Bridging phases at the morphotropic boundaries of lead oxide solid solutions

Ceramic solid solutions of PbZr1-xTixO3 (PZT) with compositions x similar or equal to 0.50 are well-known for their extraordinarily large piezoelectric responses. The latter are highly anisotropic, and it was recently shown that, for the rhombohedral compositions (x less than or similar to 0.5), the piezoelectric coefficients were largest away from the polar direction, contrary to common belief. Shortly afterwards a low-symmetry monoclinic phase was observed by synchrotron X-ray diffraction at around x = 0.50. Similar behavior and features are also present in a number of related lead-based strongly-piezoelectric single crystals, such as Pb(Mg1/3Nb2/3)(1-x)TixO3, Pb(Zn1/3Nb2/3)(1-x)TixO3, and Pb(Sn1/2Nb1/2)(1-x)TixO3, with piezoelectric coefficients of about 25 pm V-1, the highest values recorded to date. Recent experimental and theoretical work has greatly improved our understanding of these technologically important systems, but there are still some open questions. In this review, we summarize the most recent progress in the field.