$, systematically, by means of Rietveld method on neutron powder diffraction data. The shifts of iron ion valence from +2, $\delta$, are accurately determined from the occupancies of O and O$_{1-x}$F$_x$ sites obtained by the Rietveld analysis and F-concentration obtained by secondary ion-microprobe mass spectrometry. $T_\textrm{c}$-$\delta$ curve of PrFeAsO$_{1-y}$ is different from the curve of PrFeAsO$_{1-x}$F$_x$, indicating that $\delta$ is not a principal parameter for $T_\textrm{c}$ in so-called 1111 system. Structural parameters of the FeAs layers, for example, As-Fe-As bond angle and As-height from Fe layer, are different between both systems with similar $\delta$-values. Their parent compounds are also found to have different structural parameters, possibly due to the different synthetic conditions. These results suggest that the difference of structural parameters of FeAs layer is the origin of the discrepancy of $T_\textrm{c}$-$\delta$ curves of both systems and the $T_\textrm{c}$-value in the 1111 system is sensitive to the structural parameters. It may be attribute to an energy balance of the conducting bands contributing to the superconductivity. 
Katsuaki Kodama, Motoyuki Ishikado, Fumitaka Esaka, Akira Iyo, Hiroshi   Eisaki, Shin-ichi Shamoto
cond-mat.supr-con