Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba(Mg0.33Ta0.67)O3

The effect of a slight A- and B-site cation nonstoichiometry on the structure, densification, and microwave dielectric properties of Ba(Mg1/3Ta2/3)O-3 (BMT) was investigated. Magnesium and barium nonstoichiometric compositions based on Ba(Mg0.33-xTa0.67)O-3 [x = -0.015, -0.010, -0.005, 0.0, 0.005, 0.010, 0.015, 0.020, 0.025, and 0.030] and Ba1-x(Mg0.33Ta0.67)O-3 [x = -0.015, -0.010, -0.005, 0.0, 0.0025, 0.005, 0.0075, 0.010, 0.015, 0.020, 0.025, and 0.030] were prepared using the conventional solid-state ceramic route. The lattice distortion and cation ordering were determined using X-ray diffraction technique. The phase composition and surface morphology were studied by EDX and scanning electron microscopy techniques, respectively. The sintered samples were characterized in the microwave frequency range using the resonance technique. It is found that a slight barium or magnesium deficiency can improve density, microwave dielectric properties, and cation ordering, while the addition of excess ions deteriorated them. The improvement in microwave dielectric properties was more pronounced in barium nonstoichiometric samples. Microwave dielectric properties of Ba-0.9925(Mg0.33Ta0.67)O-3 [epsilon(r) = 24.7, tau(f) 1.2 ppm/degreesC, Q(u)xf = 152 580 GHz] and Ba(Mg0.3183Ta0.67)O-3 [epsilon(r) = 25.1, tau(f) = 3.3 ppm/degreesC and Q(u)xf = 120 500 GHz] were found to be better than stoichiometric BMT [epsilon(r) = 24.2, tau(f) = 8 ppm/degreesC and Q(u)xf = 100 500 GHz]. Raman spectroscopy was employed to study the effects of nonstoichiometry and related lattice distortions in BMT ceramics on their vibrational modes. Raman results clearly showed the 1:2 ordered structures of these materials with all active modes assigned. The spectra showed variations in the normal modes as a function of the composition. Also secondary phases contributed to the changes in the Raman spectra observed in compounds with x greater than or equal to 0.02.