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Thermally‐induced phase transformations in Na 0.5 Bi 0.5 TiO 3 –KNbO 3 ceramics
Author(s) -
Wang Ge,
Li Yizhe,
Murray Claire A.,
Tang Chiu C.,
Hall David A.
Publication year - 2017
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/jace.14856
Subject(s) - ferroelectricity , materials science , dielectric , metastability , phase (matter) , phase diagram , atmospheric temperature range , phase transition , ceramic , condensed matter physics , crystallography , thermodynamics , chemistry , optoelectronics , composite material , physics , organic chemistry
The structures and functional properties of Na 0.5 Bi 0.5 TiO 3 – x KNbO 3 (NBT‐xKN) solid solutions, with x in the range from 0.01 to 0.09, were investigated using a combination of high‐resolution synchrotron X‐ray powder diffraction (SXPD) and ferroelectric property measurements. For low KN contents, an irreversible transformation from cubic to rhombohedral phases was observed after the application of a high electric field, indicating that the polar nanoregions (PNRs) in the unpoled state can be transformed into metastable long‐range ordered ferroelectric domains in the poled state. In contrast, the near‐cubic phase of the unpoled ceramics was found to be remarkably stable and was retained on cooling to a temperature of −175°C. Upon heating, the field‐induced metastable ferroelectric rhombohedral phase transformed back to the nanopolar cubic state at the structural transformation temperature, T ST , which was determined as approximately 225°C and 125°C for KN contents of 3% and 5% respectively. For the field‐induced rhombohedral phase in the poled specimens, the pseudo‐cubic lattice parameter, a p , exhibited an anomalous reduction while the inter‐axial angle increased towards a value of 90° on heating, resulting in an overall increase in volume. The observed structural changes were correlated with the results of temperature‐dependent dielectric, ferroelectric and depolarization measurements, enabling the construction of a phase diagram to define the stable regions of the different ferroelectric phases as a function of composition and temperature.