Relaxor Phase Evolution of (Bi 0.5 Na 0.5- x K x )TiO 3 Ceramics due to K Ion Substitution and Their Corresponding Electrical Properties

We synthesized lead-free piezoelectric (Bi 0.5 Na 0.5- x K x )TiO 3 (BNKT) ceramics using a conventional solid-state reaction method. We have investigated the structural and electrical properties of the materials with x = 0.05 to 0.40. The X-ray diffraction (XRD) analysis suggests that the BNKT ceramics show the transition from rhombohedral to tetragonal structure. The ratio of the tetragonal structure increased continuously in accordance with the increasing composition of x . The sample of x = 0.10 showed a similar ratio between the tetragonal and rhombohedral structures. Frequency-dependent dielectric measurements showed a sort of relaxor properties emerged with increasing x composition, this effect may be interpreted in terms of the formation of polar nano-regions (PNRs) in samples. The value of remnant polarization ( P r ) decreases rapidly as x increases beyond the point of x = 0.10 from 25.3 μC/cm 2 to 5.9 μC/cm 2 . On the contrary, as for inverse piezoelectric coefficient ( d 33 *), a higher value of d 33 * (336 pm/V) at x = 0.10, was observed when compared with x = 0.05 ( d 33 * = 51 pm/V). These results can be explained by the formation of PNRs and their variations with the external applied field. We here propose a possible mechanism showing the effects of dipolar defects, which can be resulted from the K ion substitution on (Bi,Na)TiO 3 (BNT) ceramics.