Piezoelectricity of strongly polarized ferroelectrics in prototropic organic crystals†
The replacement of lead-containing piezoceramics is increasingly in demand for environmentally benign applications in various electromechanical devices. However, small organic molecules have attracted little interest in relation to their piezoelectric properties. We examined the direct and the converse piezoelectric effects at room temperature of several prototropic organic ferroelectrics having large spontaneous polarizations. The magnitude of the piezoelectric coefficient d33 showed a positive correlation with the spontaneous polarization. Whereas the maximum d33 (∼15 pC N−1 for croconic acid) is comparable to the highest level ever observed among nonferroelectric small molecules, it is about half that of ferroelectric polymers and one or two orders of magnitude smaller than those of the commercially available piezoceramics. The modest piezoelectricity of prototropic ferroelectrics, despite their strong polarization, is opposite in behavior to that of ferroelectric Rochelle salt or triglycine sulfate. Plausibly, its origin can be ascribed to thermally robust ferroelectricity and a microscopic mechanism of ferroelectric switching that minimizes molecular deformations and reorientations.