The performance of ferroelectric devices is intimately entwined with the structure and dynamics of ferroelectric domains. In ultrathin ferroelectrics, ordered nanodomains arise naturally in response to the presence of a depolarizing field and give rise to highly inhomogeneous polarization and structural profiles. Ferroelectric superlattices offer a unique way of engineering the desired nanodomain structure by modifying the strength of the electrostatic interactions between different ferroelectric layers. Through a combination of X-ray diffraction, transmission electron microscopy, and first-principles calculations, the electrostatic coupling between ferroelectric layers is studied, revealing the existence of interfacial layers of reduced tetragonality attributed to inhomogeneous strain and polarization profiles associated with the domain structure.

The local structural distortions in polydomain ferroelectric PbTiO 3 /SrTiO 3 superlattices are investigated by means of high spatial and energy resolution electron-energy-loss spectroscopy combined with high-angle annular dark field imaging. Local structural variations across the interfaces have been identified with unit-cell resolution through the analysis of the energy-loss near-edge structure of the Ti L 2,3 and O K edges. Ab initio and multiplet calculations of the Ti L 2,3 edges provide unambiguous evidence for an inhomogeneous polarization profile associated with the observed structural distortions across the superlattice. Complex oxide heterostructures offer a vast playground for exploring and combining the many functional properties of these interesting materials arising from the subtle interplay between their charge, spin, orbital, and lattice degrees of freedom. Bilayers, multilayers, and superlattices composed of ultrathin oxide layers not only shed light on our fundamental understanding of the constituent materials, but frequently reveal unexpected phases at their interfaces.1 Superlattices composed of ferroelectric and paraelectric oxides have been the subject of numerous studies, motivated by fundamental questions about ferroelectric size effects, by the possibilities these artificially layered materials offer for tailoring their functional properties, and by the interesting interface physics they display. Ultrafine period superlattices, composed of ferroelectric PbTiO 3 (PTO) and paraelectric SrTiO 3 (STO), for example, have been shown to exhibit improper ferroelectricity driven by the coupling of the polar and nonpolar distortions at the interface.2 More recently, regular ferroelectric nanodomains have been observed in PTO/STO superlattices with larger periodicities and were shown to be responsible for large enhancements in the effective dielectric constant.3 Such domains are expected to give rise to complex inhomogeneous structural distortions and polarization profiles, 4,5 departing from uniform polarization models, frequently used to describe the properties of ferroelectric/paraelectric superlattices in the absence of domains. 6 Thus a microscopic insight into the local structure is key to understanding the behavior of these artificially layered materials, and here transmission electron microscopy (TEM), with recent advances in spatial resolution, provides an invaluable tool. Individual ionic displacements within a single perovskite unit cell can now be identified from phase contrast images. The spatial resolution is high enough to determine the direction and even the magnitude of the local dipole moments in ferroelectric materials, 7-9 and has recently enabled the direct verification of the existence of polarization rotation at domain walls in Pb(Zr,Ti)O 3 ferroelectric thin films. 10In this Rapid Communication, we focus on an alternative spectroscopic technique to study local ferroelectric distortions at the single unit-cell scale. Atomically resolved electronenergy-loss spectra (EELS) and ...

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