Surface structure and energetics of low index facets of bismuth ferrite.

Bismuth ferrite (BiFeO3) is a multiferroic material that has received significant interest due to its functional properties which could lead to potential novel applications in microelectronics, spintronics, and controlled catalytic reactions. Here, we provide the results of an extensive theoretical study to understand the surface structure and describe the energetics of differently terminated BiFeO3 surfaces. We specifically evaluate low index crystal facets and surface level atomic terminations via density functional theory and ab initio thermodynamics techniques. Our findings indicate that surface stability with varying terminations is strongly dependent on the oxygen partial pressure and chemical potentials of bismuth and iron. In oxygen rich environments, the results suggest that (100)-O and (110)-O and terminated surfaces are more stable compared to other surface terminations and facets. On the other hand, in a relatively oxygen poor environments, we observe that (110)-Bi and (110)-Fe are more stable. The calculations also show that the majority of BFO surfaces exhibit metallic behavior with the exception of the O-terminated (100) and (110) surfaces.

First-principles studies of spontaneous polarization in mixed poly(vinylidene fluoride)/2,3,3,3-tetrafluoropropene polymer crystals.

Spontaneous polarization P of mixed polymer crystals based on ß poly(vinylidene fluoride) (PVDF, -CH2-CF2-) and 2,3,3,3-tetrafluoropropene (TFP, -CH2-CF(CF3)-) was evaluated for ß-PVDF/iso-PTFP, ß-PVDF/P(VDF-alt-iso-TFP) and ß-PVDF/syndio-PTFP. A plane-wave-based density-functional theory (DFT) approach, combined with the Modern Theory of Polarization formalism utilizing maximally-localized Wannier functions for calculating P, indicates that all systems exhibit similarly high or even slightly larger polarization than that of perfectly crystalline ß-PVDF (0.18 C m-2). These properties stem from the substantial dipole moment of the TFP unit, which is estimated to be ∼2.3 D in an isolated chain, but is enhanced to ∼2.8 D in the crystal.

Domain alignment within ferroelectric/dielectric PbTiO3/SrTiO3 superlattice nanostructures.

The ferroelectric domain pattern within lithographically defined PbTiO3/SrTiO3 ferroelectric/dielectric heteroepitaxial superlattice nanostructures is strongly influenced by the edges of the structures. Synchrotron X-ray nanobeam diffraction reveals that the spontaneously formed 180° ferroelectric stripe domains exhibited by such superlattices adopt a configuration in rectangular nanostructures in which domain walls are aligned with long patterned edges. The angular distribution of X-ray diffuse scattering intensity from nanodomains indicates that domains are aligned within an angular range of approximately 20° with respect to the edges. Computational studies based on a time-dependent Landau-Ginzburg-Devonshire model show that the preferred direction of the alignment results from lowering of the bulk and electrostrictive contributions to the free energy of the system due to the release of the lateral mechanical constraint. This unexpected alignment appears to be intrinsic and not a result of distortions or defects caused by the patterning process. Our work demonstrates how nanostructuring and patterning of heteroepitaxial superlattices allow for pathways to create and control ferroelectric structures that may appear counterintuitive.

Towards an accurate description of perovskite ferroelectrics: exchange and correlation effects.

Using the van der Waals density functional with C09 exchange (vdW-DF-C09), which has been applied to describing a wide range of dispersion-bound systems, we explore the physical properties of prototypical ABO3 bulk ferroelectric oxides. Surprisingly, vdW-DF-C09 provides a superior description of experimental values for lattice constants, polarization and bulk moduli, exhibiting similar accuracy to the modified Perdew-Burke-Erzenhoff functional which was designed specifically for bulk solids (PBEsol). The relative performance of vdW-DF-C09 is strongly linked to the form of the exchange enhancement factor which, like PBEsol, tends to behave like the gradient expansion approximation for small reduced gradients. These results suggest the general-purpose nature of the class of vdW-DF functionals, with particular consequences for predicting material functionality across dense and sparse matter regimes.

Topological phase transformations and intrinsic size effects in ferroelectric nanoparticles.

