Ferroelectrics as Smart Mechanical Materials

Abstract

The mechanical properties of materials are insensitive to space inversion, even when they are crystallographically asymmetric. In practice, this means that turning a piezoelectric crystal upside down or switching the polarization of a ferroelectric should not change its mechanical response. Strain gradients, however, introduce an additional source of asymmetry that has mechanical consequences. Using nanoindentation and contact-resonance force microscopy, this study demonstrates that the mechanical response to indentation of a uniaxial ferroelectric (LiNbO ) does change when its polarity is switched, and use this mechanical asymmetry both to quantify its flexoelectricity and to mechanically read the sign of its ferroelectric domains.

Flexoelectric Fracture-Ratchet Effect in Ferroelectrics

“…The black line marks the region where the gradient-induced electric field is strong enough to be able to induce local switching of the polarization Flexoelectric effects are proportional to dielectric permittivity [29] , therefore, it is to be expected that in a ferroelectric, with a higher dielectric constant, such as Lithium Niobate (LN) ( = 13; = 37), [30] the local flexoelectric switching is enhanced. The coercive field for LN is E c = 2.1x10 7 Vm -1 [31] , by using the flexocoupling coefficient estimated elsewhere [12] , we obtained that the strain gradient required to induce local switching is 4.2x10 5 m -1 . Using equations (5) - (8), and the values in the literature [32 -34] , we found that the distance from the crack tip that corresponds to this strain gradient is ~ 25 nm around the tip, which is big enough to be stable and detectable at room temperature.…”

“…[11] In the case of ferroelectrics, flexoelectricity enables qualitatively new behaviours. For example, it has recently been predicted [10] and demonstrated [12] that ferroelectrics can have an asymmetric mechanical response to inhomogeneous deformations.…”

Second Harmonic Generation Investigation of Symmetry Breaking and Flexoelectricity Induced by Nanoindentations in SrTiO3

“…In this work, we report on centrosymmetry breaking induced by nanoindentations in bulk undoped SrTiO 3 . Indentations were previously exploited to induce flexoelectric effects in polar materials . After mapping the topography of the nanoindentations with atomic force microscopy (AFM), we show with Raman spectroscopy imaging that the nanoindentations generate deformations and therefore a strain gradient in their vicinity.…”

Robust Macroscopic Polarization of Block Copolymer–Templated Mesoporous Perovskite‐Type Thin‐Film Ferroelectrics

“…Unlike in the high‐temperature paraelectric phase (space group R c ), strong internal elastic forces result in atom displacement (from the centrosymmetric position). The thus created dipoles organize themselves into domains, thereby giving rise to robust spontaneous polarization . Lithium tantalate (LiTaO 3 ) shows analogous behavior.…”