First-Principles Investigation of the Epitaxial Stabilization of Oxide Polymorphs: TiO₂ on (Sr,Ba)TiO₃

Metastable polymorphs, many of which have never been fabricated, have been predicted to exhibit interesting and technologically relevant properties. Epitaxial synthesis is a powerful structure-directing method that can produce metastable polymorphs but is typically done in a trial and error fashion. Unfortunately, the relevant thermodynamic terms governing epitaxial synthesis of new materials are unknown. Accurate calculation of the relevant thermodynamic terms and their incorporation into predictive models would accelerate the synthesis of metastable polymorphs by identifying thermodynamically favorable paths. Using density functional theory with three different functionals, we computed several relevant terms for TiO₂ anatase (A) and rutile (R) film growth on low-index surfaces of SrTiO₃ (STO) and BaTiO₃ (BTO) cubic perovskites. After identifying potential coherent epitaxial interfaces based on experimental observations, the volumetric formation, volumetric strain, and areal substrate-film interface energies were calculated for (001)_A∥(001)_(S/B)TO, (102)_A∥(011)_(S/B)TO, (100)_R∥(111)_(S/B)TO, and (112)_A∥(111)_(S/B)TO coherent interfaces. These terms were integrated into a standard model of epitaxial nucleation, and the results yielded reasonable agreement between experimental observations and DFT predictions of the preferred epitaxial polymorph. Predicted trends in epitaxial stability were essentially independent of the three functionals used in the calculations. These results are discussed in light of their promise that DFT-informed epitaxial film growth can accelerate fabrication of new polymorphs. These results also validate the recently proposed 20 kJ/mol stability window for predicting which polymorphs could be epitaxially stabilized.