A Crystal-Chemical Framework for Relaxor versus Normal Ferroelectric Behavior in Tetragonal Tungsten Bronzes

Abstract

Tetragonal tungsten bronzes (TTBs), an important class of oxides known to exhibit ferroelectricity, undergo complex distortions, including rotations of oxygen octahedra, which give rise to either incommensurately or commensurately modulated superstructures. Many TTBs display broad, frequency-dependent relaxor dielectric behavior rather than sharper frequency-independent normal ferroelectric anomalies, but the exact reasons that favor a particular type of dielectric response for a given composition remain unclear. In this contribution the influence of incommensurate/commensurate displacive modulations on the onset of relaxor/ferroelectric behavior in TTBs is assessed in the context of basic crystal-chemical factors, such as positional disorder, ionic radii and polarizabilities, and point defects. We present a predictive crystal-chemical model that rationalizes composition–structure–properties relations for a broad range of TTB systems.

Relaxor-to-Ferroelectric Crossover and Disruption of Polar Order in “Empty” Tetragonal Tungsten Bronzes

“…It is important to note that Chen and co-workers also proposed a model to predict relaxor vs. ferroelectric behavior but which considered only the difference in A-site cation radii. 37,39 As part of the proposed crystal-chemical framework 20 and in earlier 40 studies we reported the properties of a family of "empty" TTBs of general formula TTBs Ba 4 R 0.67…”

“…19 The origins of these superstructures are principally variations in the oxygen positions due to octahedral tilting out of the ab plane. [20][21][22][23][24][25][26][27][28][29][30] . When these variations are periodic, i.e.…”

Electric-field-induced phase transition and pinched P–E hysteresis loops in Pb-free ferroelectrics with a tungsten bronze structure

“…The tetragonal tungsten bronze structure is composed of ten distorted octahedral sharing corners in such a way that three different interstices (2 tetragonal A1, 4 pentagonal A2, and 4 triangle C) are available for cations in the general formula (A1) 2 (A2) 4 (C) 4 (B1) 2 (B2) 8 O 30 28 . In recent years, a number of Pb-free ferroelectrics with tetragonal tungsten bronze structures have been determined [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] 39,40 . A detailed structural study indicated that the relaxor nature is always associated with incommensurate superlattice modulation.…”

“…A detailed structural study indicated that the relaxor nature is always associated with incommensurate superlattice modulation. The decrease in the R cation size increases the driving force for commensurate superlattice modulation, which dominates the normal ferroelectric nature [29][30][31][32] .…”

Strain Mechanisms in Lead-Free Ferroelectrics for Actuators

“…456 A recent framework for frustrated ferroelectricity in tetragonal tungsten bronzes was developed based on competing instabilities. 464 This suggests that a proper understanding of structural instabilities and associated order parameters, as well as their couplings, is mandatory in order to engineer the functionality of several classes of advanced ceramics. Ferroelectrics have been traditionally designed around FD phase instabilities 169 to maximize strain output such as was treated for BZT-BCT (Section 5.1).…”