Fatigue induced effects on bipolar strain loops in PZN-PT piezoelectric single crystals

Polarization and bipolar strain hysteresis (P-E and S-E) measurements were performed simultaneously during fatigue tests performed under ac fields with a triangular wave form. Rhombohedral (1-x)Pb(Zn_1/3Nb _2/3)O₃-xPbTiO₃ (PZN-PT) piezo/ferroelectric single crystals demonstrate a wide variety of anisotropic behavior under bipolar electrical switching. Specifically, PZN-4.5 PT crystals demonstrate exceptional polarization switching fatigue resistance along [001]_C (-c; pseudocubic), as opposed to normal fatigue in [111]_C directions. This system provides an excellent opportunity to study fatigue induced effects in [111]_C orientation in comparison with [001]_C. Studies in bulk ferroelectric ceramics have reported fatigue-associated strain hysteresis asymmetry. In this study, the evolution of bipolar strain loops was studied in both "fatiguing" [111]_C and "fatigue-free" [001]_C orientations. It was found that [111]_C oriented PZN-4.5 PT crystals show the onset of a strain-field asymmetry displayed by shortage of one wing of the butterfly, which becomes more evident as cycling continues. In contrast, [001]_C crystals show symmetrical strain-field curves throughout cycling. S-E hysteresis data obtained from different points in a fatigued crystal by gently moving it between the consecutive measurements show that the shorter wing switches side indicating the local character of fatigue. In comparison to the findings in polycrystalline ceramics this observation in a single crystal ferroelectric will be helpful for a better understanding of fatigue and related effects. Rejuvenation studies in fatigued crystals are also reported in this paper.