A new layered iron arsenide oxide (Fe 2 As 2 )(Ca 4 (Mg,Ti) 3 O y ) was discovered. Its crystal structure is tetragonal with a space group of I4=mmm consisted of the anti-fluorite type FeAs layer and blocking layer of triple perovskite cells and is identical with (Fe 2 As 2 )(Sr 4 (Sc,Ti) 3 O 8 ) discovered in our previous study. The lattice constants of (Fe 2 As 2 )(Ca 4 (Mg,Ti) 3 O y ) are a ¼ 3:877A and c ¼ 33:37 A. This compound exhibited bulk superconductivity up to 43 K in susceptibility measurement without intentional carrier doping. A resistivity drop was observed at 47 K and zero resistance was achieved at 42 K. These values correspond to the second highest T c among the layered iron-based superconductors after REFeAsO system. #

We synthesized new layered iron arsenide oxides (Fe2As2)(Sr4(Sc,Ti)3O8), (Fe2As2)(Ba4Sc3O7.5), and (Fe2As2)(Ba3Sc2O5). The crystal structures of these compounds are tetragonal with a space group of I4/m m m. The structure of (Fe2As2)(Sr4(Sc,Ti)3O8) and (Fe2As2)(Ba4Sc3O7.5) consists of the alternate stacking of antifluorite Fe2As2 layers and triple perovskite-type oxide layers. The interlayer distance between the Fe planes of (Fe2As2)(Ba4Sc3O7.5) is ∼18.7 Å. Moreover, the a-axis of (Fe2As2)(Ba3Sc2O5) is the longest among the layered iron pnictides, indicating the structural flexibility of the layered iron pnictide containing perovskite-type layers. The bulk sample of (Fe2As2)(Sr4(Sc0.6Ti0.4)3O8) exhibited diamagnetism up to 28 K in susceptibility measurements.

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A new layered iron arsenide oxide (Fe 2 As 2 )(Ca 5 (Mg,Ti) 4 O y ) and its structural derivative were found in the Fe-As-Ca-Mg-Ti-O system. The crystal structure of (Fe 2 As 2 )(Ca 5 (Mg,Ti) 4 O y ) is identical to that of (Fe 2 As 2 )(Ca 5 (Sc,Ti) 4 O y ), which was reported in our previous study. The lattice constants of this compound are a = 3.86(4) Å and c = 41.05(2) Å. In addition, another phase with a thicker blocking layer was found.The structure of the compound and its derivative was tentatively assigned through STEM observation as (Fe 2 As 2 )(Ca 8 (Mg,Ti) 6 O y ) with sextuple perovskite-type sheets divided by a rock salt layer. The interlayer Fe-Fe distance of this compound is ~30 Å.The compound and its derivative exhibited bulk superconductivity, as found from magnetization and resistivity measurements.