Structural evolution and enhanced microwave dielectric properties in Sr2+/Ti4+ co-substituted SrNd2Al2O7 ceramics

Dense Sr1+2xNd2-2xAl2-2xTi2xO7 (x = 0, 0.05, 0.10, 0.15) ceramics were prepared via a standard solid state reaction method, and the structure and phase composition were investigated together with their effects on the microwave dielectric properties. The major phase with n = 2 Ruddlesden-Popper structure was confirmed from the X-ray diffraction patterns, while two ineluctable secondary phases were also observed. With increasing x, the dielectric constant εr increased monotonously from 21.4 to 23.3, and the temperature coefficient of resonant frequency τf was tuned from −6.1 ppm/°C to −1.0 ppm/°C. The Qf value was improved from 63,700 GHz at x = 0.0 to 76,500 GHz at x = 0.10 and turned to decrease thereafter. The drop of Qf value at x = 0.15 could be attributed to the increased amount of secondary phase, where stacking faults were also detected from the TEM images. Moreover, infrared reflectivity spectra analyses were applied to further understand the variation of intrinsic dielectric properties. The optimum microwave dielectric properties were achieved at x = 0.10: εr = 22.5, Qf = 76,500 GHz, τf = −1.5 ppm/°C.