MODULATED STRUCTURE OF BA6ZNIR4O15 - A COMPARISON WITH BA6CUIR4O15 AND SRMN1-XCOXO3-Y

The new phase Ba6CuIr4O15 has been shown by powder X-ray diffraction t o be a member of the perovskite-related structural family A(3n+3)A(n)' Bn+3O6n+9 with n = 1; space group R32, a = 10.1196(3), c = 13.4097(4) Angstrom. The trigonal-prismatic A' sites and the octahedral B sites a re both occupied by a disordered distribution of Ir and Cu. A combinat ion of electron microscopy and X-ray diffraction has shown that Ba6ZnI r4O15 has a related incommensurate structure with a approximate to 10. 1, c approximate to 4.4 Angstrom and a modulation vector q = a+0.35c* . Alternatively, Ba6ZnIr4O15 can be considered as a composite of trigo nal (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) Angstrom. It is shown that the same structural formalism can be used to describe the incomme nsurate hexagonal phase of SrMn1-xCoxO3-y.