Depoling phenomena in Na_0.5Bi_0.5TiO₃−BaTiO₃ : A structural perspective

Abstract

The structural complexities of the lead-free piezoelectric system ( 1 − x ) Na_0.5Bi_0.5TiO₃ − x BaTiO₃ (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 R 3 c at x ∼ 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 R 3 c - P 4 b m 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 R 3 c phase, triggered by the appearance of the P 4 b m phase. Our study suggests that partial conversion of the Cc average structure to R 3 c in unpoled NBT-BT at x ∼ 0.03 (more precisely in the range 0.03 ≤ x ≤ 0.05 ) is catalyzed by the appearance of P 4 b m phase. The overlap of T d and T 2 for this composition range is correlated with the collapse of the tetragonality of the P 4 b m phase and significantly reduced kinetic barrier associated with the R 3 c → P 4 b m 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 ( P 4 m m ) -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.