Comments on origins of enhanced piezoelectric properties in ferroelectrics.

This review discusses recent advances in understanding origins of large piezoelectric properties in some ferroelectric materials. In particular, it addresses the role of polarization rotation and monoclinic phases. It is suggested that the polarization rotation is an old concept that was proposed more than 30 years ago to explain enhanced properties of Pb(Zr,Ti) O3 in the morphotropic phase boundary region. It is further demonstrated that in addition to polarization rotation, polarization extension can also lead to large electro-mechanical properties. In fact, the largest piezoelectric coefficient has been reported in KH2PO4, which exhibits structural instability involving polarization extension and no monoclinic phases. It is also shown that substantial theoretical and experimental evidence exists to show that the highest piezoelectric response is often not observed in monoclinic phases but in the phase transition regions where polarization either changes direction or appears from the nonpolar state. Finally, it is proposed that the concept of free energy instability, which emerged from phenomenological and first principle calculations, is the most general approach that can be used to consistently interpret many experimental observations and underlies many theoretical results on enhancement of piezoelectric properties.