No abstract

The single-crystal structures of two incommensurate compounds, Ba 1+x [(Cu x Rh 1-x )O 3 ] with x ) 0.1605 and x ) 0.1695, were solved by a general method using the superspace group approach. A saw-tooth function was used to model both the occupational and displacive modulations. This approach requires very few refineable parameters and leads to low residuals, R ) 0.0370 and R ) 0.0330 for x ) 0.1605 and x ) 0.1695, respectively. The idealized structures of these two compounds consist of [(Cu x Rh 1-x )O 3 ] ∞ chains containing groups of five face-shared (RhO 6 ) octahedra separated by (CuO 6 ) trigonal prisms. These chains in turn are separated from each other by [Ba] ∞ chains. The idealized composition of both phases is Ba 7 CuRh 5 O 18 . Their modulated structures, described in the text, deviate from the ideal composition and structure. The approach used for the structure determinations of these compounds is completely general and can be applied to other modulated composite structures of this series.