Presence of a purely tetragonal phase in ultrathin BiFeO₃ films: Thermodynamics and phase-field simulations

The stability of a purely tetragonal phase relative to the nominal rhombohedral phase in ultrathin BiFeO₃ films is investigated using thermodynamics and phase-field simulations. The thermodynamic analysis demonstrates the possible presence of a purely tetragonal state primarily due to the interfacial effect from the constraint of the adjacent layer although the built-in potential and compressive in-plane strain also play a role. Phase-field simulations of the corresponding ultrathin films reveal the coexistence of tetragonal and rhombohedral phases at certain film thickness arising from strain phase separation. It is shown that the piezoelectric coefficient d₃₃ of the two-phase mixture is up to 200% higher than that of the rhombohedral single phase.