New iron-based arsenide oxides (Fe2As2)(Sr4M2O6)(M = Sc, Cr)

We have discovered new arsenide oxides (Fe 2 As 2 )(Sr 4 M 2 O 6 ) (M = Sc, Cr: M-22426). These materials are isostructural with (Fe 2 P 2 )(Sr 4 Sc 2 O 6 ), which was found in our previous study. The new compounds are tetragonal with a space group of P 4/ nmm and consist of the anti-fluorite-type FeAs layer and a perovskite-type blocking layer. These compounds have long interlayer Fe–Fe distances corresponding to the c -axis length; the 15.8 Å in Sc-22426 is the longest in the iron-based pnictide oxide systems. Chemical flexibility of the perovskite block in this system was probed by chromium-containing (Fe 2 As 2 )(Sr 4 Cr 2 O 6 ). Different trends were found in bond angle and bond length of the new pnictide oxides compared to the reported systems, such as REFePnO. The absence of superconductivity in these compounds is considered to be due to insufficient carrier concentration as in the case of undoped REFeAsO.