In this work, we demonstrate an approach for improving ferroelectric properties of La:HfO₂ thin films by shifting the grain growth regime toward heterogeneous nucleation. A dilute 0.083 M instead of a 0.25 M solution together with an annealing step after every spin-coating cycle film gives rise to a significant improvement of ferroelectric properties. While a remanent polarization of 7 μC/cm² was found for randomly oriented conventional films, the value of 15 μC/cm² for the dilute solution is a result of a mixed 111 and 002 preferential orientation. A more than 50% improved breakdown voltage stems from a global density improvement from 8.0 to 8.4 g/cm³ as obtained from x-ray reflectivity (XRR). We also find superstructure peaks in XRR hinting on periodic alternations of the local density throughout the film thickness. Scanning transmission electron microscopy and secondary ion mass spectrometry confirm this periodicity. The sensitivity of XRR for this periodicity was leveraged to gain further insights in the origin of this superstructure with additional experiments. We conclude that both orientation and the superstructure are caused by a “layered structure” according to Schuler's microstructural zone model. However, our data also provide evidence for parallel chemical effects of cap formation in each stacked sub-layer. While this work shows a significant enhancement of ferroelectric properties, it also provides insights into further optimization potential of solution deposition of HfO₂/ZrO₂ thin films. Our XRR-based approach supplemented with suitable additional analysis can be of great value for the optimization of other solution-derived thin films beyond the material class studied here.

We report on a heterogeneity study, down to the atomic scale, on a representative multiple-element-modified ceramic based on potassium sodium niobate (KNN): 0.95(Na_0.49K_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃–0.05CaZrO₃ with 2 wt % MnO₂. We show that different routes for incorporating the MnO₂ (either before or after the calcination step) affect the phase composition and finally the functionality of the material. According to X-ray diffraction and scanning electron microscopy analyses, the ceramics consist of orthorhombic and tetragonal perovskite phases together with a small amount of Mn-rich secondary phase. The addition of MnO₂ after the calcination results in better piezoelectric properties, corresponding to a ratio between the orthorhombic and tetragonal perovskite phases that is closer to unity. We also show, using microscopy techniques combined with analytical tools, that Zr-rich, Ta-rich and Mn-rich segregations are present on the nano and atomic levels. With this multi-scale analysis approach, we demonstrate that the functional properties are sensitive to minor modifications in the synthesis route, and consequently to different material properties on all scales. We believe that detecting and learning how to control these modifications will be a step forward in overcoming the irreproducibility problems with KNN-based materials.

The dynamics of domain walls (DWs) during polarization switching has a great influence on the electromechanical response of ferroelectrics. To understand the influence of DWs on the functional performance of the material, their local structure and properties must be revealed. There are many investigations of the DWs in ferroelectric thin films, while investigations of the DWs in polycrystalline ferroelectrics are fewer. Here, we report on a comparative study of uncharged and charged (“tail-to-tail”) DWs in BiFeO₃ ceramics from the point of view of the atomically resolved strain and structure using scanning-transmission electron microscopy. Both types of DWs have a step-like morphology and have a non-Ising behavior. The strain, expressed as the unit-cell distortion, is higher and more concentrated in the case of uncharged walls, which we associate with a narrower transition region of the Fe-displacement vectors. Conversely, in the case of “tail-to-tail” charged DWs, the unit-cell distortion is smaller and more dispersed because of the wider transition region of the Fe-displacement vectors. All the types of step-like DWs reported here, regardless of their charge state, exhibit a comparable amount of Bi-vacancies segregation. Further details about the structural differences are discussed from the point of view of the Fe-displacement evolution through the wall for uncharged/“tail-to-tail” charged DWs. The results are useful as they provide an insight into the local structure and chemistry of charged and uncharged DWs in polycrystalline BiFeO_3.