First-principles investigation of auxetic piezoelectric effect in nitride perovskites

, where both the longitudinal and transverse piezoelectric coefficients are negative. These unusual positive transverse piezoelectric coefficients originate from the domination of the positive internal-strain contribution. We further confirm the auxetic piezoelectric effect with finite electric field calculations. Our research enriches the understanding of the piezoelectric properties of nitride perovskites and provides a new compositional space for the design of novel auxetic piezoelectric materials.