Coexistence of ferroelectric phases and electric field induced structural transformation in sodium potassium bismuth titanate ceramics

• Published in: Journal of applied physics 2018-06, Vol.123 (23)
• Main Authors: Veera Gajendra Babu, M., Bagyalakshmi, B., Giridharan, N. V., Dhayanithi, D., Sundarakannan, B.
• Format: Article
• Language: English
• Subjects: Applied physics; Bismuth titanate; Ceramics; Coercivity; Curie temperature; Curie-Weiss law; Deoxidizing; Dielectrics; Diffraction; Ferroelectric materials; Ferroelectricity; Hysteresis loops; Inhomogeneity; Lead free; Phase transitions; Piezoelectricity; Potassium; Relaxors; Symmetry; Transition temperature
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.5021830

Lead-free sodium potassium bismuth titanate (1−x)[Na0.5Bi0.5TiO3]-x[K0.5Bi0.5TiO3] (NKBTx) ceramics have been believed to be a rhombohedral crystal system with R3c symmetry up to x = 0.16 mole fraction, right now. For the first time, on the contrary, we have disclosed the existence of a low-symmetry (MA)-type monoclinic phase (69%) along with rhombohedral R3c (31%) in the NKBT0.10 sample from Rietveld studies on conventional and high-energy synchrotron powder diffraction data in the unpoled state. The peak splitting and reduced FWHM of {110}pc and {111}pc reflections after poling indicate the field-induced structural transformation. Rietveld analysis reveals that the monoclinic (Cc) phase is partially transformed to rhombohedral (R3c) at 50 kV/cm. Two distinct dielectric anomalies are noticed in the dielectric curve at 120 and 310 °C corresponding to freezing temperature (Tf) and glass-like transition temperature (Tm), respectively. The dielectric diffusivity (γ) estimated from the modified Curie-Weiss law reveals that the sample has a relaxor behavior. Structural inhomogeneity in the nanoscale region due to cation disorder in the A-site causes relaxing nature. A well-saturated square-like ferroelectric hysteresis loop is obtained with a remnant polarization and a coercive field of 35.8 μC/cm2 and 47 kV/cm, respectively. A piezoelectric constant (d33) of 85 pC/N is obtained for the sample which is somewhat higher than that of pure Na0.5Bi0.5TiO3 ceramics.