Modulated structure of Ba6ZnIr4O15; a comparison with Ba6CuIr4O15 and SrMn1−xCoxO3− y

The new phase Ba 6 CuIr 4 O 15 has been shown by powder X-ray diffraction to be a member of the perovskite-related structural family A 3n+3 A′ n B n+3 O 6n+9 withn=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 Ba 6 ZnIr 4 O 15 has a related incommensurate structure with a≈10.1, c≈4.4 Å and a modulation vector q = a *+0.35 c *. Alternatively, Ba 6 ZnIr 4 O 15 can be considered as a composite of trigonal (cell 1) and rhombohedral (cell 2) substructures with a 1 =a 2 =10.1228(9) c 1 =4.4099(6), c 2 =2.6982(2) Å. It is shown that the same structural formalism can be used to describe the incommensurate hexagonal phase of SrMn 1-x Co x O 3-y .