Depoling Phenomena In Na0.5bi0.5tio3-Batio3: A Structural Perspective

The structural complexities of the lead-free piezoelectric system (1-x)Na0.5Bi0.5TiO3-xBaTiO(3) (NBT-BT) continues to pose challenge regarding understanding the mechanisms underlying several interesting phenomena. Issues like (i) whether thermal depoling across compositions is triggered by a structural transformation event or not, (ii) what causes the average Cc structure to partially transform to R3c at x similar to 0.03 in unpoled specimens, (iii) what makes complete depoling of the compositions 0.03 <= x <= 0.05 occur in a considerably small temperature interval as compared to those for x < 0.03, (iv) what makes the R3c-P4bm transition temperature (T-2) abruptly become smaller than the depolarization temperature (T-d) at x = 0.06, etc., have remain unresolved. Here, we offer structural insights on these issues by carrying out a detailed investigation using a set of complementary tools involving temperature-dependent x-ray powder diffraction, neutron powder diffraction, dielectric, ferroelectric, piezoelectric, and thermally induced depoling current measurements. We show that onset of thermal depoling in NBT (x = 0) well below its depolarization temperature is caused by abrupt reduction of intrinsic polarization in the ferroelectric R3c phase, triggered by the appearance of the P4bm phase. Our study suggests that partial conversion of the Cc average structure to R3c in unpoled NBT-BT at x similar to 0.03 (more precisely in the range 0.03 <= x <= 0.05) is catalyzed by the appearance of P4bm phase. The overlap of T-d and T-2 for this composition range is correlated with the collapse of the tetragonality of the P4bm phase and significantly reduced kinetic barrier associated with the R3c -> P4bm transformation. We show that the abrupt crossover between T-d and T-2 at x = 0.06 is due to takeover of the thermal depoling process by an emergent tetragonal (P4mm)-like ferroelectric distortion. We present updated phase diagrams of poled and unpoled specimens which highlight all the subtle details needed to explain the temperature-dependent properties of this complex piezoelectric alloy system.