Origin of the stable 6x6x4 superlattice in PrNiO2 thin films

Determine the physical mechanism responsible for the appearance and stability of the giant 6x6x4 unit-cell superlattice (with in-plane commensurabilities δh ≈ δk ≈ 1/6 and out-of-plane commensurability δℓ ≈ 1/4) in annealed PrNiO2 thin films, and explain why this superstructure persists over a ~300 K temperature range irrespective of whether the SrTiO3 capping layer is crystalline or amorphous.

Background

The paper reports, via high-energy grazing-incidence x-ray diffraction, a robust giant superlattice in PrNiO2 thin films characterized by period-six in-plane order and period-four out-of-plane stacking. This superstructure emerges upon heating above room temperature, displays long correlation lengths, and remains stable across a broad temperature window before disappearing at high temperatures where the system reverts to the ANiO3 perovskite stoichiometry.

While the authors suggest that oxygen diffusion and ordering (e.g., partial apical oxygen occupation) could be involved—consistent with observed c-axis expansion and the ability to quench the state—the specific reason that a commensurate 6x6x4 superlattice is favored and maintained across ~300 K is not established, motivating a targeted investigation into its underlying mechanism and energetics.