Abstract
This paper systematically investigated the influence of Ti4+ substitution for Ta5+ on the phase composition and microwave dielectric properties of Ba3Ta4-4xTi4+5xO21 (x = 0.1, 0.2, and 0.3) ceramics with hexagonal tungsten bronze-like structures. X-ray diffraction and Rietveld refinement results indicated that single-phase Ba3Ta4Ti4O21 could be obtained only with the x values of 0.1 and 0.2, and a secondary phase was detected at an x value of 0.3. The valence state of Ba3Ta4-4xTi4+5xO21 (x = 0.2) ceramics was analyzed through X-ray photoelectron spectroscopy. Increasing Ti4+/Ta5+ ratios could reduce sintering temperature and improve the microwave dielectric properties of Ba3Ta4-4xTi4+5xO21 solid solutions. However, the dielectric properties, particularly the quality factor, of Ba3Ta4-4xTi4+5xO21 ceramics deteriorated severely as a result of oxygen vacancy defects caused by the transition of the valence state from Ti4+ to Ti3+ when x = 0.2 and the coexistence of the secondary phase when x = 0.3. Infrared reflectivity spectroscopy was performed to explore the intrinsic dielectric properties of Ba3Ta4-4xTi4+5xO21 (x = 0.1) ceramics. The measured and extrapolated microwave dielectric properties of Ba3Ta4-4xTi4+5xO21 (x = 0.1) ceramics sintered at 1240 °C for 6 h were εr ~ 46.5, Q × f = 13,900 GHz, τf ~ +49.4 ppm/°C, and εr ~ 44, Q × f = 34,850 GHz.
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