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Combinatorial substrate epitaxy (CSE) was used to study the orientation relationships (ORs) and polytypic stability of AEMnO3 (AE = Ca, Sr) thin films grown on polycrystalline SrMnO3 and SrTiO3 substrates. SrMnO3 films with the stable four‐layered hexagonal (4H) and metastable three‐layered cubic (3C) structures were also grown on (111) and (100) SrTiO3 single crystal substrates, respectively. Electron backscatter diffraction data were used to determine the following ORs, which hold true regardless of the substrate surface orientation: false(001false)[100]4HSrMnO3false|false|false(001false)[100]4HSrMnO3, false(111false)[11¯0]3CCaMnO3false|false|false(001false)[100]4HSrMnO3, and false(001false)[100]4HSrMnO3false|false|false(111false)[11¯0]3CSrTiO3. These are all simply the eutactic OR, which aligns the eutactic planes and directions; its ubiquity indicates that the interface energy is generally lower for the eutactic OR than for all other possible ORs. 3C SrMnO3 was found to grow only on very near (100) 3C SrTiO3 grains. This narrow range of epitaxial stabilization suggests that the penalties of higher interfacial and/or strain energies between polytypic perovskites adopting the eutactic OR are not significant enough to overcome the volumetric formation energy of the stable phase in these growth conditions, except for very special orientations.

Combinatorial substrate epitaxy (CSE) was used to investigate the epitaxial phase competition between the threelayered cubic (3C) and the four-layered hexagonal (4H) perovskite polymorphs of SrMnO 3 . Films were deposited on polycrystalline 3C-SrTiO 3 and 4H-SrMnO 3 substrates as a function of substrate orientation, temperature, and oxygen pressure. Electron backscatter diffraction data was analyzed using a dictionary-based indexing technique to determine that the eutactic orientation relationship (OR) was the preferred epitaxial orientation for both polymorphs across all conditions and substrates. An increase in substrate temperature and a decrease in oxygen pressure were found to expand the range of substrate orientations that support 3C growth from ( 100) to ( 110) and, finally, to (111) when 3C appeared as the stable phase on all SrTiO 3 substrate orientations. In these conditions, the metastable 4H phase was then epitaxially stabilized on nonbasal orientations of 4H-SrMnO 3 . These results highlight how CSE can be used to understand phase competition and prepare novel metastable epitaxial films.