Lead‐Free Relaxor Ferroelectrics

Abstract

Feature size is a natural determinant of material properties. Its design offers the technological perspectives for material improvement. Grain size, crystallite size, domain width, and structural defects of different nature constitute the classical design elements. Common ferroelectric ceramics contain micrometer grain sizes and submicrometer domain widths. Domain wall mobility is a major contribution to their macroscopic material properties providing approximately half of the macroscopic output in optimized materials. The extension into the dynamic nanoworld is provided by relaxor ferroelectrics. Ionic and nanoscale field disorders form the base to a state with natural nanometer‐size polar structures even in bulk materials. These polar structures are highly mobile and can dynamically change over several orders of magnitude in time and space being extremely sensitive to external stimuli. This yields among the largest dielectric and piezoelectric constants known. In this feature article, we want to outline how lead‐free relaxors will offer a route to an environmentally safer option in this outstanding material class. Properties of uniaxial, planar, and volumetric relaxor compositions will be discussed. They provide a broader and more interesting scope of physical properties and features than the classical lead‐containing relaxor compositions.

Macroscopic response and directional disorder dynamics in chemically substituted ferroelectrics

“…The shift of the permittivity maxima with respect to the corresponding T C and the dispersion in frequency of the (T ) curves around the maxima signal a broad-type transition. This kind of frequency dependence is found in agreement with that reported in the literature on lead-free relaxor ferroelectrics [5,13]: We note that it does not manifest the features of a canonical relaxor response but instead those of a sharp-transition relaxor [5] (or "incipient relaxor" of [46]). …”

“…The local ordering and crystalline symmetry of PNRs have been extensively investigated using various techniques, the principal being diffractometric [20,[27][28][29][30][31][32][33][34][35]. Results indicate that the behavior and response of PNR-hosting samples is strongly affected by disorder [5,13,21,36,37]. Several works have been devoted to investigating PNR dynamics in correspondence to the cubic-to-tetragonal phase transition using NMR and EXAFS techniques [29][30][31].…”

“…Among these, an important role is played by solidsolution perovskites, which are commonly accepted as a paradigm of disordered ferroelectric crystals [5,11,12]. The behavior of these compounds can continuously span from that of a canonical relaxor, diffuse-transition relaxor, sharptransition relaxor, to a true ferroelectric [5,12], depending on the composition and stoichiometry [5,13]. A key feature of disordered ferroelectrics is that they can manifest many of the properties of standard crystals, in terms of electrical, optical, and mechanical response, and still host a population of nanosized reconfigurable polar regions, the so-called polar nanoregions (PNRs).…”

Strain Mechanisms in Lead-Free Ferroelectrics for Actuators

“…At the microscopic scale, atomic disorder as well as electrostatic and local strain fields determine to a great extent functional properties. 51,53,57 In the mesoscopic scale, intergranular interactions are also relevant and must be considered. 448 Investigations on the BNT-0.25ST were performed for both the long and mesoscopic length scales, as will be treated in the next paragraphs.…”

“…[51][52][53][54][55][56][57] The information of many textbooks and review papers related to the physics of materials science was employed throughout these sections. Unless explicitly stated otherwise, mathematical relationships displayed throughout this work hold for linear and homogeneous materials.…”