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

Abstract

Polycrystalline samples of bulk composition Ba ₅ Mn ₃ PdO ₁₂ , Ba ₆ Mn ₄ PdO ₁₅ , and Ba ₇ Mn ₅ PdO ₁₈ have been synthesised and characterised by X-ray diffraction, electron diffraction and magnetometry. These compounds adopt a 2H-related crystal structure consisting of [001] chains of MnO ₆ and PdO ₆ polyhedra, with Ba ²⁺ cations separating the chains. The three compounds, each of which is commensurate 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≈10.02 Å, c ₁ ≈4.3 Å, c ₂ ≈2.61 Å. The homogeneity and crystallinity 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 susceptibility of all three phases deviates from the Curie-Weiss law below 100 K, but no magnetic phase transition is apparent above 5 K.