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)nOy) (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)nOy) (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 Å, which are close to those of FeSe and LiFeAs. (Fe2As2)(Ca6(Al, Ti)4Oy) exhibited bulk superconductivity in magnetization measurement with Tc(onset) ∼ 36 K and resistivity drop was observed at ∼ 39 K. (Fe2As2)(Ca5(Al, Ti)3Oy) also showed large diamagnetism at low temperatures. These new compounds indicate that considerable room still remains for new superconductors in layered iron pnictides.