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...

The piezoelectric compositions (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) span a model lead-free morphotropic phase boundary (MPB) between room temperature rhombohedral and tetragonal phases at approximately x = 0.5. In the present work, in situ X-ray diffraction measurements during electric field application are used to elucidate the origin of electromechanical strain in several compositions spanning the tetragonal compositional range 0.6 ≤ x ≤ 0.9. As BCT concentration decreases towards the MPB, the tetragonal distortion (given by c/a-1) decreases concomitantly with an increase in 90° domain wall motion. The increase in observed macroscopic strain is predominantly attributed to the increased contribution from 90° domain wall motion. The results demonstrate that domain wall motion is a significant factor in achieving high strain and piezoelectric coefficients in lead-free polycrystalline piezoelectrics.

Domain wall motion in the tetragonal phase is also readily apparent and exhibits a degree of frequency dispersion similar to that measured in both the relative permittivity and piezoelectric coefficients at similar conditions.

A high energy synchrotron x-ray study of crystallographic texture and lattice strain in soft lead zirconate titanate ceramics J. Appl. Phys. 96, 4245 (2004); 10.1063/1.1787590 Crossover between nucleation-controlled kinetics and domain wall motion kinetics of polarization reversal in ferroelectric filmsPolarization reversal in polycrystalline ferroelectrics is shown to occur via two distinct and sequential domain reorientation steps. This reorientation sequence, which cannot be readily discriminated in the overall sample polarization, is made apparent using time-resolved high-energy x-ray diffraction. Upon application of electric fields opposite to the initial poling direction, two unique and significantly different time constants are observed. The first (faster time constant) is shown to be derived by the release of a residual stress due to initial electrical biasing and the second (slower time constant) due to the redevelopment of residual stress during further domain wall motion. A modified domain reorientation model is given that accurately describes the domain volume fraction evolution during the reversal process. V C 2014 AIP Publishing LLC.