(1 − x)(Bi1/2Na1/2)TiO3-xBaTiO3 (BNT-100xBT) single crystals with three different compositions of x = 0.036, 0.065, and 0.088, covering the rhombohedral to predominantly tetragonal region of the phase diagram and encompassing the morphotropic phase boundary (MPB), were grown by top-seeded solution growth. Dielectric and ferroelectric measurements were performed on samples with different orientations with respect to the crystallographic axes. While the results complied with the current understanding of the crystallographic structure, no enhancement of electromechanical properties based on transient polarization rotation was observed. This clearly sets BNT-100xBT apart from other relaxor ferroelectric systems with a rhombohedral-tetragonal MPB such as (1 − x) Pb(Mg1/3Nb2/3)O3-xPbTiO3. An anomaly was observed in the poling behavior of the strain in 〈001〉 oriented BNT-100xBT in the immediate vicinity of the MPB with x = 0.065, resulting in a giant small-signal piezoelectric coefficient d33 of 4600 pm/V. This effect is hypothesized to be due to an irreversible phase change from rhombohedral polar nanoregions to tetragonal ferroelectric microdomains.

The direction-dependence of pyroelectric properties of (1 -x) Bi 1/2 Na 1/2 TiO 3 -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

Lead-free piezoelectric single crystals of Fe-doped 0.95(Bi 1/2 Na 1/2)TiO 3-0.05BaTiO 3 are shown to combine multiple mechanisms for high strain and an effective piezoelectric coefficient of up to 3260 pm/V. This is explained based on the analysis of superstructure reflections from diffuse synchrotron x-ray scattering. It depends on three factors: (1) the stabilization of a local tetragonal structure by (Fe'-V •• O) • defect dipoles, (2) the reversible creation of a morphotropic phase boundary under field, and (3) the field-induced reversible transition between a short-range ordered relaxor and a long-range ordered ferroelectric state. IMPACT STATEMENT The observation that a combination of three different physical mechanisms produces unprecedently high piezoelectric response in a lead-free piezoelectric presents a breakthrough in the fundamental understanding of optimizing piezoelectric properties.