Revisiting Properties of Ferroelectric and Multiferroic Thin Films under Tensile Strain from First Principles

First-principles calculations are performed to revisit properties of (001) epitaxial BiFeO3 (BFO) and PbTiO3 thin films under tensile strain. While these two films possess different ground states when experiencing no misfit strain, they both exhibit the same, previously unknown phase for tensile strains above similar or equal to 5% at T = 0 K. This novel state is of orthorhombic Pmc2(1) symmetry and is macroscopically characterized by a large in-plane polarization coexisting with oxygen octahedra tilting in-phase about the out-of-plane direction. On a microscopic point of view, this Pmc2(1) state exhibits short atomic bonds and zigzag cation displacement patterns, unlike conventional ferroelectric phases and typical domains. Such unusual inhomogeneous patterns originate from the coexistence of polar and antiferroelectric distortions having the same magnitude and lead BFO films to be the first known material for which orbital ordering coexists with a large polarization. Furthermore, this Pmc2(1) state is also found in other perovskite films under tensile strain, which emphasizes its generality.