BaTi1−2yGayNbyO3 (BTGN) (0≤y≤0.35) powders were synthesized at 1300°C by the conventional solid-state method. Room temperature x-ray diffraction patterns for y≤0.025 and 0.05≤y≤0.30 can be indexed as the tetragonal (P4mm) and cubic (Pm m) polymorphs of BaTiO3, respectively, whereas y=0.35 consists of a mixture of the cubic polymorph of BaTiO3 and an 8H hexagonal-type perovskite (P63/mcm) isostructural with Ba8Ti3Nb4O24. Scanning electron microscopy shows the microstructures of BTGN ceramics (y≤0.30) sintered at 1500°C to consist of fine grains (1–3 m) within a narrow grain size and shape distribution. Room temperature transmission electron microscopy for y≤0.08 reveals core–shell structures and (111) twins in some grains; however, their relative volume decreases with y. Energy dispersive spectroscopy reveals the cores to be Ga and Nb deficient with respect to y. For y>0.08 there is no evidence of core–shell structures, however, some grains have a high density of dislocations, consistent with chemical inhomogeneity. BTGN ceramics exhibit a diverse range of dielectric behavior in the temperature range 120–450 K and can be subdivided into two groups. 0.025≤y≤0.15 display modest ferroelectric relaxor-type behavior, with high room temperature permittivity, 25′, (>300 at 10 kHz), whereas 0.25≤y≤0.30 are temperature and frequency stable dielectrics with 25′<100 that resonate at microwave frequencies with modest quality factors, Q×f, ∼3720 GHz (at ∼5 GHz) for y=0.30.
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Document Type: Research Article
Department of Engineering Materials, The University of Sheffield, Sheffield S1 3JD, U.K.
Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802
Publication date: November 1, 2005