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A. Belous, O. V’yunov, and O. Yanchevskij Department of Solid State Chemistry, Institute of General and Inorganic Chemistry, 32/43 Palladina Ave., Kyiv-142, Ukraine (Received 31 July 2002; accepted 19 September 2002)

Phase composition, degree of cation ordering, and dielectric properties of complex perovskites with general formula Ba(B⬘1/3B⬙2/3)O3, where B⬘ ⳱ Mg, Zn, and Ni and B⬙ ⳱ Nb and Ta, were analyzed. It was found that all the studied complex perovskites attained high degrees of 1:2 cation ordering at temperatures specific to each composition. A high temperature order–disorder phase transition in Ba(Zn1/3Nb2/3)O3 occurred below 1380 °C. Ba(Ni1/3Nb2/3)O3 (BNN) and Ba(Mg1/3Nb2/3)O3 (BMN) pervoskites remained 100% ordered at temperatures as high as 1500 and 1620 °C, respectively. It was found that in BMN and BNN extrinsic factors, such as the second phase (i.e., Ba3Nb5O15) and point defects, dominated the dielectric loss at microwave frequencies. Ba(Mg1/3Ta2/3)O3 (BMT) remained single phase up to 1630 °C. Above this temperature, the Ba3Ta5O15 second phase was detected. A decrease in the 1:2 cation ordering and increase of dielectric loss in BMT occurred at sintering temperatures above 1590 °C. It was also revealed by electron paramagnetic resonance that all samples studied contained a substantial amount of paramagnetic point defects. These defects contributed to extrinsic dielectric loss at microwave frequencies, thus degrading the Q factor. I. INTRODUCTION

Low-loss temperature-stable dielectric resonators are used extensively as input and output multiplexers in communication satellites, band-stop and band-pass filters, feedback circuits for microwave oscillators, etc.1 Recent breakthroughs in the design of dielectric resonators shielded with high-temperature superconductor walls show promise in fulfilling new requirements for future communication systems.2 Complex perovskites with the general formula Ba(B⬘1/3 B⬙2/3)O3, where B⬘ ⳱ Mg and Zn and B⬙ ⳱ Ta, have proved to be the most suitable candidates for application in X, Ku, and K frequency bands (i.e., 8–27 GHz). They satisfy three important requirements imposed on microwave dielectrics, i.e., high dielectric constant (⑀ ≈ 24–30), high Q × f value (Q × f ≈ 100–300 THz, where Q is the quality factor and f is the resonance frequency), and near-zero temperature coefficient of the resonance frequency, ␶f. Tremendous research efforts were undertaken to improve the properties3,4 as well as to understand the dominant mechanism of the dielectric loss in these materials.5,6

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Address all correspondence to this author. e-mail: [email protected]

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http://journals.cambridge.org

J. Mater. Res., Vol. 17, No. 12, Dec 2002 Downloaded: 24 Nov 2014

Intrinsic microwave dielectric loss in Ba(B⬘1/3B⬙2/3)O3 originates from the two-phonon difference absorption process.7 The only way to minimize this loss is to achieve a complete 1:2 ordering of B⬘ and B⬙ cations. However, recent studies suggest that extrinsic dielectric loss due to the point defects or second phase may