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

All the high-T_c oxide superconducors known to date are copper-based oxides (cuprates) in the layered perovskite crystal structures that contain a planar network of copper and oxygen (CuO₂ plane). In searching for possible new superconducting metal oxides without Cu ions, we have examined the layered tetragonal pnictides M₄Pn₂O (M = Ca, Sr, Ba and Pn = 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.