According to recent experiments and predictions, the orientation of the polarization at the surface of a ferroelectric material can affect its surface chemistry. Here we demonstrate the converse effect: the chemical environment can control the polarization orientation in a ferroelectric film. In situ synchrotron x-ray scattering measurements show that high or low oxygen partial pressure induces outward or inward polarization, respectively, in an ultrathin PbTiO 3 film. Ab initio calculations provide insight into surface structure changes observed during chemical switching. DOI: 10.1103/PhysRevLett.102.047601 PACS numbers: 77.80.Fm, 68.43.Àh, 68.47.Gh, 77.84.Dy Ferroelectric materials are fascinating and useful because the spontaneous polarization which appears below the Curie temperature T C is strongly coupled to long-range electric and stress fields, leading to outstanding properties such as piezoelectricity and electrically switchable structure [1,2]. Understanding the behavior of ultrathin ferroelectric films, for which interfacial effects begin to dominate over the physics of the film interior, has been an area of major progress recently [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. One of the most important interfacial effects is the screening of the intrinsic surface charge of the polar phase [19], since this bound charge produces an electric field opposing the bulk polarization. The energy of this depolarizing field can be reduced by stripe domain formation [3][4][5][6][7][8][9][10] or by compensation via free charge at the interfaces [8,[11][12][13][14][15][16][17]. In both cases, incomplete screening leads to a depression of T C for thinner films and a critical thickness below which the polar phase is not stable. While electronic charge in metallic electrodes provides screening adequate to stabilize the polar phase down to nanometer dimensions [8,11,12,17], similar small critical thicknesses have been observed for films without surface electrodes [2,13,15,16]. Ab initio calculations [15,16] have indicated that extra ions or point defects could be providing charge compensation at these surfaces. Such ionic compensation of ferroelectric surfaces has also been inferred from electric force microscopy measurements [20]. The electronic or ionic nature of the compensating charge at interfaces has become a subject of debate for polar oxides in general [19,21,22].Because of this evidence that ions can provide surface charge compensation for ferroelectrics, potentially giving new device functionality, several recent studies have focused on the interaction between the chemistry of the environment and the polarization orientation. Experiments have shown that ferroelectric surfaces with opposite polarity have different properties for adsorbing molecules [23,24]. Ab initio calculations have found that catalytic activity [25] and equilibrium surface stoichiometry [26] depend upon polarization orientation. In this work, we demonstrate the converse effect-that the chemical environment can control the p...
Modern ferroelectric materials (e.g. PbMg1/3Nb2/3O3 - PbTiO3) are often based on ternary and quaternary oxides where disorder constitutes a groundwork for exceptional electromechanical properties. While the basic role of the inhomogeneity (strain and/or local electric field generated by occupational disorder locks and enhances the 'transition state' and thus electromechanical coupling) is widely accepted, precise description of mechanisms governing polar correlations is still lacking. In particular, indicating the role of ferroelectric soft phonon modes, their 'interaction' with inhomogeneous medium and relation to (quasi) static polar state at low temperatures constitutes an intriguing and fundamental problem [1,2]. Diffuse scattering analysis providing information about atomic to mesoscale correlations is a method of choice for studying of these phenomena; the fact that has been proven by an abundance of X-ray, neutron and electron diffraction results for ferroelectric materials in the recent years. At the same time, interpretation has been very often ambiguous, calling for better understanding of how the experimental data should be approached. We address some principal questions related to the diffuse scattering analysis of ferroelectric and related compounds. Can we gain some insight into the local structure treating data 'ab-initio', without assuming any model of polar correlations [3]? Can we separate static and dynamic contributions to the diffuse scattering (e.g. by calculating theoretical phonon-related thermal diffuse scattering)? The study is carried out using atomistic computational methods including reverse Monte Carlo, molecular dynamics and density functional theory based calculations.
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