Strain-induced ferroelectricity in orthorhombicCaTiO3from first principles

First-principles calculations are used to investigate the effects of the epitaxial strain on the structure of the perovskite oxide ${\text{CaTiO}}_3$. At 1.5% tensile strain, we find an epitaxial orientation transition between the $ab\text{−}ePbnm$ phase favored for compressive strains and the $c\text{−}ePbnm$ phase. While no ferroelectric instability is found for compressive strains, larger tensile strains are found to stabilize a ferroelectric phase related to a polar instability identified in previous first-principles studies of the ideal cubic perovskite high-symmetry reference structure but hidden in the orthorhombic equilibrium bulk $Pbnm$ structure. This strain-induced ferroelectric $c\text{−}ePbnm$ phase has polarization along a $⟨110⟩$ direction with respect to the primitive perovskite lattice vectors of the square substrate with a magnitude of $0.46\text{C}/{\text{m}}^2$ at 4% tensile strain.