Superspace description of NaCa₄Nb₅O₁₇ (a perovskite-related compound of the type A _n B _n O_3n+2) as a modulated layered structure

Abstract

The recently determined structure of NaCa 4 Nb 5 O 17 is reanalyzed within the superspace framework. The material follows the general features of the superspace model proposed for the perovskite-related compounds of the general formula A n B n O 3 n +2 [Elcoro et al. (2001). Acta Cryst. B 57 , 471–484]. It can be described as a commensurate modulated structure with discontinuous atomic domains given by occupational crenel functions. These atomic domains automatically introduce the layered configuration of the actual structure in real space. However, the displacive modulations follow a quite different pattern from that observed in the Sr n (Nb,Ti) n O 3 n +2 series, discussed in the above-mentioned reference. The superspace group relevant in this new case has been identified through a systematic search of all the possible groups and a comparison of the resulting superspace embeddings of the experimental three-dimensional structure. Being a commensurate structure, the fundamental ambiguity of the superspace description was broken by choosing the highest possible symmetry that yields smooth displacive atomic modulations and hence minimizes the number of parameters. The efficiency of the proposed superspace model is demonstrated with a new refinement of the structure. Assuming the model's general relevance, symmetry properties for the whole [Ca,Na] n Nb n O 3 n +2 series are predicted.