Orientation-Dependence of Thermal Depolarization and Phase Development in Bi1/2Na1/2TiO3-BaTiO3 Single Crystals

The direction-dependence of pyroelectric properties of (1 - x) Bi1/2Na1/2TiO3 - xBaTiO(3) (BNT - 100xBT) is investigated, using single crystal samples with well-defined orientations for x = 0.036 and x = 0.063. The results are compared with those of temperature-dependent measurements of the ferroelectric and dielectric hysteresis. The depolarization temperature T-d of each crystal composition is found to depend on crystal orientation, a fact that is explained by differences in the stability of respective domain configurations. A rationalization is offered for the observation that T-d differs from the ferroelectric-relaxor transition temperature, depending on orientation. The hysteresis curves of BNT - 3.6BT are typical for a rhombohedral system with a ferroelectric-relaxor transition, with polarization reversal close to T-d occurring in a multistep process that includes decay of ferroelectric domains into polar nanoregions and re-formation of domains. BNT - 6.3BT, a composition in the region of the morphotropic phase boundary, shows the same feature, but additionally is characterized by a field-induced transition between rhombohedral and tetragonal symmetry. This combination results in an effective piezoelectric coefficient of d(33)* = 1886 pm/V.