Two-dimensional ferroelectricity has garnered significant research interest owing to its potential for enabling novel miniaturized devices. While conventional perovskite ferroelectrics inherently belong to three-dimensional structural systems, preserving robust ferroelectric properties in perovskite-derived structures down to the monolayer limit is important. In this work, we conduct first-principles calculations to systematically investigate the ferroelectricity of $\mathrm{BaTi}{\mathrm{O}}_3$ monolayer film, explicitly considering the influences of strain and electrode configuration. For a freestanding $\mathrm{BaTi}{\mathrm{O}}_3$ monolayer film with $\mathrm{Ti}{\mathrm{O}}_2$ termination, in-plane ferroelectricity along the [110] orientation is energetically favored. In the case of a $\mathrm{Ti}{\mathrm{O}}_2$-terminated $\mathrm{BaTi}{\mathrm{O}}_3$ monolayer interfaced with $\mathrm{SrRu}{\mathrm{O}}_3$ electrodes, the thermodynamically stable phase undergoes sequential transitions from an in-plane ferroelectric state to a paraelectric intermediate and then to an out-of-plane ferroelectric state, as the in-plane strain varies from tensile to compressive. We further reveal that both electrode and strain significantly modify the Ti-O bond lengths, which plays a crucial role in governing ferroelectric behavior. Besides, even when the thickness of freestanding $\mathrm{Ti}{\mathrm{O}}_2$-terminated $\mathrm{BaTi}{\mathrm{O}}_3$ film reaches eight unit cells, in-plane ferroelectricity along the [110] orientation remains energetically favored under strain-free conditions. However, the out-of-plane antiferroelectric state emerges as the thermodynamically stable state under in-plane compressive strain. Our research contributes to a more comprehensive understanding of ferroelectricity in monolayer perovskite oxides.

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• Ferroelectricity
• First-principles calculations
• Ferroelectrics
• Monolayer films
• Perovskites