Composite materials comprised of ferroelectric nanoparticles in a dielectric matrix are being actively investigated for a variety of functional properties attractive for a wide range of novel electronic and energy harvesting devices. However, the dependence of these functionalities on shapes, sizes, orientation and mutual arrangement of ferroelectric particles is currently not fully understood. In this study, we utilize a time-dependent Ginzburg-Landau approach combined with coupled-physics finite-element-method based simulations to elucidate the behavior of polarization in isolated spherical PbTiO3 or BaTiO3 nanoparticles embedded in a dielectric medium, including air. The equilibrium polarization topology is strongly affected by particle diameter, as well as the choice of inclusion and matrix materials, with monodomain, vortex-like and multidomain patterns emerging for various combinations of size and materials parameters. This leads to radically different polarization vs. electric field responses, resulting in highly tunable size-dependent dielectric properties that should be possible to observe experimentally. Our calculations show that there is a critical particle size below which ferroelectricity vanishes. For the PbTiO3 particle, this size is 2 and 3.4 nm, respectively, for high- and low-permittivity media. For the BaTiO3 particle, it is ∼3.6 nm regardless of the medium dielectric strength.

Screening mechanisms at polar oxide heterointerfaces.

The interfaces of polar oxide heterostructures can display electronic properties unique from the oxides they border, as they require screening from either internal or external sources of charge. The screening mechanism depends on a variety of factors, including the band structure at the interface, the presence of point defects or adsorbates, whether or not the oxide is ferroelectric, and whether or not an external field is applied. In this review, we discuss both theoretical and experimental aspects of different screening mechanisms, giving special emphasis to ways in which the mechanism can be altered to provide novel or tunable functionalities. We begin with a theoretical introduction to the problem and highlight recent progress in understanding the impact of point defects on polar interfaces. Different case studies are then discussed, for both the high thickness regime, where interfaces must be screened and each interface can be considered separately, and the low thickness regime, where the degree and nature of screening can be manipulated and the interfaces are close enough to interact. We end with a brief outlook toward new developments in this rapidly progressing field.

Giant optical enhancement of strain gradient in ferroelectric BiFeO3 thin films and its physical origin.

Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 10(5)-10(6) m(-1) that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. These findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics.

Piezoelectricity in the dielectric component of nanoscale dielectric-ferroelectric superlattices.

The origin of the functional properties of complex oxide superlattices can be resolved using time-resolved synchrotron x-ray diffraction into contributions from the component layers making up the repeating unit. The CaTiO3 layers of a CaTiO3/BaTiO3 superlattice have a piezoelectric response to an applied electric field, consistent with a large continuous polarization throughout the superlattice. The overall piezoelectric coefficient at large strains, 54 pm/V, agrees with first-principles predictions in which a tetragonal symmetry is imposed on the superlattice by the SrTiO3 substrate.

Polarization effects and phase equilibria in high-energy-density polyvinylidene-fluoride-based polymers.

Using first-principles calculations, the phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers under an applied electric field are studied and phase transitions between their nonpolar alpha and polar beta phases are discussed. The results show that the degree of copolymerization is a crucial parameter controlling the structural phase transition. In particular, for tetrafluoroethylene (TeFE) concentration above 12%, PVDF-TeFE is stabilized in the beta phase, whereas the alpha phase is stable for lower concentrations. As larger electric fields are applied, domains with smaller concentrations (< or = 12%) undergo a transition from the alpha to the beta phase until a breakdown field of approximately 600 MV m(-1) is reached. These structural phase transitions can be exploited for efficient storage of electrical energy.

Suppressed dependence of polarization on epitaxial strain in highly polar ferroelectrics.

A combined experimental and computational investigation of coupling between polarization and epitaxial strain in highly polar ferroelectric PbZr(0.2)Ti(0.8)O3 (PZT) thin films is reported. A comparison of the properties of relaxed (tetragonality c/a approximately 1.05) and highly strained (c/a approximately 1.09) epitaxial films shows that polarization, while being amongst the highest reported for PZT or PbTiO3 in either film or bulk forms P(r) approximately 82 microC/cm(2)), is almost independent of the epitaxial strain. We attribute this behavior to a suppressed sensitivity of the A-site cations to epitaxial strain in these Pb-based perovskites, where the ferroelectric displacements are already large, contrary to the case of less polar perovskites, such as BaTiO3. In the latter case, the A-site cation (Ba) and equatorial oxygen displacements can lead to substantial polarization increases.