Preferential orientation relationships in Ca₂MnO₄ Ruddlesden-Popper thin films

A high-throughput investigation of local epitaxy (called combinatorial substrate epitaxy) was carried out on Ca₂MnO₄ Ruddlesden-Popper thin films of six thicknesses (from 20 to 400 nm), all deposited on isostructural polycrystalline Sr₂TiO₄ substrates. Electron backscatter diffraction revealed grain-over-grain local epitaxial growth for all films, resulting in a single orientation relationship (OR) for each substrate-film grain pair. Two preferred epitaxial ORs accounted for more than 90% of all ORs on 300 different microcrystals, based on analyzing 50 grain pairs for each thickness. The unit cell over unit cell OR ([100][001]_film ∥ [100][001]_substrate, or OR1) accounted for approximately 30% of each film. The OR that accounted for 60% of each film ([100][001]_film ∥ [100][010]_substrate, or OR2) corresponds to a rotation from OR1 by 90° about the a-axis. OR2 is strongly favored for substrate orientations in the center of the stereographic triangle, and OR1 is observed for orientations very close to (001) or to those near the edge connecting (100) and (110). While OR1 should be lower in energy, the majority observation of OR2 implies kinetic hindrances decrease the frequency of OR1. Persistent grain over grain growth and the absence of variations of the OR frequencies with thickness implies that the growth competition is finished within the first few nm, and local epitaxy persists thereafter during growth