Picturing the elephant: Giant piezoelectric activity and the monoclinic phases of relaxor-ferroelectric single crystals

Understanding the relationship between the structure and “giant” piezoelectric properties of relaxor ferroelectric solid solutions (1–x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ [PMN–xPT] and (1–x)Pb(Zn_1/3Nb_2/3)O₃–xPbTiO₃ [PZN–xPT] is an extremely difficult task. In this article, three main paradigms are reviewed. In the first, the monoclinic phases present at the morphotropic phase boundary (MPB) are responsible for the giant piezoelectric response in that they allow, or at least facilitate, polarization rotation. In the second, a strong polarization rotation effect is explained by the large piezoelectric shear coefficients of zero-field rhombohedral and orthorhombic phases due to the near degeneracy at the MPB and the intrinsic softness of the relaxor state; zero-field monoclinic symmetries are explained by residually distorted rhombohedral and orthorhombic phases in the presence of internal stresses and/or residual bias fields. In the third, the monoclinic “phases” are composed of very finely twinned rhombohedral or tetragonal domains. In this “adaptive phase” model, based on that for ferroelastic martensites, the large electric-field induced strains are extrinsic in nature and result from the progressive switching of the component “nano-twins”; the ease of polarization rotation is explained by a high domain wall mobility. These paradigms remain to be mutually reconciled. The article includes a thorough review of the history of PMN–xPT and PZN–xPT single crystals and, particularly, the most important work done over the last decade.