Depolarisation of Na0.5Bi0.5TiO3-based relaxors and the resultant double hysteresis loops

Abstract

The temperature-induced depolarisation of (100-x-y)Na0.5Bi0.5TiO3-xBaTiO3-yK0.5Na0.5NbO3 ceramics has been investigated using polarisation-electric field (P-E) loops, current density-electric field loops, dielectric measurements, thermally stimulated depolarisation current measurements, and x-ray diffraction. The depolarisation temperature (Td) values were measured using the thermally stimulated depolarisation current on each furnace to allow the comparison of results between techniques. Td closely agreed with the values determined from the dielectric anomalies resulting from the ferroelectric to relaxor transition (TF-R). Td determined using pinching of P-E loops was 2-9 K higher, and as the maximum applied electric field was increased, the value of Td increased. The pinching of the loop was shown not to be from an antiferroelectric structure; pinching occurs as the induced ferroelectric phase becomes unstable and returns to its unpoled relaxor state. This is the tuning of the transition from the poled non-ergodic state to the ergodic relaxor state with the applied electric field. Above Td, the P-E loops return to a relaxor state before a reverse field is applied, giving the appearance of a classic antiferroelectric P-E loop.