Piezoelectricity in n = 2 Ruddlesden-Popper Ferroelectric Oxides

We present first-principles calculations of piezoelectric tensors for the n = 2 Ruddlesden-Popper ferroelectrics Ca₃Ti₂O₇, Sr₃Zr₂O₇, Sr₃Sn₂O₇, and Li₂SrNb₂O₇. Our computed longitudinal piezoelectric coefficients are comparable to the intrinsic coefficients of prototypical perovskite ferroelectrics. We identify an unusual form of transverse piezoelectricity in Ca₃Ti₂O₇ and Li₂SrNb₂O₇, where one transverse piezoelectric coefficient is negative and the other is positive. Furthermore, we show that octahedral rotation distortions, which are coupled to the polarization in this family of layered ferroelectrics, can tune the amplitudes of the piezoelectric coefficients.