Electric field-induced strain in core-shell structured BiFeO3K0.5Bi0.5TiO3PbTiO3 ceramics

A coherent core-shell structure in 0.24BiFeO3-0.56K0.5Bi0.5TiO3-0.20PbTiO3 ceramics, with Tm around 385 °C, was revealed using a combination of characterisation methods including scanning and transmission electron microscopy. The core regions consist of a cubic matrix with minor monoclinic phase with limited coherence length, while the shell regions are a tetragonal phase with characteristic long-range ordered ferroelectric domain structures. The electric field-induced strains for both core and shell regions were investigated by conducting an in-situ poling study using high energy synchrotron X-ray diffraction (XRD). A methodology is developed to evaluate both intrinsic (lattice strain) and extrinsic (domain switching) contributions for each phase based on diffraction peak profile fitting for multiple reflections. It is shown that there were no significant variations in the cubic and tetragonal phase fractions under an electric field in the range from -6 to +6 kV/mm. An irreversible strain 0.04% was attributed mainly to ferroelectric domain switching in the tetragonal shell regions, while a reversible lattice strain 0.12% was identified in the pseudo-cubic core, leading to significant levels of intra-granular stress. Strain anisotropy, dependent on the crystallographic orientation of different grain families, was also evident. Macroscopic strain measurements, conducted using a novel digital image correlation (DIC) method, demonstrated the formation of typical ‘butterfly’-type strain-electric field loops and were consistent with the total strain values calculated from the XRD results.