Atomic-Level Response of the Domain Walls in Bismuth Ferrite in a Subcoercive-Field Regime.

The atomic-level response of zigzag ferroelectric domain walls (DWs) was investigated with in situ bias scanning transmission electron microscopy (STEM) in a subcoercive-field regime. Atomic-level movement of a single DW was observed. Unexpectedly, the change in the position of the DW, determined from the atomic displacement, did not follow the position of the strain field when the electric field was applied. This can be explained as low mobility defect segregation at the initial DW position, such as ordered clusters of oxygen vacancies. Further, the triangular apex of the zigzag wall is pinned, but it changes its shape and becomes asymmetric under electrical stimuli. This phenomenon is accompanied by strain and bound charge redistribution. We report on unique atomic-scale phenomena at the DW level and show that in situ STEM studies with atomic resolution are very relevant as they complement, and sometimes challenge, the knowledge gained from lower resolution studies.

Heterogeneity Challenges in Multiple-Element-Modified Lead-Free Piezoelectric Ceramics.

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(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)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.

Ferroelectric order driven Eu3+ photoluminescence in BaZrxTi1-xO3 perovskite.

The ability to tune and enhance the properties of luminescent materials is essential for enlarging their application potential. Recently, the modulation of the photoluminescence emission of lanthanide-doped ferroelectric perovskites by applying an electric field has been reported. Herein, we show that the ferroelectric order and, more generally the polar order, has a direct effect on the photoluminescence of Eu3+ in the model BaZrxTi1-xO3 perovskite even in the absence of an external field. The dipole arrangement evolves with increasing x from long-range ferroelectric order to short-range order typical of relaxors until the non-polar paraelectric BaZrO3 is achieved. The cooperative polar interactions existing in the lattice (x < 1) promote the off-center displacement of the Eu3+ ion determining a change of the lanthanide site symmetry and, consequently, an abrupt variation of the photoluminescence emission with temperature. Each type of polar order is characterized by a distinct photoluminescence behaviour.

The ultrastructural features of the premalignant oral lesions.

Premalignant oral lesions are among the most important risk factors for the development of oral squamocellular carcinoma. Recent population studies indicate a significant rise in the prevalence of leukoplakia, erythroplakia/erythroleukoplakia, actinic cheilitis, submucous fibrosis and erosive lichen planus. Since standard histopathological examination has numerous limitations regarding the accurate appreciation of potential malignant transformation, the present study aims to aid these evaluations using the transmission electron microscopy (TEM) technique, which emphasizes ultrastructural changes pertaining to this pathology. Oral mucosa fragments collected from 43 patients that were clinically and histopathologically diagnosed with leukoplakia, erosive actinic cheilitis and erosive lichen planus have been processed through the classic technique for the examination using TEM and were examined using a Philips CM100 transmission electron microscope. The electron microscopy study has confirmed the histopathological diagnosis of the tissue samples examined using photonic microscopy and has furthermore revealed a series of ultrastructural details that on the one hand indicate the tendency for malignant transformation, and on the other reveal characteristic features of tumor development. All the details furnished by TEM complete the overall picture of morphological changes, specific to these lesions, indicating the importance of using these techniques in establishing both a correct diagnosis and prognosis.