  We have discovered new layered oxyarsenides (Fe2As2)(Sr4M2O6) (M = Sc, Cr:
M-22426). These materials are isostructural with (Fe2P2)(Sr4Sc2O6), which was
found in our previous study. The new compounds are tetragonal with a space
group of P4/nmm and consist of the anti-fluorite type FeAs layer and
perovskite-type blocking layer. The lattice constants are a = 4.050 A, c =
15.809 A for M = Sc and a = 3.918 A, c = 15.683 A for M = Cr. These compounds
have long interlayer Fe-Fe distances corresponding to the c-axis length, the
15.8 A in Sc-22426 is the longest in the iron-based oxypnictide systems.
Chemical flexibility of the perovskite block in this system was probed by
chromium containing (Fe2As2)(Sr4Cr2O6). Different trends were found in bond
angle and bond length of the new oxypnictides compared to the reported systems,
such as REFePnO. Absence of superconductivity in these compounds is considered
to be due to insufficient carrier concentration as in the case of undoped
REFeAsO.
