Can oxygen deficient SmFeAsO$_{1-}_{x}$ be synthesized?: Unintentional incorporation of hydride ion at oxygen vacancy site

Hydrogen substitution and introduction of oxygen vacancy are effective electron doping methods for the \textit{Ln}FeAsO. However, their $T_{c}$ vs e$^{-}$/Fe diagrams do not entirely overlap each other, while $T_{c}$ vs lattice dimension relationships are very similar. These contradictions can be understood by assuming that unintentional hydrogen is incorporated into the oxygen vacancy. To examine the preferred electron-dopant species in \textit{Ln}FeAsO and the influence of the atmosphere during synthesis on the formation of \textit{Ln}FeAsO$_{1-}_{x}$, we try to synthesize the SmFeAsO$_{1-}_{x}$ under three well-controlled atmosphere (H$_{2}$O, H$_{2}$ and H$_{2}$O- and H$_{2}$-free). Under H$_{2}$O and H$_{2}$ atmosphere, hydrogen were incorporated at oxygen sites as the hydride ion and SmFeAsO$_{1-}_{x}$H$_{x}$ was formed. On the other hand, when H$_{2}$O and H$_{2}$ were removed from synthetic process, nearly stoichiometric SmFeAsO was formed. Furthermore, DFT calculations showed that H-substituted samples are more stable than oxygen deficient samples. These results strongly imply that the reported \textit{Ln}FeAsO$_{1-}_{x}$ was contaminated with unintentional hydrogen and \textit{Ln}FeAsO$_{1-}_{x}$H$_{x}$ was misidentified as \textit{Ln}FeAsO$_{1-}_{x}$.