Structure determination of two new incommensurate compounds, Ba1+x[(CuxRh1-x)O-3] with x=0.1605 and x=0.1695, using a general method based on the superspace group approach

The single-crystal structures of two incommensurate compounds, Ba1+x [(CuxR h1-x)O-3] With x = 0.1605 and x = 0.1695, were solved by a general method u sing the superspace group approach. saw-tooth function was used to model bo th 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 o f these two compounds consist of [(CulambdaRh1-x)O-3](infinity) chains cont aining groups of five face-shared (RhO6) octahedra separated by (CuO6) trig onal prisms. These chains in turn are separated from each other by [Ba](inf inity) chains. The idealized composition of both phases is Ba7CuRh5O18. The ir modulated structures. described in the text, deviate from the ideal comp osition and structure. The approach used for the structure determinations o f these compounds is completely general and can be applied to other modulat ed composite structures of this series.