Superspace description of the crystal structures of Ca[n] (Nb,Ti)[n]O[3n+2] ( n = 5 and 6)
What is it about?
A common (3+1)-dimensional superspace model is used to describe the crystal structures of both compositions within a particular homologous series, with the modulation wavevector and the width of the atomic domains systematically varying with composition. The two crystal structures are characterized as commensurately modulated structures consisting of discontinuous atomic domains described by occupational crenel functions.
Why is it important?
For n = 6, the centrosymmetric (3+1)-dimensional superspace symmetry provides a natural explanation for the pseudo-symmetries that are present in the non-centrosymmetric (three-dimensional) superstructure of this compound. The efficiency of the superspace approach is demonstrated by structure refinements in (3+1)-dimensional superspace and by comparing these results with the refinements in their three-dimensional superstructures.