Structure-property relationships of BaTi _1-2yGa _yNb _yO ₃ (0≤y≤0.35) ceramics

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 (Pmm) polymorphs of BaTiO3, respectively, whereas y=0.35 consists of a mixture of the cubic polymorph of BaTiO3 and an 8H hexagonal-type perovskite (P63mcm) 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 μm) 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, ε25′, (>300 at 10 kHz), whereas 0.25≤y≤0.30 are temperature and frequency stable dielectrics with ε25′<100 that resonate at microwave frequencies with modest quality factors, Q×f, ∼3720 GHz (at ∼5 GHz) for y=0.30.