A NEW MANGANITE WITH AN ORIGINAL COMPOSITE TUNNEL STRUCTURE - BA6MN24O48

A new manganite Ba6Mn24O48 with an original tunnel structure has been synthesized, Its electron diffraction study shows that this new phase exhibits a composite structure, built up of two tetragonal I sublattic es with a = 18.2 Angstrom, c(1) = 2.8 Angstrom, and c(2) = 4.6 Angstro m. The high-resolution electron microscopy investigation shows that th e octahedral [Mn4O8](infinity) host lattice consists of hollandite- an d rutile-type columns sharing the edges of their MnO6 octahedra so tha t a new kind of ten-sided tunnel is generated, The powder X-ray diffra ction study confirms that the first sublattice (a, c(1)) corresponds t o the [Mn4O8] framework, The second sublattice (a, c(2)) is associated with the barium network. The incommensurate modulated superstructural spots in the ''a, c(2)'' sublattice are correlated to both the occupa ncy and displacive modulation of the barium ions in the two kinds of t unnels. (C) 1997 Academic Press.