Neutron diffraction study of the 'doubled perovskite' phases Ba ₂M_2-xCu_xO_4+δ (M = In, Sc)

Members of the series Ba₂M_2-xCu_xO _4+δ (M = In, x = 0.5, 1; M = Sc, x = 1) have been synthesized by solid-state reaction methods and characterised by powder neutron diffraction. Selected compositions were annealed under oxygen at 1 atm and 160 atm. The evolution of the structure with oxygen stoichiometry δ was studied. The materials form 'doubled perovskite' phases, A₂BB′O _6-x crystallising in tetragonal space group P4/mmm. The structures essentially consist of two ABO_3-x cubes stacked along the c axis in which the B cations are ordered on two crystallographically different sites. The divalent Ba cation is on average nine-coordinate and the trivalent M cation octahedrally coordinated to oxygen. The copper oxidation state and coordination varies with the oxygen level of the structure.