Epitaxial-strain-induced polar-to-nonpolar transitions in layered oxides

Abstract

Epitaxial strain can induce collective phenomena and new functionalities in complex oxide thin films. Strong coupling between strain and polar lattice modes can stabilize new ferroelectric phases from nonpolar dielectrics or enhance electric polarizations and Curie temperatures. Recently, strain has also been exploited to induce novel metal-insulator transitions and magnetic reconstructions through its coupling to nonpolar modes, including rotations of BO6 transition-metal octahedra. Although large strains are thought to induce ferroelectricity, here we demonstrate a polar-to-nonpolar transition in (001) films of layered A3B2O7 hybrid-improper ferroelectrics with experimentally accessible biaxial strains. We show the origin of the transition originates from the interplay of trilinear-related lattice mode interactions active in the layered oxides, and those interactions are directly strain tunable. Our results call for a careful re-examination of the role of strain-polarization coupling in ferroelectric films with nontrivial anharmonicities and offer a route to search for new functionalities in layered oxides.

Ferroelectric Sr3Zr2O7: Competition between Hybrid Improper Ferroelectric and Antiferroelectric Mechanisms

“…Most remarkably, we discover a metastable paraelectric polymorph (space group Pnab ), which competes with the equilibrium ferroelectric phase ( A 2 1 am ), and an unusual first‐order ferroelectric–paraelectric phase transition involving a change in the “sense” of the OOR while the OOT persists at all temperatures. We show that the paraelectric Pnab phase appears through a trilinear coupling of OOR and OOT modes interacting with an antipolar mode, a recently predicted hybrid improper “antiferroelectric” mechanism . In addition to being of technological significance for the development of new ferroelectrics, our results highlight the importance of multimode anharmonic interactions in producing functional acentric material properties.…”

Hidden Antipolar Order Parameter and Entangled Néel-Type Charged Domain Walls in Hybrid Improper Ferroelectrics

“…3a signify their important roles in the ground-state structure [38] and the shallower Γ-point phonon is consistent with the HIF by X-point instabilities [17,22]. Notably, P-point instability is undocumented in the earlier theoretical [17,22,29,30] and X-ray and neutron powder studies of the HIF [15,20,21], while admissible for Ruddlesden-Popper phases [16].…”

“…Indeed, Ruddlesden-Popper oxides, (AO)-(A n B n O 3n ), naturally crystallize into two-dimensional perovskites (n, perovskite-unit number) [15], with the rock-salt AO layer sectioning the three-dimensional corner-shared octahedra into two-dimensional perovskite slabs that nurture more octahedral degrees of freedom [15,16]. The Ruddlesden-Popper phases hence display rich octahedral distortions [15,16] and the recently coined hybrid improper ferroelectricity (HIF) in n = 2 Ca 3 Ti 2 O 7 (CTO) and (Ca,Sr) 3 Ti 2 O 7 is particularly enlightening with the zone-center Γ-point ferroelectricity being driven by the hybrid condensation of two zone-boundary octahedral instabilities that transform like two-dimensional X-point irreps [17][18][19][20][21][22].…”

“…Compared to the paraelectric parent phase (I4/mmm), the antiferrodistortive octahedral tilting (order parameter, T) in a!b!-plane in Figs. 1a and 1b originates from the lattice instability against 3 X − irrep and the order parameter of octahedral rotation (R) along c axis represents the 2 X + -irrep distortion mode, altogether known as the hybrid condensation of the two X-point irreps [17][18][19][20][21][22].…”

“…4 [5,16,28,39]. The order-parameter direction of P 5 is thus denoted as (a,a,b,b) [5,39], suggesting that the b!-oriented antipolar order parameter, i.e., (b,b), shall have an a!-degenerate counterpart and the physics of CSTO is composed by multicomponent order-parameter spaces of P 5 , 5 − Γ , 2 X + , and 3 X − (one primary and one secondary order parameters for the latter three two-dimensional irreps), undocumented in the symmetry analyses before [17,22,29,30].…”