A new homologous series of iron pnictide oxide superconductors (Fe2As2)(Can+2(Al, Ti)nOy) (n=2, 3, 4)

We have discovered a new homologous series of iron pnictide oxides (Fe2As2)(Can+2(Al, Ti)(n)O-y) (n = 2, 3, 4). These compounds have perovskite-like blocking layers between Fe2As2 layers. The structure of the new compounds is tetragonal with space groups of P4/nmm for n = 2 and 4 and P4mm for n = 3, which are similar to those of (Fe2As2)(Can+1(Sc, Ti)(n)O-y) (n = 3, 4, 5) found in our previous study. Compounds with n = 3 and 4 have new crystal structures with three and four sheets of perovskite layers, respectively, including a rock salt layer in each blocking layer. The a-axis lengths of the three compounds are approximately 3.8 angstrom, which are close to those of FeSe and LiFeAs. (Fe2As2)(Ca-6(Al, Ti)(4)O-y) exhibited bulk superconductivity in magnetization measurement with T-c(onset) similar to 36 K and resistivity drop was observed at similar to 39 K. (Fe2As2)(Ca-5(Al, Ti)(3)O-y) also showed large diamagnetism at low temperatures. These new compounds indicate that considerable room still remains for new superconductors in layered iron pnictides.