Computationally guided epitaxial synthesis of (Sr/Ba)MnO[formula omitted] films on (Sr/Ba)TiO[formula omitted] substrates

Abstract

Density functional theory (DFT) was used in the computation of thermodynamic terms relevant to the competition between epitaxial polytypes during nucleation of (Sr/Ba)MnO3 on (100), (110), and (111) cubic (Sr/Ba)TiO3 substrates. Values for volumetric formation energies, volumetric strain energies, and area-specific interface energies were computed for different polytypes (3C, 4H, and 2H) and were incorporated in a standard (capillarity) model for epitaxial nucleation. Experimental orientation relationships (ORs) for SrMnO3 were used in the construction of strained and interface cells for (Sr/Ba)MnO3. For 3C polytypes, the OR is simply cube-on-cube, or (111)[11̄0]3C,film||(111)[11̄0]3C,sub, and is isostructural with cubic (Sr/Ba)TiO3. For 4H/2H polytypes, the orientation relationship is (001)[100]4H/2H,film||(111)[11̄0]3C,sub, which can only be modeled with a coherent interface on the (111) substrate. Results indicate that 3C SrMnO3 has increased energies (becomes less stable) on moving from SrTiO3 substrate orientations (100) to (110) to (111), consistent with experimental observations. For BaMnO3, similar trends are predicted, although no experimental data is available for comparison. We use the DFT results to discuss the different thermodynamic contributions to polytype stability, and assess the feasibility of stabilizing a 3C BaMnO3 film on (Sr/Ba)TiO3 substrates.