BALA4CU5O13+DELTA AN OXYGEN-DEFICIENT PEROVSKITE BUILT UP FROM CORNER-SHARING CUO6 OCTAHEDRA AND CUO5 PYRAMIDS

The structue of an oxygen-deficient perovskite BaLa//4Cu//5O//1//3// plus // delta has been determined by neutron powder diffraction and high-resolution electron microscopy. The framework left bracket Cu//5O//1//3 right bracket is built up from corner-sharing CuO//5 pyramids and CuO//6 octahedra forming hexagonal tunnels and perovskite cages where the La**3** plus and Ba**2** plus ions are located in an ordered manner. The barium ions are located in the perovskite tunnels where the lanthanum ions are located in the hexagonal tunnels. One typical feature of the host lattice left bracket Cu//5O//1//3 right bracket deals with the geometry of the hexagonal tunnels which is rather different from the ideal model derived from the stoichiometric perovskite.