Search for new superconducting oxides in M4Pn2O phases (M=Ca, Sr, Ba and Pn= As, Sb, Bi)

All the high-Tc oxide superconducors known to date are copper-based oxides (cuprates) in the layered perovskite crystalstructures that contain a planar network of copper and oxygen (CuO2 plane). In searching for possible new superconducting metaloxides without Cu ions, we have examined the layered tetragonalpnictides M4Pn2O (M¼ Ca, Sr, Ba and Pn ¼ As, Sb, Bi) because their structures are considered as an anti-perovskite with alternate stacking of two kinds of layers; conducting M2Pn2 block layers sandwiched by insulating perovskite-based M2O layers. Compounds were prepared by melting stoichiometric amounts of the components in tantalum tube all sealed under an argon atmosphere. The samples have been characterised by scanning electron microscopy, powder X-ray diffraction (XRD), metallographic examination and electrical resistivity. For these stoichiometric compounds no superconductive transition is observed.