Superstructure of Locally Disordered Na<sub>2</sub>Zn<sub>2</sub>TeO<sub>6</sub>

This work expands\non the superstructure in the layered\noxide Na<sub>2</sub>Zn<sub>2</sub>TeO<sub>6</sub> (NZTO). The diffractogram\ncontains\nweak peaks that are not fitted by the average structure but are found\nto belong to a 2 × 2 × 3 commensurate modulation of the\nstructure. The modulated structure was found by randomizing the starting\npoints to ensure that the global minima are identified. The resulting\nsuperstructure is then used to generate a starting structure for reversed\nMonte Carlo (RMC) fitting of the total scattering data. We identify\ntwo structural features that are preserved through both fittings.\nOne is that two of the Zn/Te columns in the layer direction have a\nsignificantly higher degree of disorder, with the exchange of Zn and\nTe. The second feature is that one of the crystallographic Na sites\n(2<i>a</i>) was found to be empty. The Na atoms are located\nmidway between crystallographic 2<i>a</i> and 6<i>g</i> sites, which surrounds half of the 2<i>a</i> sites. This\ntype of off-centering is found both in the modulated structure and\nin the corresponding structure preserved through RMC modeling. The\nresults are discussed in relation to variable-temperature solid-state\nNMR data measured between 100 and 293 K. We suggest that the phenomenon\nis a feature of a dynamic situation and an important clue to progress\nthe understanding of ion dynamics in more detail.