Topological defects at octahedral tilting plethora in bi-layered perovskites

Abstract

Oxygen octahedral distortions including tilts/rotations, deformations and off-centering in

Seeing is believing: visualization of antiferromagnetic domains

“…29,30 The α-Fe 2 O 3 magnetic vortex/antivortex domains resemble crystallographic vortex/antivortex domains observed in, for example, hexagonal R(Mn,Fe) O 3 (R = rare earths), Fe 1/3 TaS 2 , and Ca 2 SrTi 2 O 7 . [31][32][33][34] It is a theoretical and experimental challenge to unveil such AFM vortex/antivortex domains.…”

“…The probably first demonstration occurred on hematite, α-Fe 2 O 3 , in 1958. Antiferromagnets exhibiting a parasitic magnetization are restricted to certain symmetry classes, however, 34 and despite the smallness of the magnetization, they may behave as ferromagnets rather than antiferromagnets. 45 Secondary effects allowing optical detection of the AFM order may also be introduced artificially.…”

From first- to second-order phase transitions in hybrid improper ferroelectrics through entropy stabilization

“…This hybrid mechanism is responsible for the polar symmetry (A2 1 am space group) observed at room temperature in the Ruddlesden-Popper perovskites (A n+1 B n O 3n+1 ) with n = 2 (RP2) in low-Sr doped Ca 3−x Sr x Ti 2 O 7 (0 x 0.9), with an experimentally verified switchable polarization for the members with x = 0, 0.54, and 0.85 of 8, 4.2, and 2.4 μC cm −2 , respectively [11]. There has been significant interest in the unusual domain structures accessible in these materials, including head-to-head and tail-to-tail charged domain walls, and topologically protected Z 4 vortex domain structures [11][12][13][14]. This richness of domain structure arises from the multidimensional nature of the OPs that have tilt and rotation character and span the two-dimensional irreducible representations (irreps) X − 3 and X + 2 .…”

“…In the isostructural Ca 3 Mn 2 O 7 , a complex competition between lattice modes of different symmetry leads to pronounced uniaxial negative thermal expansion driven by a phase coexistence between A2 1 am and Acaa over a large temperature range [15,16], hampering any measurement of ferroelectric properties. Phase transitions in HIF Ca 3−x Sr x Ti 2 O 7 with increasing x and temperature have been reported from A2 1 am to a phase with only octahedral tilts (P4 2 /mnm) [12]; however, how the multidimensional OPs that give rise to the HIFs phase evolve and its rich domain physics remain unclear.…”

Aperiodic topological order in the domain configurations of functional materials

“…In this Review, we introduce a new scheme of Zm×Zn domains and Zl vortices to classify various vortex-domain configurations in a wide range of complex materials such as multiferroics 14,34,39,48,49 , improper ferroelectrics 44,[50][51][52][53][54][55] , layered transition-metal dichalcogenides [56][57][58][59] , bilayer graphene 60 , and magnetic superconductors 61 . Vortex-like domains due to surface (or charge) boundary conditions or spatial confinement constitute 'extrinsic' domains (sometimes they are also called closure or circular domains) and were reported in various nanoscale particles or heterostructures [62][63][64][65][66] .…”