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.

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

Strong coupling between local polar displacements and a commensurate octahedral tilting is proposed to explain the onset of classic ferroelectric behavior in tetragonal tungsten bronzelike dielectrics Ba2LaxNd1−xNb3Ti2O15. The ferroelectric phase transition is associated with a discontinuous non-lock-in transformation of an incommensurate tilted structure to a commensurate superstructure. In a manner reminiscent of perovskitelike oxides, the driving force for commensurate tilting increases as the average ionic radius of the rare-earth ion decreases; no classical ferroelectric transition is observed for compositions with x>0.75, which remain incommensurate and exhibit only relaxor behavior below room temperature.

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Coupling between octahedral tilting and ferroelectric order in tetragonal tungsten bronze-structured dielectrics

A polycrystalline ceramic with a new type of complex tungsten-bronze type structure, having a general formula K2Ba2Nd2Ti4Nb4W2O30 has been prepared using a high temperature solid-state reaction route after optimizing the calcinations conditions on the basis of thermal analysis results. The material has been characterized by different experimental techniques. The formation of the compound has been confirmed using X-ray diffraction analysis. Dielectric properties (er and tanδ) of the compound as a function of temperature at different frequencies have been carried out. Temperature dependence of dielectric constant indicates the presence of ferroelectric phase transition well above the room temperature. Complex impedance spectroscopic analysis has been carried out as a function of frequency at different temperatures to establish some correlation between the microstructure and electrical properties of the material. The nature of frequency dependence of ac conductivity obeys the Jonscher’s power law. The dc conductivity calculated from the ac conductivity spectrum shows the negative temperature coefficient of resistance behavior like a semiconductor.

Studies of dielectric and electrical properties of a new type of complex tungsten bronze electroceramics

Complex impedance studies of tungsten-bronze structured Ba5SmTi3Nb7O30 ferroelectric ceramics

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 M6−pRpTi2+pNb8−pO30 (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 A1- and A2-site ions, ∆r. 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 ∆r generally results in the normal ferroelectric, and the smaller ∆r 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 ∆r should be the guidelines for designing the possible multiferroic tungsten bronzes.

X.H. Zheng

Papers

Crystal structure and dielectric properties of ferroelectric ceramics in the BaO-Sm2O3-TiO2-Nb2O5 system