The correlation between structure and electrical properties of lead-free ͑1−x͒͑Bi 1/2 Na 1/2 ͒TiO 3 -xBaTiO 3 ͑BNT-100xBT͒ polycrystalline piezoceramics was investigated systematically by in situ synchrotron diffraction technique, combined with electrical property characterization. It was found that the morphotropic phase boundary ͑MPB͒ between a rhombohedral and a tetragonal phase evolved into a morphotropic phase region with electric field. In the unpoled material, the MPB was positioned at the transition from space group R3m to P4mm ͑BNT-11BT͒ with optimized permittivity throughout a broad single-phase R3m composition regime. Upon poling, a range of compositions from BNT-6BT to BNT-11BT became two-phase mixture, and maximum piezoelectric coefficient was observed in BNT-7BT. It was shown that optimized electrical properties are related primarily to the capacity for domain texturing and not to phase coexistence.

The electromechanical coupling in ferroelectric materials is controlled by several coexisting structural phenomena which can include piezoelectric lattice strain, 180° and non‐180° domain wall motion, and interphase boundary motion. The structural mechanisms that contribute to electromechanical coupling have not been readily measured in the past, particularly under the low‐to‐medium driving electric field amplitudes at which many piezoelectric materials are used. In this feature, results from in situ, high‐energy, and time‐resolved X‐ray diffraction experiments are interpreted together with macroscopic piezoelectric coefficient measurements in order to better understand the contribution of these mechanisms to the electromechanical coupling of polycrystalline ferroelectric materials. The compositions investigated include 2 mol% La‐doped PbZr0.60Ti0.40O3, 2 mol% La‐doped PbZr0.52Ti0.48O3, 2 mol% La‐doped PbZr0.40Ti0.60O3, undoped PbZr0.52Ti0.48O3, and 2 mol% Fe‐doped PbZr0.47Ti0.53O3. In all compositions, a strong correlation is found between the field‐amplitude‐dependence of the macroscopic piezoelectric coefficient and the contribution of non‐180° domain wall motion determined from the diffraction data. The results show directly that the Rayleigh‐like behavior of d33 piezoelectric coefficient is predominantly due to a Rayleigh‐like behavior of non‐180° domain wall motion. Furthermore, after separating contributions from lattice (atomic level) and domain wall motion (nanoscale level) to the measured macroscopic piezoelectric properties, we show that previously ignored intergranular interactions (microscopic level) account for a surprisingly large portion of the electromechanical coupling. These results demonstrate that electromechanical coupling in polycrystalline aggregates is substantially different from that observed in single crystalline materials. The construct of emergence is used to describe how averaged macrolevel phenomena are different from the material response observed in an isolated subcomponent of the material. Consequently, and due to its size‐scale complexity, the description of grain‐to‐grain interactions is presently inaccessible in most ab initio and phenomenological approaches. Results presented here demonstrate the need to account for these interactions in order to completely describe macroscopic electromechanical properties of polycrystalline materials.

Dedicated to Prof. Dr. Marija Kosec, our Mari cka, who left us after a long struggle in her tenacious spirit in December 2012.Bismuth ferrite (BiFeO 3 ), a perovskite material, rich in properties and with wide functionality, has had a marked impact on the field of multiferroics, as evidenced by the hundreds of articles published annually over the past 10 years. Studies from the very early stages and particularly those on polycrystalline BiFeO 3 ceramics have been faced with difficulties in the preparation of the perovskite free of secondary phases. In this review, we begin by summarizing the major processing issues and clarifying the thermodynamic and kinetic origins of the formation and stabilization of the frequently observed secondary, nonperovskite phases, such as Bi 25 FeO 39 and Bi 2 Fe 4 O 9 . The second part then focuses on the electrical and electromechanical properties of BiFeO 3 , including the electrical conductivity, dielectric permittivity, high-field polarization, and strain response, as well as the weak-field piezoelectric properties. We attempt to establish a link between these properties and address, in particular, the macroscopic response of the ceramics under an external field in terms of the dynamic interaction between the pinning centers (e.g., charged defects) and the ferroelectric/ferroelastic domain walls. J ournalFeature BiFeO 3 ceramics. Among the most interesting are the BiFe-O 3 -PbTiO 3 (BFPT) 14,16 and BiFeO 3 -BaTiO 3 (BFBT) 15,17,18 systems, which provide both enhanced piezoelectricity and a high T C at the MPB, the latter exceeding that of Pb(Zr,Ti)O 3 (PZT) (T C~6 50°C for BFPT, T C~6 00°C for BFBT and T C~3 50°C for PZT at the MPB). In addition, a number of other BiFeO 3 -based lead-free compositions are presently the subject of intensive studies, including BiFeO 3 -REFeO 3 (RE = La, Nd, Sm, Gd, Dy), 19-21 BiFeO 3 -AETiO 3 (AE = Mg, Ca, Sr), 22-26 BiFeO 3 -Bi 0.5 K 0.5 TiO 3 27 , and BiFe-O 3 -Bi(Zn 0.5 Ti 0.5 )O 3 . 28 The piezoelectric properties of many of these ceramic systems have not yet been characterized systematically.The processing of single-phase BiFeO 3 ceramics is difficult; however, significant progress has been made recently, particularly in relation to the identification of the origins of the frequently formed nonperovskite, secondary phases. In addition, the complex relationship between processing and defects, on one hand, and the high-and weak-field electrical and electromechanical properties, on the other, has been addressed to some extent. Up to now, a lot of these new findings, in particular those relating to processing and domain-switching behavior, have not been considered sufficiently or are even ignored in the literature. Along with the aim of presenting a detailed overview of the past and recent results on BiFeO 3 ceramics, this absence or poor coverage of some important topics was one of the motivations that led us to prepare a comprehensive article, which also includes new data.The review comprises two topics on BiFeO 3 that are the most controvers...