Moiré superlattice formation by natural twistronics of coexisting variants in Ca ₂ MnO ₄ ( <i>n</i>  = 1) Ruddlesden–Popper member

Abstract Moiré superlattice architecture by stacking two‐dimensional (2D) monolayers with a relative angle of twist keeps delivering emerging functionalities. Since the discovery of this novel lattice architecture, this concept has rapidly progressed in 2D van der Waals materials. Synthesis of freestanding single‐crystalline oxide membranes made it possible to extend this concept to predominantly ionic nano membranes. Our work demonstrates the observation of natural moiré superlattice formation in as‐synthesized bulk Ca 2 MnO 4 ( n = 1) Ruddlesden–Popper (RP) member. Our investigation confirms the coexistence of tetragonal I4 1 /acd, orthorhombic Acam structural phases, and the 90° orientational variant of the orthorhombic phase along the [001] axis with a well‐defined orientation relationship. These coexisting variants are present in nano lamellae type morphology stacked along the [001] axis. Relative in‐plane rotation among these stacked nano lamellae by a fixed angle of approximately 2.9° leads to the formation of a moiré superlattice with chessboard‐type nanodomains contrast, each nanodomain is approximately 3.66 × 3.66 nm in size, sharing {220} type interfaces. The modulation wavelength of the moiré superlattice along the &lt;110&gt; type direction is approximately 7.32 nm. A decrease in bandgap value of Ca 2 MnO 4 by approximately 0.16 eV is observed.