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Unit-cell scale mapping of ferroelectricity and tetragonality in epitaxial ultrathin ferroelectric films


Typically, polarization and strain in ferroelectric materials are coupled, leading to the generally accepted direct relation between polarization and unit-cell tetragonality. Here, by means of high-resolution transmission electron microscopy we map, on the unit-cell scale, the degree of tetragonality and the displacements of cations away from the centrosymmetry positions in an ultrathin epitaxial PbZr0.2Ti0.8O3 film on a SrRuO3 electrode layer deposited on a SrTiO3 substrate. The lattice is highly tetragonal at the centre of the film, whereas it shows reduced tetragonality close to the interfaces. Most strikingly, we find that the maximum off-centre displacements for the central area of the film do not scale with the tetragonality. This challenges the fundamental belief in a strong polarization–tetragonality coupling in PbTiO3-based ferroelectrics, at such thicknesses. Furthermore, a systematic reduction of the atomic displacements is measured at the interfaces, suggesting that interface-induced suppression of the ferroelectric polarization plays a critical role in the size effect of nanoscale ferroelectrics.

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Figure 1: Schematic view of the crystal structures.
Figure 2: HRTEM image of the PZT/SRO thin-film heterostructure.
Figure 3: Comparison of image contrast in experiment and calculation.
Figure 4: Quantitative comparison between experimental and calculated data.
Figure 5: Compilation of the measurements for PZT.
Figure 6: Spontaneous polarization.


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The authors thank A. Thust, J. Barthel and H. Kohlstedt for fruitful discussions. This work was partially supported by the National Science Foundation (NSF) under Grants DMR-0132918, NSF-MRSEC DMR-0080008 and an NSF US–Europe program DMR-0244288. V.N. also acknowledges the support of the Alexander von Humboldt Foundation for his stay in Germany and the financial support of an Australian Research Council Discovery Grant 0666231.

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Correspondence to Chun-Lin Jia.

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Jia, CL., Nagarajan, V., He, JQ. et al. Unit-cell scale mapping of ferroelectricity and tetragonality in epitaxial ultrathin ferroelectric films. Nature Mater 6, 64–69 (2007).

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