Analysis of the Phase Transition and the Domain Structure in K_0.5Bi_0.5TiO₃ Perovskite Ceramics by In Situ XRD and TEM

Abstract

The local crystal and domain structures of K0.5Bi0.5TiO3 ceramics were investigated by transmission electron microscopy (TEM) and selected‐area electron diffraction (SAED). The individual grains showed a lamellar domain structure, and on the basis of spot splitting along the characteristic crystallographic directions, 90°a–a‐ and 90°a–c‐type domains were identified. Furthermore, lamellar features within the 90° domains were observed, which we presumed were 180° domains; however, in the case of P4mm structures they cannot be distinguished by SAED. The 90° domain boundaries were (011) and (101) twin planes, typical for tetragonal perovskites. The domains could be designated as rotation twins, where the symmetry element describing the relationship between two twin domains is a twofold twin axis [011]/[101] and the symmetry operation is a 180° rotation. In order to determine the dynamics and the temperature of the phase transformation from the tetragonal to the cubic structures, high‐temperature XRD and in situ heating TEM analyses were performed. The results showed a gradual phase transformation in the temperature range from ∼280° to ∼450°C, where some grains lost their polar domains at a lower temperature than others. These findings confirmed the existence of a binary stability field in which the cubic and tetragonal structures coexist.

Lead-Free Relaxor-Like 0.75Bi0.5K0.5TiO3– 0.25BiFeO3Ceramics with Large Electric Field-Induced Strain

“…The combination of these two reported tendencies suggests at least two phase boundaries in the system: one connecting the pseudocubic phase with polar rhombohedral nanodomains and the rhombohedral (R3c) phase, as reported in [4], and one connecting a pseudocubic phase with the tetragonal phase [2] isostructural to BKT (P4mm) [17]. Our preliminary structural analysis suggests that the composition BKTF-25 belongs either to a tetragonal phase with P4mm crystal structure (being isostructural with BKT prototype at room temperature [17,18]) or to a pseudocubic phase with nano-or micro-domains of non-polar cubic structure and polar tetragonal structure (similar to the high-temperature (280-400 °C) isostructural type of BKT [6,17,18]). The latter case would presume an existence of one more phase boundary demarcating two pseudocubic phases: one with polar tetragonal and one with polar rhombohedral nanodomains.…”

“…The relative density was measured to be ~ 98%. Most of the grains appear with some regular image contrast that may indicate local compositional inhomogenieties or, more likely, a lamellar domain structure, similar to that observed in BKT [18]. 3 shows characterization of the bipolar polarization (P) and strain (S) hysteretic responses to applied electric field (E) with various amplitudes E0.…”

Polarization and strain response in Bi0.5K0.5TiO3-BiFeO3 ceramics

“…Note, that BKT itself, as relaxor ferroelectric, 5,8 exhibits such boundary at temperature Tm ≈ 280 °C, where the polar tetragonal structure gradually changes upon heating to the cubic state with some polar tetragonal areas persisting up to 400 °C. 4,25 Finally, refinement of the patterns using the space groups of the two end members or a cubic perovskite was challenging due to peak broadening and strain effects. Further evaluation of the corresponding XRD powder patterns is underway.…”

Dielectric and Piezoelectric Properties of Bi0.5K0.5TiO3-BaNb2O6 Lead-Free Piezoelectric Ceramics

“…Moreover, it is worth noting that the addition of BN tends to change the crystal symmetry of the KBT sample. It is known that pure KBT has tetragonal symmetry with low tetragonality (c/a = 1.016), 25 as characterized by the splitting of the (200) and (002) diffraction peaks. On addition of BN, the (200) and (002) diffraction peaks merge gradually into a single one, meaning that the phase structure changes from tetragonal to rhombohedral symmetry, as can be clearly seen from the (200) reflection line in Fig.…”