  Temperature dependent structural changes in a nearly pure monoclinic phase
composition (x=0.525) of Pb(Zr_xTi_1-x)O_3 (PZT) have been investigated using
Rietveld analysis of high-resolution synchrotron powder x-ray diffraction data
and correlated with changes in the dielectric constant and planar
electromechanical coupling coefficient. Our results show that the intrinsic
piezoelectric response of the tetragonal phase of PZT is higher than that of
the monoclinic phase. It is also shown that the high piezoelectric response of
PZT may be linked with an anomalous softening of the elastic modulus (1/S_11)
of the tetragonal compositions closest to the morphotropic phase boundary.
