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

Abstract

First-principles calculations are performed to revisit properties of (001) epitaxial BiFeO(3) (BFO) and PbTiO(3) 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 ≃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.

Electric Field Control of Jahn-Teller Distortions in Bulk Perovskites

“…This is the case for BiFeO 3 which was recently proposed to adopt an unusual Pb2 1 m symmetry under large epitaxial tensile strain [18,[33][34][35]]. This particular phase was shown to develop polar, antipolar, and a 0 a 0 c þ AFD motions [33], which were later demonstrated to be coupled together through the third term of Eq. (1) [18].…”

“…(1) [18]. Amazingly, the authors reported the existence of an orbital ordering of the Fe 3þ 3d orbitals, explained from the coexistence of the polar and the antipolar motion yielding a particular lattice distortion pattern [33]. This orbital ordering is unusual since, in this system, no Jahn-Teller effect is required to form a Mott insulating state (Fe 3þ are in a half filled high spin t 3 2g e 2 g configuration).…”

“…The predicted highly strained Pb2 1 m phase in bulk perovskites is not restricted to BiFeO 3 , and it was predicted to occur also in some titanates (CaTiO 3 and EuTiO 3 ) [33], in BaMnO 3 [33], and even in a Jahn-Teller active compound TbMnO 3 [36]. The highly strained bulk perovskites are then an ideal playground to demonstrate our coupling between the polarization and the Jahn-Teller distortion.…”

“…Strain engineering, through thin film epitaxy, for example, can provide a powerful tool to unlock a polar mode in perovskites [10,[27][28][29][30][31][32]. This is the case for BiFeO 3 which was recently proposed to adopt an unusual Pb2 1 m symmetry under large epitaxial tensile strain [18,[33][34][35]]. This particular phase was shown to develop polar, antipolar, and a 0 a 0 c þ AFD motions [33], which were later demonstrated to be coupled together through the third term of Eq.…”

“…Regarding the electronic structure, we checked for the appearance of an orbital ordering as observed in BiFeO 3 [33]. For the four compounds, we report the projected density of states on the d levels of two neighboring B sites in the ðabÞ plane (see Fig.…”