Directed Lifting of Inversion Symmetry in Ruddlesden–Popper Oxide–Fluorides: Toward Ferroelectric and Multiferroic Behavior

Abstract

The cooperative tilting distortions of n = 2 Ruddlesden−Popper oxides can be utilized to break the inversion symmetry of the host lattice and induce ferroelectric behavior. Unfortunately the desired a − a − c + /a − a − c + structural deformation is only stabilized in phases with extremely small structural tolerance factors, limiting the chemical scope of this symmetry breaking approach. Here we describe the influence of topochemical fluorination on the structural distortions of n = 2 Ruddlesden−Popper oxides and demonstrate that the conversion of La 3 Ni 2 O 7 to La 3 Ni 2 O 5.5 F 3.5 breaks the inversion symmetry of the perovskite double layers which constitute the Ruddlesden−Popper framework, by driving a change from an a − a − c 0 /a − a − c 0 distortion in the parent phase to an a − a − c + /a − a − -(c + ) distortion in the oxide−fluoride. In this instance, the symmetry breaking distortions of adjacent acentric perovskite sheets are antialigned, and as a result the inversion symmetry of the host lattice is broken locally, but not globally, resulting in an antiferroelectric structure. The breaking of local inversion symmetry in layered perovskite phases, in the absence of second-order Jahn−Teller active "distortion centers", is an important step toward the realization of ferroelectric and multiferroic behavior in phases of this structure type.

Ferroelectric Sr3Zr2O7: Competition between Hybrid Improper Ferroelectric and Antiferroelectric Mechanisms

“…The Pnam as well as A 2 1 am structures are induced by the coupled irrep (), and the OOR/OOT patterns in both the structures are represented by a − a − c + . The Pnam structure can be distinguished from the A 2 1 am structure through the relative rotation sense of OORs in adjacent n = 2 perovskite blocks; a − a − c + / a − a − c + for A 2 1 am and a − a − c + / a − a − –( c + ) for Pnam . For the Pnam phase, the two‐against‐one Sr cation displacements in adjacent perovskite blocks are in opposite direction, making the crystal structure centrosymmetric and antipolar.…”

“…Here, we demonstrate that ferroelectricity emerges by layering nonpolar perovskite SrZrO 3 with nonpolar rock salt‐structured SrO to form the n = 2 Ruddlesden–Popper (RP) A 3 B 2 O 7 structure . We show that Sr 3 Zr 2 O 7 is a hybrid improper ferroelectric (HIF), whereby ferroelectricity with an electric polarization P occurs from a combination of two nonpolar lattice modes, Q 1 and Q 2 , interacting through a trilinear term of the form PQ 1 Q 2 .…”

High cycle life all-solid-state fluoride ion battery with La2NiO4+d high voltage cathode

“…Chemical fluorination. Attempts on chemical fluorination of the La 2 NiO 4+d has been done by different fluorinating agents: CuF 2 has been used as the fluorinating agent according to literature 77 . For that purpose, two separated boat-like crucibles (one was filled with CuF 2 and the other one with La 2 NiO 4+d ) were placed close to each other in an air-tight tube furnace under a flow of O 2 (0.1 SLM) and heated up at various temperatures from 200 to 350°C for 48 h. Fluorine gas diluted with argon (10% F 2 by volume) was also used as the fluorinating agent at different temperatures and reaction times, which are stated directly in the figure captions (see Supplementary Fig.…”

Unraveling the Suppression of Oxygen Octahedra Rotations in A3B2O7 Ruddlesden-Popper Compounds: Engineering Multiferroicity and Beyond

“…They can nowadays be grown with good atomic control [13,14] and recently have attracted a growing interest for accessing functional properties inaccessible in the parent ABO 3 perovskites [15][16][17][18][19][20]. Intriguingly, experiments focusing on different compounds seem to suggest a common feature: the rotations of BO 6 octahedra appearing in ABO 3 perovskites are often partly or completely suppressed in related RP structures [19][20][21][22][23]. Moreover, decreasing the tolerance factor, which typically enhances the rotations in ABO 3 perovskites, seems inefficient to recover the full rotation patterns in RP systems like SrR 2 Fe 2 O 7 (R ¼ rare-earth ion) [20].…”