Ultrahigh Negative Longitudinal Piezoelectricity in Rhombohedral GeTe and Its Group IV-VI Analogues

Conventional piezoelectric materials typically exhibit positive longitudinal piezoelectric coefficients, yet recent studies have identified exceptions with negative piezoelectric responses. Using density functional theory, we demonstrate for the first time that rhombohedral GeTe (r-GeTe) possesses an ultrahigh negative piezoelectric strain coefficient (d₃₃) of -70.87 pC/N, surpassing all previously reported negative piezoelectric materials. This phenomenon arises from the "quasi-layered" structure of r-GeTe, comprising alternating strong and weak bonds, which induces a pronounced negative internal-strain contribution and an exceptionally low elastic constant. We further extend our investigation to other IV-VI rhombohedral materials, identifying GeS, GeSe, and SiTe as promising candidates for ultrahigh negative piezoelectricity. In contrast to prior reports, where negative piezoelectricity stems from a negative clamped-ion term that dominates a small positive internal-strain contribution, our findings propose a new material design strategy for large negative piezoelectricity by introducing a significantly negative internal strain, along with the negative clamped-ion term.