Rotation of the polarization and easy shear cooperate to boost the piezoelectric effect

Ferroelectrics are materials exhibiting spontaneous polarization and strain below the Curie temperature, so that the application of an electric field produces a deformation and the application of a stress produces an electric field, the so-called piezoelectric effect. For sensor and actuators it is desired that the magnitude of the piezoelectric effect is maximized, and this is often the case near transitions between ferroelectric phases whose spontaneous polarizations are in different crystallographic directions.
In the article by F. Cordero, H. T. Langhammer, T. Müller, V. Buscaglia and P. Nanni, in Phys. Rev. B 93, 064111 (2016), the role of the elastic response in this maximization is clarified. It is shown that when the direction of the polarization is unstable, the response to a shear response, called compliance, exhibits a maximum of the Curie-Weiss type at the structural transition, so contributing to the maximization of the piezoelectric effect. The concept works for the materials with optimized piezoelectric properties like PZT, but is shown to work always, for example with the classical barium titanate. This concept may be useful in the search for new materials with high piezoelectricity, because a maximum in the elastic compliance can be measured also on defective samples with too high an electrical conductivity to be measured with traditional dielectric and piezoelectric methods.

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