Negative piezoelectric behaviors in hybrid improper ferroelectric Ca3-xNaxTi2O7 (x=0, 0.01) ceramics

Hybrid improper ferroelectric Ca3Ti2O7 (CTO) and Ca2.99Na0.01Ti2O7 (NaCTO) ceramics were synthesized using the solid-state reaction method. X-ray powder diffraction technique was used to determine the phase purity and crystal structure of the prepared ceramics. We investigated their ferroelectric polarization, dynamic switching current, and piezoelectric properties, and confirmed their ferroelectric reversal property with a coercive field of 60-80 kV/cm. A negative piezoelectric behavior was observed with a maximum strain of -0.01% and -0.007% for CTO and NaCTO, respectively. This abnormal piezoelectric behavior can be used to prepare composites with conventional oxide materials that generally exhibit positive piezoelectricity. Mechanism of the negative piezoelectricity effect was ascribed to its special quasi-two-dimensional (quasi-2D) crystal structure with weak van der Waals (vdW) layers alternating with covalent layers.