%0 Journal Article
%K Transmission electron microscopy
%K Electrodes
%K Polarization
%K Ferroelectricity
%K Interfaces (materials)
%K Ultrathin films
%K Nanoscale ferroelectrics
%K Off-centre displacements
%A C.-L Jia
%A V Nagarajan
%A J.-Q He
%A L Houben
%A T Zhao
%A Ramamoorthy Ramesh
%A K Urban
%A R Waser
%B Nature Materials
%D 2007
%G eng
%I Nature Publishing Group
%P 64-69
%R 10.1038/nmat1808
%T Unit-cell scale mapping of ferroelectricity and tetragonality in epitaxial ultrathin ferroelectric films
%V 6
%X 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 PbZr"0"."2Ti"0"."8O"3 film on a SrRuO"3 electrode layer deposited on a SrTiO"3 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 PbTiO"3-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. © 2007 Nature Publishing Group.