Imaging the Emergent Functionality in Bulk Ferroelectrics

Emergent behaviour in ferroelectrics can be imaged at surfaces by scanning probe microscopy (SPM), and in thin lamella by transmission electron microscopy (TEM). Despite tremendous advances in these techniques over the past decades, SPM and TEM remain unsuitable for investigation of the bulk. X-ray-based techniques, on the other hand, penetrate through the material and are well suited for investigation of the bulk. Multiple X-ray techniques are now able to image strain and orientation deep in the bulk. Dark-field X-ray microscopy (DF-XRM) is one such technique, which has previously been applied to ferroelectrics and has produced images of domains and strain deep in the bulk [1]. Here, we present our efforts to functionalize DF-XRM and image the polarization in addition to strain. Functionalized DF-XRM will allow us to image exotic topological structures deep inside the sample or at a buried interface. A more expansive analysis of the interplay between strain, stress, and polarization can then be explored in the 3D bulk of the material. The interplay between ferroelectric domain walls, line defects, and vacancies in the bulk is of particular interest, and this is difficult to approach using other techniques.<br/><br/>We will present proof-of-concept experimental results from our ongoing efforts to image the polarization of BaTiO₃ in-situ using DF-XRM. Our experimental work is guided by a theoretical framework, which we will also present together with simulations of realistic experimental conditions. Furthermore, we will describe how a stroboscopic configuration can be used to image material dynamics. Finally, this work has implications for any material imaged in DF-XRM with an applied electric field. In a general case, the electric field will perturb the X-ray structure factor, which is assumed to be constant when calculating strain maps from a DF-XRM image series. We will show how this perturbation can be quantified and addressed, so that accurate strain maps may still be produced in the presence of an electric field.<br/><br/>[1] Simons, Hugh, et al. "Long-range symmetry breaking in embedded ferroelectrics." Nature materials 17.9 (2018): 814-819.