Negative piezoelectric response of van der Waals layered bismuth tellurohalides

The polarization and piezoelectric response of the BiTe$X$ ($X$=Cl, Br, and I) layered tellurohalides are computed from first principles. The results confirm a mixed ionic-covalent character of the bonding, and demonstrate that the internal structure within each triple layer is only weakly affected by the external stress, while the changes in the charge distribution with stress produce a substantial negative piezoelectric response. This suggests a new mechanism for negative piezoelectric response that should remain robust even in ultra-thin film form in this class of materials.