Cubic alkaline-earth manganite perovskites have been the focus of many computational and physical studies because of their potential multiferroic properties, in which the ferromagnetic and ferroelectric order originate from the electrons on the Mn⁴⁺ ions and their bonding with oxygen, resulting in a potentially strong magnetoelectric coupling. Depending on the composition, especially the size of the alkaline earth cation and oxygen content, the ground state structure can adopt various polytypic perovskite structures [1-4], which differ in the amount of cubic or hexagonal stacking of the eutactic (nearly close-packed) planes, which also controls the amount of corner-sharing and face-sharing Mn-O octahedra. BaMnO₃ has a ground state structure called 2H, which has all hexagonal stacking and all face shared octahedra, while SrMnO₃ adopts the 4H structure, which has half hexagonal / half cubic stacking and half corner-shared / half face-shared octahedra. The 3C (or cubic) versions of these materials have all cubic stacking and all corner-shared octahedra, and are known or predicted to be strongly coupled multiferroics. In the Sr_1-xBa_xMnO₃ system, the highest known Ba-content to form in the 3C structure is x = 0.5.

In this work we revisit the Sr_1-xBa_xMnO₃ system with the goal of understanding the fundamental challenges to the epitaxial synthesis of cubic polytypes. While controlling thermodynamic parameters, e.g. temperature, pressure, component activity, etc., during bulk synthesis has proven invaluable in accessing some specific metastable polytypes, it is clear that stabilizing a 3C BSMO compound will be increasingly difficult with higher Ba concentrations (x > 0.5), and that additional parameters are needed.  Epitaxial stabilization is one method that offers an additional structure-directing thermodynamic parameter: the interface with the substrate. Furthermore, kinetic processes during epitaxial growth also can impact nucleation outcomes, and in some cases improve the possibilities of accessing new metastable materials. Nevertheless, the largest known value of x found for thin films is 0.5 [5], identical to bulk material, for very thin (< 10 nm) on particular substrate (DyScO₃) [5]. 

We will review a high-throughput deposition method we call combinatorial substrate epitaxy (CSE), where films are deposited upon epi-polished polycrystalline substrates, and describe its utility for understanding thermodynamic influences on 3C and 4H growth for (Ca,Sr,Ba)MnO₃. We demonstrate that all film-substrate perovskite polytype pairs align their eutactic planes and directions, regardless of substrate type or orientation. Finally, we show that certain orientations lead to improved metastable film formation than others. Focusing on pushing the stability boundary of Sr_1-xBa_xMnO₃ beyond x=0.5, we focus on films of x = 0.5 and 0.6. We will demonstrate that kinetic aspects control polymorph formation as well, with 4H seeming to have a kinetic advantage over 3C. Interval pulsed laser depostion (iPLD), where growth is interrupted after deposition of approximately a monolayer [6] to allow for kinetic relaxation processes to occur, was combined with CSE to demonstrate that Sr_0.4Ba_0.6MnO_3-y can be stabilized as a 3C polytype when controlling both thermodynamic and kinetic factors.  Both film flatness and, more importantly, volume of the 3C polymorph improved with iPLD, resulting in 40 nm single-phase Sr_0.4Ba_0.6MnO_3-y films on single-crystal DyScO₃ and polycrystalline GdScO_3. The results imply that iPLD improves persistent nucleation of highly metastable phases and offers a new approach to epitaxial stabilization of novel materials, including more Ba-rich BSMO compositions in the 3C structure.