Incommensurate phases in the Ba-Mn-Pd-O system

Polycrystalline samples of bulk composition Ba5Mn3PdO12, Ba6Mn4PdO15, and B a7Mn5PdO18 have been synthesised and characterised by X-ray diffraction, el ectron diffraction and magnetometry. These compounds adopt a 2H-related cry stal structure consisting of [001] chains of MnO6 and PdO6 polyhedra, with Ba2+ cations separating the chains. The three compounds, each of which is c ommensurate in the xy plane but incommensurate along the z axis, differ in the ratio of octahedra to trigonal prisms in the chains (3 : 1, 4 : 1 and 5 : 1 respectively). The lattice parameters of the four dimensional trigonal unit cell are typically a approximate to 10.02 Angstrom, c(1) approximate to 4.3 Angstrom, c(2) approximate to 2.61 Angstrom. The homogeneity and cry stallinity of the samples increases with the ratio of octahedra to trigonal prisms and is greater in the xy plane than parallel to z. The magnetic sus ceptibility of all three phases deviates from the Curie-Weiss law below 100 K, but no magnetic phase transition is apparent above 5 K.