Electron microscopy characterization of Ba(Cd 1/3 Ta 2/3 )O 3 microwave dielectrics with boron additive
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Shaojun Liu and N. Newman Department of Chemical and Materials Engineering, Science and Engineering Materials Program, Arizona State University, Tempe, Arizona 85287
M.R. McCartney Center for Solid State Science, Arizona State University, Tempe, Arizona 85287
David. J. Smith Center for Solid State Science, and Department of Physics and Astronomy, Arizona State University, Tempe, Arizona 85287 (Received 19 October 2003; accepted 15 January 2004)
The microstructure of Ba(Cd1/3Ta2/3)O3 ceramics with boron additive was investigated by high-resolution and analytical electron microscopy. Superlattice reflections were present at positions of (h ± 1/3, k ± 1/3, l ± 1/3) away from the fundamental reflections in the [11¯0] zone diffraction pattern for the pseudocubic perovskite unit cell. Lattice images showed a well-ordered structure with hexagonal symmetry. No boron segregation and amorphous phase was observed along grain boundaries. An amorphous phase rich in boron-oxide was observed to form pockets partially penetrating along multiple grain junctions.
I. INTRODUCTION
Ceramics with the formula Ba(B⬘1/3 B⬙2/3)O3 (B⬘ ⳱ Mg and Zn, B⬙ ⳱ Ta) are recognized for their potential applications in microwave communication systems because of their excellent high-frequency properties, which include high-dielectric constant, ultra-low loss tangent, and low-temperature coefficient of resonant frequency.1–3 In Ba(B⬘1/3Ta2/3)O3, the B-site cations are stoichiometrically ordered, in the sequence of one B⬘ layer and two Ta layers along directions of the parent perovskite cubic cell.4 It has been shown experimentally that the B-site ordering arrangement in the Ba(B⬘1/3Ta2/3)O3 ceramics is crucial for optimizing the dielectric properties.5,6 Ba(Cd 1/3 Ta 2/3 )O 3 is another member compound among the family of Ba(B⬘1/3Ta2/3)O3 perovskite ceramics. Galasso and Pyle synthesized various niobate and tantalate perovskites and proposed that the B-site ordering in A(B⬘1/3 B⬙2/3)O3 perovskites increased as the difference in charge and size between B⬘ and B⬙ atoms increased.4,7 The difference in ionic size between B⬘ and B⬙ of 0.29 Å for Ba(Cd1/3Ta2/3)O3 is much larger than
DOI: 10.1557/JMR.2004.0185 J. Mater. Res., Vol. 19, No. 5, May 2004
http://journals.cambridge.org
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the corresponding values of 0.06 Å and 0.01 Å for Ba(Zn1/3Ta2/3)O3 and Ba(Mg1/3Ta2/3)O3, respectively. Thus, Ba(Cd1/3Ta2/3)O3 should be ordered much more easily than these other materials. On the other hand, recent theoretical investigations suggest that the relative contribution of d-electron bonding can provide enhanced directional covalent bonding, a property that is absent in classic ionic compounds. This directionality can strengthen the soft anharmonic lattice modes that are believed to play an important role in the microwave loss for Ba(Cd1/3Ta2/3)O3.7 Ba(Cd1/3Ta2/3)O3 ceramics with addition of zinc have also experimentally shown a reasonably high-dielectric constant and low loss tangent.8,9 Because significant volatilization of cadmium occurs during
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