Superconductivity at 39 K in New Iron Pnictide Oxide (Fe2As2)(Sr4(Mg,Ti)2O6)

We have discovered a new iron pnictide oxide superconductor (Fe2As2)(Sr4(Mg,Ti)2O6). This material is isostructual with (Fe2As2)(Sr4M2O6) (M = Sc, Cr), which was found in our previous study. The structure of this compound is tetragonal with a space group of P4/nmm and consists of the anti-fluorite type FeAs and perovskite-type blocking layers. The lattice constants are a = 3.935 A and c = 15.952 A for (Fe2As2)(Sr4MgTiO6). In both magnetization and resistivity measurements, this compound exhibits superconductivity below 10 K. Moreover, ratio of Mg and Ti in this compound can be changed toward Ti-rich composition. (Fe2As2)(Sr4Mg1-xTi1+xO6) phase is obtained at 0 < x as a main phase, and Tc and superconducting volume fraction increase with increasing x. The highest Tc(onset) was confirmed at 39 K for x = 0.6 in resistivity measurement.