Experimental demonstration of hybrid improper ferroelectricity and the presence of abundant charged walls in (Ca,Sr)3Ti2O7 crystals

Abstract

On the basis of successful first-principles predictions of new functional ferroelectric materials, a number of new ferroelectrics have been experimentally discovered. Using trilinear coupling of two types of octahedron rotation, hybrid improper ferroelectricity has been theoretically predicted in ordered perovskites and the Ruddlesden-Popper compounds (Ca3Ti2O7, Ca3Mn2O7 and (Ca/Sr/Ba)3(Sn/Zr/Ge)2O7). However, the ferroelectricity of these compounds has never been experimentally confirmed and even their polar nature has been under debate. Here we provide the first experimental demonstration of room-temperature switchable polarization in bulk crystals of Ca3Ti2O7, as well as Sr-doped Ca3Ti2O7. Furthermore, (Ca, Sr)3Ti2O7 is found to exhibit an intriguing ferroelectric domain structure resulting from orthorhombic twins and (switchable) planar polarization. The planar domain structure accompanies abundant charged domain walls with conducting head-to-head and insulating tail-to-tail configurations, which exhibit a conduction difference of two orders of magnitude. These discoveries provide new research opportunities, not only for new stable ferroelectrics of Ruddlesden-Popper compounds, but also for meandering conducting domain walls formed by planar polarization.

Improper ferroelectric polarization in a perovskite driven by intersite charge transfer and ordering

“…There has been a large amount of renewed interest in improper ferroelectric mechanisms recently, particularly in studying the coupling between two nonpolar zone-boundary lattice modes with octahedral tilt character in the RuddlesdenPopper layered perovskites, which has been discussed in the context of "hybrid improper ferroelectricity" [2,[35][36][37]. Here we reveal a mechanism by which two zone-boundary modes, implicitly dependent on orbital degrees of freedom, combine to give polar zone-centered displacements.…”

Designing new ferroelectrics with a general strategy

“…15 The hybrid improper ferroelectricity (HIF) was recently discovered in the artificial superlattice PbTiO 3 /SrTiO 3 , 16 where the ferroelectricity is induced by a trilinear coupling (PQ 1 Q 2 ) between the FE mode (P) and two oxygen octahedral rotational modes (Q 1 and Q 2 , respectively). The HIF was also found in the double-layered Ruddlesden-Popper (RP) perovskite A 3 B 2 O 7 (A=Ca, Sr; B=Mn,Ti), [17][18][19] the 1:1 A-cation ordering perovskite-type superlattice, [20][21][22][23] A-cation ordering RP NaRTiO 4 (R = Y, La, Nd, Sm-Ho), 24 the 2:2 B-cation ordered superlattice, 25 and metal-organic perovskite material. 26 In the above-mentioned theoretically designed FEs, one usually starts from a high-symmetry structure (e.g., cubic perovskite structure), then the effect of atomic substitution and soft phonon modulations are examined to see whether the ferroelectricity can be induced or not.…”

“…But for the improper FEs, the FE mode is no longer the primary order parameter, i.e., the polarization is induced by one or two rotational modes. [13][14][15][16][17][18][19][20][21][22][23][24][25] To verify whether the ½111 -superlattice LaCoO 3 /LaAlO 3 is an improper FE or not, the stability of the FE mode will be examined. We adopt the ISOTROPY software 33 to obtain the symmetry-adapted phonon modes in the low-symmetry FE structure.…”

Multiferroic Materials: Physics and Properties

“…Experimental demonstration of hybrid improper ferroelectricity has been achieved in some Ruddlesden-Popper phases including Ca 3 Ti 2 O 7 and Sr-doped Ca 3 Ti 2 O 7 , which are not magnetically ordered but exhibit relatively large spontaneous polarization (8 mC cm À2 for Ca 3 Ti 2 O 7 ). 53 Another example of hybrid improper ferroelectricity is the room temperature multiferroic (1 À x)(Ca y Sr 1Ày ) 1 Here, the structural design of octahedral rotations also tunes the magnetization to stabilize a weak ferromagnetic state in addition to the ferroelectric displacements induced by the same rotations. 54 The recent mechanism of hybrid improper ferroelectricity shows potential for obtaining new multiferroic materials with enhanced functionalities.…”