The giant piezoelectric effect: electric field induced monoclinic phase orpiezoelectric distortion of the rhombohedral parent?

Abstract

Lead zinc niobate–lead titanate (PZN–PT) single crystals show very largepiezoelectric strains for electric fields applied along the unit cell edges e.g. [001]R.It has been widely reported that this effect is caused by anelectric field induced phase transition from rhombohedral(R3m) tomonoclinic (Cmor Pm)symmetry in an essentially continuous manner. Group theoretical analysis usingthe computer program ISOTROPY indicates phase transitions betweenR3m andCm (orPm) mustbe discontinuous under Landau theory. An analysis of the symmetry of a strained unitcell in R3mand a simple expansion of the piezoelectric strain equation indicate thatthe piezoelectric distortion due to an electric field along a cell edge inrhombohedral perovskite-based ferroelectrics is intrinsically monoclinic(Cm),even for infinitesimal electric fields. PZN–PT crystals have up to nine times theelastic compliance of other piezoelectric perovskites and it might be expected thatthe piezoelectric strains are also very large. A field induced phase transition istherefore indistinguishable from the piezoelectric distortion and is neithersufficient nor necessary to understand the large piezoelectric response of PZN–PT.