Figure 4.

(a–c) Jahn–Teller (DFT-JT) and (d–f) spin disproportionated (SD) structural models for the P^′3 phase. Na⁺/vacancy zigzag ordering in the Na-layer (pink/green zigzags overlaid) is present in both models (a, d). Honeycomb (or equivalent) ordering of Ni sites on the triangular Ni lattice is also present in both models (b, e). Note that in the DFT-JT model, while there are 3 crystallographic Ni sites, Ni₍₁₎ (grey), Ni_(2A) (dark blue)/Ni_(2B) (light blue), their NiO₆ environments are essentially identical; thus, they are equivalent for the purpose of charge ordering. Ni–O coordination within the unit cell (cutoff for long bonds drawn = 2.0 Å) differs between the two models (c, f). In these “ball and stick” style figures, the atoms are enlarged for visual clarity. In all panels, 1 × 3 × 3 unit cells are shown, while the unit cell itself is denoted by the “black box outline”.