Nonhexagonal Na Sublattice Reconstruction in the Super-Ionic Conductor Na2Zn2TeO6: Insights from Ab Initio Molecular Dynamics

In this work, we examine the distribution of Na+ ions in the interlayer of the super-ionic conductor Na2Zn2TeO6 by means of atomistic first-principle modeling based on density functional theory. This layered structure presents a variety of partially occupied interstitial Na sites forming a triangular prismatic coordination group with the neighboring O atoms. We examine the energetics of Na periodic arrangements, considering distinct occupation patterns of these interstitial sites. We then simulate high-temperature annealing of the system by means of ab initio molecular dynamics simulations, finding that the Na sublattice prefers a disordered distributions along the available interstitial sites rather than a honeycomb arrangement conformal to the periodicity of the Zn–Te layers. Suitable supercells are constructed, reproducing the arrangement patterns observed in the most favorable configurations obtained by simulated annealing. We report a structure with rhomboidal reconstruction of the Na sublattice as the most energetically favorable, and note that the associated occupation of the interstitial sites is compatible with experimental data. We also report the tendency for the formation of a pentagon–triangle reconstruction, predicted to become increasingly favorable as the system is compressed.