Role of Ni and Zr doping on the electrical, optical, magnetic, and structural properties of barium zinc tantalate cerami
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Role of Ni and Zr doping on the electrical, optical, magnetic, and structural properties of barium zinc tantalate ceramics G. Rong, N. Newman, and B. Shaw Electrical and Computer Engineering Department, Northwestern University, Evanston, Illinois 60208
D. Cronin Trans-Tech Inc., Adamstown, MD 21710 (Received 2 November 1998; accepted 27 July 1999)
Properties of Ni- and Zr-doped Ba(Zn1/3Ta2/3)O3 ceramics are reported. The nickel ion has an effective paramagnetic moment of 3.22 ± 0.05. Optical spectra of Ni-doped Ba(Zn1/3Ta2/3)O3 are dominated by discrete internal transitions between Ni2+ 3d orbitals. Absorption from the 3⌫2(F) ground state to 4⌫3(F), 3⌫1(D), 5⌫1(D), 4⌫3(P) excited states occurs at approximately 1.55, 1.75, 2.50, and 2.80 eV, respectively. The ligand field strength of neighboring oxygen ions ranges from about 7300 cm−1 (0.25% Ni) to about 7700 cm−1 (1.0% Ni). A significant increase in the visible continuum background is correlated with increased tan ␦. This effect is attributed to point defects in the Ni environment, suggesting that point defects play an important role in microwave loss in practical dielectric material. I. INTRODUCTION
Ultrahigh-performance microwave dielectrics will revolutionize communication satellite and emerging personal communication system/cellular technology if significant improvements in material quality can be realized (see Ref. 1 and references therein). The development of high dielectric-constant material with diminished microwave loss and a near-zero f will result in miniaturization of devices operating in currently used microwave bands and development of higher frequency high-Q devices. This report describes a systematic effort to understand the electronic, structural, and chemical properties of practical microwave dielectrics. The ultimate goal of this effort is to improve the quality factor (Q), thermal stability, and manufacturability of high-performance microwave devices with dielectric materials. Ba(Zn1/3Ta2/3)O3 is a compound that shows tremendous potential for these applications because of its large dielectric constant (⑀ ∼ 35) and ultralow loss tangent (tan ␦ < 10−4 at 10 GHz).2–5 When Ba(Zn1/3Ta2/3)O3 ceramic material is doped with Ni, the temperature coefficient of resonant frequency f can be tuned to near zero.2 Zr is also commonly used in conjunction with Ni doping because it is found to significantly decrease annealing times required to attain high Q as well as to facilitate highquality factors over a range of stoichiometries.5 Despite the practical importance of these transition-metal dopants, a fundamental understanding of their role on the material performance is still lacking. Another important question arises concerning the physical origin of high-frequency loss. A number of experimental and theoretical investigations have concluded J. Mater. Res., Vol. 14, No. 10, Oct 1999
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that the dominant microwave loss mechanism in practical dielectrics arises
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