Confocal Raman Microscopy, Synchrotron X-ray Diffraction, and Photoacoustic Study of Ba0.85Ca0.15Ti0.90Zr0.10O3: Understanding Structural and Microstructural Response to the Electric Field

The synergetic role of the phase transitions taking place between long-range, polar ordered, ferroelectric phases in lead-free ceramics with Ba0.85Ca0.15Ti0.90Zr0.10O3 composition on their electromechanical behavior has been analyzed. Synchrotron X-ray diffraction (XRD) and Rietveld analysis revealed a room temperature coexistence of rhombohedral (R), tetragonal (T), and orthorhombic (O) phases. The proportions of these phases in the investigated samples vary from the order of 42R-45T-13O in the nonpolarized condition to an estimated 65R-26T-9O in the polarized, significantly textured sample. These characteristics are related to the complex ferroelectric domain distribution that electron scanning microscopy exposes, which favors domain wall mobility and provides a large extrinsic contribution to the electromechanical response of the material. A confocal Raman spectroscopy and imaging study was carried out on both unpoled and poled samples and reveals that, under the action of the electric field, the E(LO3) + A1(LO2) + E(TO4) mode of the Raman spectra shifts from 517.8 to 516.4 cm–1, indicating the lengthening of the distance between B5+ type ions and their coordinated oxygens for the poled samples. Besides, the mentioned spatially resolved model of the Raman spectra shows a narrower distribution in the poled sample, compatible with the strong texture revealed by XRD. Photoacoustic signal evolution with the temperature, in agreement with the dielectric permittivity curve, reveals the diffusive character of the phase transition taking place at 40 °C between the long-range polar ordered phase at low temperature and the high temperature one, which influences the phase coexistence at room temperature. Moreover, an evaluation of the resultant complex piezoelectric, dielectric, and elastic coefficients at resonance, including all losses, is presented. The obtained results indicate the remarkable electromechanical activity, its anisotropy, and the soft character of this ferro-piezo-electric type ceramic.