Coexistence of local structural heterogeneities and long-range ferroelectricity in Pb-free (1−x)Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 ceramics

Environmentally benign $(1\text{\ensuremath{-}}x)\mathrm{Ba}({\mathrm{Ti}}_{0.8}{\mathrm{Zr}}_{0.2}){\mathrm{O}}_{3}\text{\ensuremath{-}}x({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_{3}$ (BZT-BCT) ceramics are promising materials due to their remarkable high piezoresponse [Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)]. In this Letter, by focusing on local and average structure in combination with macroscopic electromechanical and dielectric measurements we demonstrate the structure property relationship in the tetragonal BZT-BCT ceramic. During high-temperature cubic to tetragonal phase transformation, polar nanoregions are manifested through the spontaneous volume ferroelectrostriction at temperatures below $\ensuremath{\sim}477$ K. Temperature-dependent local structural investigations across the Zr $K$ edge extended x-ray absorption fine-structure spectroscopy reveal an anomalous collaboration between the ${\mathrm{ZrO}}_{6}$ and ${\mathrm{TiO}}_{6}$ octahedra. These octahedra compromise their individuality during polarization development. The presence of domains of submicron size embedded inside the macroscopic ferroelectric regions below ${T}_{m}$, as well as their hierarchical arrangement, is observed by piezoresponse force microscopy. Effects of the existence of the structural/polar heterogeneities below ${T}_{m}$ are observed also when polarizabilities of the poled and unpoled samples are compared; the poled sample is found to be more susceptible to the electric field. In addition, by using electric field dependent x-ray diffraction studies we also show that this ceramic under field exhibits a reduction of tetragonal distortion, which is consistent with earlier reports.