Modulated structure of Ba₆ZnIr₄O₁₅; a comparison with Ba₆CuIr₄O₁₅ and SrMn_1-xCo_xO_{3- y}

Abstract

The new phase Ba₆CuIr₄O₁₅ has been shown by powder X-ray diffraction to be a member of the perovskite-related structural family A_3n+3A′_n B_n+3O_6n+9withn=1; space group R32, a=10.1196(3), c=13.4097(4) Å. The trigonal-prismatic A′ sites and the octahedral B sites are both occupied by a disordered distribution of Ir and Cu. A combination of electron microscopy and X-ray diffraction has shown that Ba₆ZnIr₄O₁₅ has a related incommensurate structure with a≈10.1, c≈4.4 Å and a modulation vector q=a*+0.35c*. Alternatively, Ba₆ZnIr₄O₁₅ can be considered as a composite of trigonal (cell 1) and rhombohedral (cell 2) substructures with a₁=a₂=10.1228(9) c₁=4.4099(6), c₂=2.6982(2) Å. It is shown that the same structural formalism can be used to describe the incommensurate hexagonal phase of SrMn_1-xCo_xO _3-y.

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