Origin of the High Piezoelectric Response in ${\mathrm{PbZr}}_{1-\mathit{x}}{\mathrm{Ti}}_{\mathit{x}}{O}_3$

High resolution x-ray powder diffraction measurements on poled ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ (PZT) ceramic samples close to the rhombohedral-tetragonal phase boundary (the so-called morphotropic phase boundary) have shown that for both rhombohedral and tetragonal compositions the piezoelectric elongation of the unit cell does not occur along the polar directions but along those directions associated with the monoclinic distortion. This work provides the first direct evidence for the origin of the very high piezoelectricity in PZT.