Structure-property relationships of BaTi1-2yGayNbyO3 (0<=y<=0.35) ceramics

BaTi1-2yGayNbyO3 (BTGN) (0 <= y <= 0.35) powders were synthesized at 1300 degrees 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 (3) over barm) polymorphs of BaTiO3, respectively, whereas y=0.35 consists of a mixture of the cubic polymorph of BaTiO3 and an 8H hexagonal-type perovskite (P6(3)/mcm) isostructural with Ba8Ti3Nb4O24. Scanning electron microscopy shows the microstructures of BTGN ceramics (y <= 0.30) sintered at 1500 degrees C to consist of fine grains (1-3 mu 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, epsilon(25)', (> 300 at 10 kHz), whereas 0.25 <= y <= 0.30 are temperature and frequency stable dielectrics with epsilon(25)'< 100 that resonate at microwave frequencies with modest quality factors, Q x f, similar to 3720 GHz (at similar to 5 GHz) for y=0.30.