Ferroelectric Transition and Low‐Temperature Dielectric Relaxations in Filled Tungsten Bronzes

Abstract

A comprehensive review on the latest development of the ferroelectric transition and low-temperature dielectric relaxations of filled tungsten bronze ceramics are presented together with some new issues. In the filled tungsten bronze ceramics M 6 -p R p Ti 2+p Nb 8 -p O 30 (p = 1, 2; M = Ba or Sr; R = rare earth or Bi), a ferroelectric transition is generally indicated together with up to three low-temperature dielectric relaxations. The ferroelectric transition is determined as 4/ mmm ? 4 mm, and the low-temperature dielectric relaxations are deeply concerned with the structure modulations due to the order/disorder of ions in A1 and A2 sites, their random cross occupancy, and the order/disorder of B-site ions. Both the ferroelectric transition and low-temperature dielectric relaxations are dominated by the composition and radius difference between A1and A2-site ions, Δr. The normal ferroelectric transition might be expected if the ratio of the biggest ion and second big ion is 2:1, otherwise the diffuse or relaxor ferroelectric is expected. Meanwhile, the larger Δr generally results in the normal ferroelectric, and the smaller Δr will lead to the diffuse or relaxor ferroelectric. Moreover, the effects of A sites order/disorder and the random cross occupancy of A-site ions are primary, and the effects of B-site ordering/disordering are secondary. The right ratio of 2:1 for A2-and A1-site ions and the large Δr should be the guidelines for designing the possible multiferroic tungsten bronzes.

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

“…19,37,[64][65][66][67][68][69][70][71] However, the relaxor-like response in these is often subtle or convoluted with the main permittivity peak associated with T C and has not been demonstratively linked changes in the structural or P-E properties and so the origins of such behavior has not been convincingly assigned. The pronounced separation of the features presented here for Ba 4 La 0.67 !…”

“…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…”

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

“…One such case is presented in Figure 3 for Sr 5 SmTi 3 Nb 7 O 30 . In the context of this contribution, we note only that the complex interaction of multiple crystallographic sites with the incommensurate/commensurate modulation coupled to site disorder is likely to lead to so called 're-entrant' relaxor behavior, a topic dealt with by Zhu et al (2014). 54 However, the primary dielectric behavior discussed in this contribution is dominated by the average A-site ionic radius and the A1 tolerance factor, as illustrated in Figure 17.…”

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.…”