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Lead titanate (PbTiO3) is a classical example of a ferroelectric perovskiteoxide illustrating a displacive phase transition accompanied by a softening ofa symmetry-breaking mode. The underlying assumption justifying the soft-modetheory is that the crystal is macroscopically sufficiently uniform so that ameaningful free energy function can be formed. In contrast to PbTiO3,experimental studies show that the phase transition behaviour oflead-zirconate-titanate solid solution (PZT) is far more subtle. Most of thestudies on the PZT system have been dedicated to ceramic or powder samples, inwhich case an unambiguous soft-mode study is not possible, as modes withdifferent symmetries appear together. Our Raman scattering study ontitanium-rich PZT single crystal shows that the phase transitions in PZT cannotbe described by a simple soft-mode theory. In strong contrast to PbTiO3,splitting of transverse E-symmetry modes reveals that there are differentlocally-ordered regions. The role of crystal defects, random distribution of Tiand Zr at the B-cation site and Pb ions shifted away from their idealpositions, dictates the phase transition mechanism. A statistical modelexplaining the observed peak splitting and phase transformation to a complexstate with spatially varying local order in the vicinity of the morphotropicphase boundary is given.

A statistical model approximation for perovskite solid-solutions: a Raman study of lead-zirconate-titanate single crystal