Article

Structure, dielectric and magnetic properties of Ba6FeNb9O30 tungsten bronze ceramics

Journal of Materials Science Materials in Electronics (Impact Factor: 1.97). 01/2011; 22(7):866-871. DOI: 10.1007/s10854-010-0227-3

ABSTRACT Ba6FeNb9O30 ceramics were synthesized by a standard solid-state sintering process. X-ray powder diffraction (XRD) refinements were carried
out to analyze the crystal structure, and the dielectric, ferroelectric and magnetic properties were investigated and discussed.
The tetragonal tungsten bronze structure in space group P4bm was determined with the lattice parameters: a=12.597(6) Å, b=12.597(6) Å, c=3.989(1) Å. An extremely high dielectric constant was indicated at higher temperatures, and it dropped quickly when the
sample was cooled down through a critical temperature, and this critical temperature showed strong frequency dependence. This
dielectric relaxation was more obviously observed in the dielectric loss curve. The nonlinear magnetic hysteresis curve was
observed in the present ceramics at 5K, which was related to the magnetic ions (Fe3+) in tungsten bronze structure.

0 Followers
 · 
107 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Several ferrum oxides of formula Ba4Gd2Fe2Nb8−xTaxO30 (x = 2, 4, 6 and 8) with tetragonal tungsten-bronze structure have been synthesized by the conventional solid state sintering method. Dielectric response of the samples was studied using AC impedance spectroscopy and universal dielectric relaxation law in detail. The dielectric anomaly at high temperature is attributed to the mobility of oxygen vacancies. Arrhenius activation energy associated with the migration of oxygen vacancies is estimated from impedance spectroscopy, which is 0.77, 1.11, 1.26 and 1.02 eV for x = 2, 4, 6 and 8, respectively.
    Journal of Materials Science Materials in Electronics 01/2014; 25(1). DOI:10.1007/s10854-013-1553-z · 1.97 Impact Factor
  • [Show abstract] [Hide abstract]
    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 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.
    Journal of the American Ceramic Society 02/2014; 97(2). DOI:10.1111/jace.12790 · 2.43 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In-depth analysis of the relaxor behaviour of Ba 6 MNb 9 O 30 (M = Ga, Sc, In) tetragonal tungsten bronze (TTB) ceramics was carried out. Powder X-ray diffraction and scanning electron microscopy were performed in order to confirm the formation of desired phases and to determine the microstructure. Low-temperature dielectric spectros-copy was used in order to characterise the dielectric properties of these materials; the degree of relaxor behaviour was investigated in relation with the increase of ionic radius of the M cation on the B-site of the TTB structure. The dynamics of dielectric relaxation of dipoles was studied by fitting the dielectric permittivity data to the Vogel–Fulcher (VF) model in order to monitor the repro-ducibility and validity of the physical results. Restrictions to the VF fit were attempted besides the regular ''free-fit'' by constraining some of the fundamental relaxation parameters to physically sensible values. We show that VF fits are very sensitive to the fitting range resulting in a large range of fundamental parameters for the dielectric relaxation processes, and that the restriction of the frequency domain due to experimental noise or instrumenta-tion limits has a dramatic influence on the values obtained.
    Journal of Thermal Analysis and Calorimetry 05/2015; DOI:10.1007/s10973-014-4355-5 · 2.21 Impact Factor