Coexistence of local structural heterogeneities and long-range ferroelectricity in Pb-free $(1−x) Ba (Zr_{0.2}Ti_{0.8})O_{3−x}(Ba_{0.7}Ca_{0.3})TiO_{3}$ ceramics

Abstract

Environmentally benign (1−x) Ba (Ti$_{0.8}$Zr$_{0.2}$) O$_3$−x (Ba$_{0.7}$Ca$_{0.3}$) 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 ∼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 ZrO$_6$ and 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