Superconductivity in new iron pnictide oxide Fe$_{2}$As$_{2}$Sr$_{4}$(Mg,Ti)$_{2}$O$_{6}$

A new iron arsenide oxide Fe$_{2}$As$_{2}$Sr$_{4}$MgTiO$_{6}$, which is isostructural with the iron-based superconductor Fe$_{2}$\textit{Pn}$_{2}$Sr$_{4}M_{2}$O$_{6}^{[1,2]}$, has been successfully synthesized by the solid-state reaction in quartz ampoules. Fe$_{2}$As$_{2}$Sr$_{4}$MgTiO$_{6}$ has antifluorite-type iron arsenide layer and K$_{2}$NiF$_{4}$-type oxide layer, while the $M$-site is composed of a combination of divalent (Mg$^{2+})$ and tetravalent (Ti$^{4+})$ cations as in the case of a double perovskite La(Mg,Ti)O$_{3}$. This fact indicates chemical flexibility of the perovskite-related layer in this system. This compound showed bulk superconductivity with $T_{c}$ of $\sim $20 K by partial substitution of Co for Fe. Moreover, high $T_{c}$ above 35 K was recorded by samples starting from Co-free and Ti-rich compositions, Fe$_{2}$As$_{2}$Sr$_{4}$(Mg$_{1-x}$Ti$_{x})_{2}$O$_{6}$ ($x$ =0.7$\sim $0.8). [1] H. Ogino \textit{et al}., \textit{Supercond. Sci. Technol.} \textbf{22} (2009) 075008. [2] X. Zhu \textit{et al}., \textit{Phys. Rev. B} \textbf{79} (2009) 220512(R).