Superstructure of Locally Disordered Na₂Zn₂TeO₆

This work expands on the superstructure in the layered oxide Na2Zn2TeO6 (NZTO). The diffractogram contains weak peaks that are not fitted by the average structure but are found to belong to a 2 × 2 × 3 commensurate modulation of the structure. The modulated structure was found by randomizing the starting points to ensure that the global minima are identified. The resulting superstructure is then used to generate a starting structure for reversed Monte Carlo (RMC) fitting of the total scattering data. We identify two structural features that are preserved through both fittings. One is that two of the Zn/Te columns in the layer direction have a significantly higher degree of disorder, with the exchange of Zn and Te. The second feature is that one of the crystallographic Na sites (2a) was found to be empty. The Na atoms are located midway between crystallographic 2a and 6g sites, which surrounds half of the 2a sites. This type of off-centering is found both in the modulated structure and in the corresponding structure preserved through RMC modeling. The results are discussed in relation to variable-temperature solid-state NMR data measured between 100 and 293 K. We suggest that the phenomenon is a feature of a dynamic situation and an important clue to progress the understanding of ion dynamics in more detail.