Observation of rhombohedral nanostructures in the orthorhombic phase of KNbO3 using convergent-beam electron diffraction

Abstract

Symmetries of nanometer-scale local structures in the rhombohedral and orthorhombic phases of potassium niobate (KNbO3) have been examined using convergent-beam electron diffraction. Nanometer-size local structures with rhombohedral symmetry have been discovered in the orthorhombic phase of KNbO3. It has been found that the structure of the orthorhombic phase of KNbO3 is formed as an average of two variants with rhombohedral symmetry. This fact indicates that the phase transformation between the orthorhombic and rhombohedral phases has an order-disorder character. The result is analogous to the case of BaTiO3, which we already reported [Tsuda et al., Phys. Rev. B 86, 214106 (2012)].

Macroscopic response and directional disorder dynamics in chemically substituted ferroelectrics

“…We associate the detected temperatures T C = 282 K, T C = 202 K, and T C = 156 K with the cubic/tetragonal, tetragonal/orthorhombic, and orthorhombic/rhombohedral transitions, respectively, in agreement with other well established disordered perovskite structures [26][27][28] and with previous studies into different compositions of KLTN [17,25]. We note that dielectric dispersion is detected along the entire range of the four observed structural phases, here indicated as P cub , P tet , P ort , P rho for decreasing temperatures.…”

“…Finally, in the rhombohedral phase there is no diffusion (only this symmetry shows a behavior of a true crystal): in this phase the displacements are correlated and the B cations are displaced in the same direction. Therefore, the rhombohedral phase is found to be the sole ordered and stable phase for the cited perovskites [19,20,27,28]. In our specimen, when the order/disorder states are characterized using Fröhlich entropy (Fig.…”

“…Altogether these phenomena are able to cause a local symmetry breaking in what would be normally considered a high-symmetry paraelectric phase, a breaking that generates a wide landscape of PNR-driven properties whose nature and temperature dependence is still under study. The local ordering and crystalline symmetry of PNRs have been extensively investigated using various techniques, the principal being diffractometric [20,[27][28][29][30][31][32][33][34][35]. Results indicate that the behavior and response of PNR-hosting samples is strongly affected by disorder [5,13,21,36,37].…”

“…Several works have been devoted to investigating PNR dynamics in correspondence to the cubic-to-tetragonal phase transition using NMR and EXAFS techniques [29][30][31]. Moreover, also in the lower temperature orthorhombic and rhombohedral phases, the internal symmetry of the PNRs was recently explored [27,28]. Nonetheless the features of the PNRs in the low-temperature crystalline phases remain, at present, only partially explored for a very restricted number of compounds (e.g., BaTiO 3 , SrTiO 3 , KNbO 3 , and PbTiO 3 ) [5,11,12,23].…”

Local Structural Study of Ferroelectric Domain Boundaries Using STEM-CBED with a Fast Pixelated STEM Detector

“…Convergent-beam electron diffraction (CBED) has been applied for symmetry determination [1] and crystal structure and electrostatic potential analysis [2][3][4] of nano-scale local specimen areas. In order to examine specimens with local structural fluctuations, we proposed a combined use of scanning the transmission electron microscopy (STEM) and the CBED (STEM-CBED method [5]), which can be categorized into the 4D-STEM techniques. In the STEM-CBED method, CBED patterns are acquired pixel-by-pixel by scanning the convergent-beam electron probe with a sub-nanometer scan step.…”