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.
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Acknowledgements
The work at Rutgers University was supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4413 to the Rutgers Center for Emergent Materials, and that at Postech by the Max Planck POSTECH/KOREA Research Initiative Program (Grant No. 2011-0031558) through the NRF of Korea funded by MEST.
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Y.S.O. carried out P(E), IP-PFM, IP-PFM before/after poling, cAFM, I–V measurements and statistical analysis, and conceived the hypothetical model. X.L. synthesized single crystals and performed XRD. F-T.H. performed the structure refinement. Y.S.O. and S-W.C. analysed the data and wrote the manuscript. S-W.C. initiated and supervised the research.
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Oh, Y., Luo, X., Huang, FT. et al. Experimental demonstration of hybrid improper ferroelectricity and the presence of abundant charged walls in (Ca,Sr)3Ti2O7 crystals. Nature Mater 14, 407–413 (2015). https://doi.org/10.1038/nmat4168
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DOI: https://doi.org/10.1038/nmat4168
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