Structure–Property Relationships of BaTi1−2yGayNbyO3 (0≤y≤0.35) Ceramics

BaTi_{1?2 y} Ga _y Nb _y O₃ (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 ( P 4 mm ) 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 ( P 6₃/ 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 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, ?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.