The new phase Ba6CuIr4O15 has been shown by powder X-ray diffraction to be a member of the perovskite-related structural family A3n+3Aâ ²n Bn+3O6n+9withn=1; space group R32, a=10.1196(3), c=13.4097(4) Ã . The trigonal-prismatic Aâ ² 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 aâ 10.1, câ 4.4 Ã 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) Ã . It is shown that the same structural formalism can be used to describe the incommensurate hexagonal phase of SrMn1-xCoxO 3-y.