Dynamic flexoelectric effect in perovskites from first principles calculations

Using the dynamical matrix of a crystal obtained from ab initio calculations, we have evaluated for the first time the strength of the dynamic flexoelectric effect and found it comparable to that of the static bulk flexoelectric effect, in agreement with earlier order-of-magnitude estimates. We also proposed a method of evaluation of these effects directly from the simulated phonon spectra. This method can also be applied to the analysis of the experimental phonon spectra, being currently the only one enabling experimental characterization of the static bulk flexoelectric effect

Positive effective Q12 electrostrictive coefficient in perovskites

It is demonstrated that for classical perovskites such as BaTiO3, SrTiO3 and PbTiO3 electrostrictive strain induced by an electric field may not obey traditionally considered "extension along the field, contraction perpendicular to it" behavior if a sample is cut obliquely to the cubic crystallographic directions

Flexoelectric effect in finite samples

Static flexoelectric effect in a finite sample of a solid is addressed in terms of phenomenological theory for the case of a thin plate subjected to bending. It has been shown that despite an explicit asymmetry inherent to the bulk constitutive electromechanical equations which take into account the flexoelectric coupling, the electromechanical response for a finite sample is "symmetric". "Symmetric" means that if a sensor and an actuator are made of a flexoelectric element, performance of such devices can be characterized by the same effective piezoelectric coefficient. This behavior is consistent with the thermodynamic arguments offered earlier, being in conflict with the current point of view on the matter in literature. This result was obtained using standard mechanical boundary conditions valid for the case where the polarization vanishes at the surface. It was shown that, for the case where there is the polarization is nonzero at the surface, the aforementioned symmetry of electromechanical response may be violated if standard mechanical boundary conditions are used, leading to a conflict with the thermodynamic arguments. It was argued that this conflict may be resolved when using modified mechanical boundary conditions. It was also shown that the contribution of surface piezoelectricity to the flexoelectric response of a finite sample is expected to be comparable to that of the static bulk contribution (including the material with high values of the dielectric constant) and to scale as the bulk value of the dielectric constant (similar to the bulk contribution). This finding implies that if the experimentally measured flexoelectric coefficient scales as the dielectric constant of the material, this does not imply that the measured flexoelectric response is controlled by the static bulk contribution to the flexoelectric effect

Evidence for dielectric aging due to progressive 180 domain wall pinning in polydomain Pb(Zr0.45Ti0.55)O3 thin films

An evidence that the dielectric ageing in the polydomain Pb(Zr0.45Ti0.55)O3 thin films is controlled by progressive pinning of 180 domain walls is presented. To provide such a conclusion, we use a general method, which is based on the study of the time evolution of the nonlinear, but anhysteretic, dielectric response of the ferroelectric to a weak electric field. A thermodynamic model of the ferroelectric system where the dielectric response is controlled by bending movements of pinned 180 domain walls is developed. Within this model, the nonlinear permittivity of the ferroelectric is expressed as a function of the microstructural parameters of the domain pattern. It is shown that using the analysis of the time evolution of the nonlinear permittivity, it is possible to estimate changes in the concentration of the pinning centers that block the movements of the 180 domain walls during aging in polydomain perovskite ferroelectrics.Comment: This version is modifed and corrected according to recently published Erratum: Phys. Rev. B 79, 219903(E) (2009). 21 pages, 3 figure

Growth-mode induced defects in epitaxial SrTiO3 thin films grown on single crystal LaAlO3 by a two-step PLD process

We grew epitaxial SrTiO3 (STO) thin films on (001) LaAlO3 substrates via a two-step procedure using pulsed laser deposition and studied them with transmission electron microscopy in plane-view and cross-sectional samples. We found that partial misfit dislocations are the main interfacial defects, whereas planar defects are the main defects in STO films. Our results suggest that a three-dimensional island mode dominates the growth of the STO fil