Synthesis and dielectric properties of tungsten-based complex perovskites
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Ba2MeWO6 (Me⳱Mg, Ni, Zn) double perovskites were prepared by the conventional solid-state reaction in a wide temperature range. Single-phase ceramics were obtained only at low temperatures approximately 1200 °C, whereas a small amount of second phases existed in the samples sintered at higher temperatures. All the compounds are characterized by the cubic perovskite structure (space group Fm3m) with a complete NaCl type ordering between B-site ions. Anomalous temperature variation of the dielectric loss tangent found in the Ba2NiWO6 perovskite is supposed to be connected with a dielectric relaxation due to electronic hopping within thermally activated Ni3+–6W(6−1/6)+/W5+–6Ni(2+1/6)+ clusters. Dielectric measurements showed that the other two perovskites—Ba2ZnWO6 and Ba2MgWO6—exhibit a positive value of the temperature coefficient of permittivity. Such temperature variation is assumed to be caused by a considerable influence of the second polar mode involving B-site ion vibrations on the low-frequency dielectric properties.
I. INTRODUCTION
The transition metal oxides with a perovskite structure are known to exhibit a fascinating variety of useful properties, covering a wide range of scientific areas, from electrical (ferroelectricity, superconductivity, ionic, and electronic conductivity) to magnetic, optical, and catalytic properties. Due to their comparatively simple crystal structure and large possibility of ionic substitutions, perovskite compounds have been used as models to investigate the relationship between the crystal structure and the physical properties in the last fifty years. This has been the case of the studies on the dielectric properties at high frequencies where ceramic components have been fabricated for microwave applications. For example, complex perovskites of the A2+2(B/2+B//5+2)O9 family (where A ⳱ Ba, Sr, B/ ⳱ Mg, Zn, and B// ⳱ Nb, Ta), with partial order in the B-site positions, were produced with exceptionally low-dielectric losses at microwave frequencies.1–3 It has been shown that the excellent dielectric properties of these materials are closely related to the ionic order between the B-site elements.3–5 However, to obtain a high degree of ordering, long annealing
a)
Present address: Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, People’s Republic of China. b) Address all correspondence to this author. e-mail: [email protected] 2600
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J. Mater. Res., Vol. 18, No. 11, Nov 2003 Downloaded: 12 Mar 2015
time is often necessary, which is not compatible with manufacturing production. The promotion of ordering is therefore a very real challenge, and the aim of the present work was to study the dielectric properties of perovskite compounds with easily achieved B-site ordering. Barium tungstates, with general chemical formula BaMe0.5W0.5O6, where Me is a 2+ cation (Me ⳱ Mg, Zn, or Ni), were chosen with the idea that the significant difference between B-site cation charges would promote the ordering in the materials. If order
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