Structural transformations in morphotropic-phase-boundary composition of the lead-free piezoelectric system Ba(Ti0.8Zr0.2)O3 - (Ba0.7Ca0.3)TiO3

The exceptionally large piezoelectric response of the morphotropic-phase-boundary (MPB) composition of the lead-free piezoelectric system $(1\ensuremath{-}x)\mathrm{Ba}(\mathrm{T}{\mathrm{i}}_{0.8}\mathrm{Z}{\mathrm{r}}_{0.2}){\mathrm{O}}_{3}\ensuremath{-}x(\mathrm{B}{\mathrm{a}}_{0.7}\mathrm{C}{\mathrm{a}}_{0.3})\mathrm{Ti}{\mathrm{O}}_{3}$ has attracted great attention in recent years. Here in this paper we report a detailed investigation of the structural phase transformation behavior of the MPB composition $(x=0.50)$ driven by electric field, stress, and temperature. We show that the system exhibits metastable phases in a wide temperature range, and that the large piezoresponse at room temperature has a significant contribution from the increased fraction of the metastable phases induced by the poling field. Using a ``powder poling'' technique we also demonstrate the equivalence of stress and electric field with regard to the nature of the structural transformation. The fundamental significance of this interesting observation is discussed.