Rhombohedral and Monoclinic Phases of PZT near the Antiferroelectric and the Morphotropic Boundaries

Anelastic and dielectric spectroscopy measurements are presented, which, together with previous measurements [1], clarify some controversial aspects of the phase diagram of PbZr1-xTixO3 close to the border with the antiferroelectric (AFE) phase, and at the morphotropic phase boundary (MPB). No evidence is found of a border separating monoclinic (M) from rhombohedral (R) phases, supporting recent structural studies according to which the two phases coexist, with the fraction of M prevailing near the MPB. A large frequency independent softening at the MPB indicates a genuine M phase over only finely twinned R phase. A new phase transition is found in both the anelastic and dielectric spectra at x = 0.1, at a temperature TIT between the Curie temperature TC and the boundary TT to the phases with tilted octahedra. Such a diffuse transition is interpreted as onset of disordered tilts, which finally become ordered below TT. In this manner, the phase diagram of PZT is rationalised with respect to the common tendency of perovskites to undergo tilting when the mismatch between the cation sizes exceeds a threshold.