Piezoelectricity in n = 2 Ruddlesden−Popper ferroelectric oxides from first principles

We present first-principles calculations of piezoelectric tensors for n = 2 Ruddlesden-Popper ferroelectrics, specifically Ca₃Ti₂O₇, Sr₃Zr₂O₇, Sr₃Sn₂O₇ and Li₂SrNb₂O₇. Our results show that the longitudinal piezoelectric coefficients are comparable to those of well-known perovskite ferroelectrics. Conventional piezoelectric materials generally exhibit positive longitudinal piezoelectric coefficients along with two negative transverse coefficients. However, our analysis reveals an unconventional form of transverse piezoelectricity in Ca₃Ti₂O₇ and Li₂SrNb₂O₇, where one transverse coefficient is positive while the other is negative. Additionally, we demonstrate that octahedral rotation distortions coupled to the polarization in these layered materials, can effectively tune the amplitude of their piezoelectric coefficients.