Abstract
AbstractOxygen octahedral distortions, including tilts/rotations, deformations and off-centring in (layered) perovskites, have the key role in their numerous functional properties. Near the polar-centrosymmetric phase boundary in bi-layered perovskite Ca3−xSrxTi2O7withx≈1, we found the presence of abundant topological eight-state vortex-antivortex pairs, associated with four oxygen octahedral tilts at domains and another four different oxygen octahedral tilts at domain walls. Our discovery opens a new revenue to unveil real-space topological defects associated with the possible vector choices in one specific lattice mode.
Highlights
Copious functional phenomena, including high Tc superconductivity,[1] ferroelectricity,[2,3] novel magnetism[4,5,6] and giant photovoltaic effects,[7,8] have been observed in perovskite (ABO3)-related compounds, where those physical properties can be closely associated with oxygen octahedral distortions, including tilts/rotations, deformations and off-centring
In this article, utilising in situ heating transmission electron microscopy (TEM) studies, synchrotron powder X-ray diffraction experiments and dielectric measurements, we report the discovery of a new intermediate tetragonal state in Ca3 − xSrxTi2O7 (CSTO), which demonstrates a displacive nature of hybrid improper ferroelectricity mechanism different from those predicted from the group–subgroup relation.[21,23]
When octahedral rotation is suppressed by chemical doping/ionic ordering, Z4 vortices emerge in the narrow compositional range, where domain and domain walls are intricately intertwined with the T′ and O′ states, arising from the active X3− mode
Summary
Copious functional phenomena, including high Tc superconductivity,[1] ferroelectricity,[2,3] novel magnetism[4,5,6] and giant photovoltaic effects,[7,8] have been observed in perovskite (ABO3)-related compounds, where those physical properties can be closely associated with oxygen octahedral distortions, including tilts/rotations, deformations and off-centring. Even when A-site ions are rather small, the perovskite-related structure can be still stabilised through oxygen octahedral tilts/rotations.[11] It turns out that superconductivity in (La,Ba)2CuO4 is significantly influenced by oxygen octahedral tilts,[12,13] and canted magnetic moments appear in antiferromagnetic perovskites with tilted/ rotated oxygen octahedra through Dzyaloshinskii–Moriya interaction.[2,14] High dielectric response in the vicinity of the morphotropic phase boundary is a consequence of the continuously rotating polarisation with various octahedral tilts.[15,16] Crystal field split can be considerably influenced by compression or elongation of oxygen octahedra, which is the origin of the Jahn–Teller effects for B = Cu2+ or Mn3+ (refs 1,17,18)
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