Crystal structure and dielectric properties of a new Na2O-Nd2O3-CeO2 ceramic system at microwave frequencies

Na2O-Nd2O3-CeO2 ceramic systems of the form Na3x-1Nd1-xCeO3 (x = 0.4, 0.5, and 0.6) were prepared via a solid-state reaction method. X-ray diffraction analyses indicated that the samples with x = 0.4, 0.6 and those with x = 0.5 formed Nd0.25Ce0.75O1.875 and Nd0.5Ce0.5O1.75 solid solutions, respectively, which resulted from a large degree of Na+ ion evaporation. Both the Nd0.25Ce0.75O1.875 and Nd0.5Ce0.5O1.75 phases belonged to the same cubic structure with an Fm-3m (225) space group. Additionally, the microstructures, sintering behaviors, and microwave dielectric properties were considered to be strongly related to the Na content in the Na3x-1Nd1-xCeO3 ceramics. A relatively large degree of Na+ ion evaporation could increase the negative temperature coefficient of the resonant frequency (tau(f)) for these ceramic systems. Moreover, after sintering at 1380 degrees C for 2 h, the x = 0.6 specimen possessed a low permittivity (epsilon(r)) of approximately 9.13, a moderate quality factor (Q x f) of approximately 23,800 GHz (at 9.14 GHz) and a negative tau(f) value of approximately -61.7 ppm/degrees C. As a result, it could be a promising microwave dielectric material for achieving a near-zero tau(f) value by solidifying one or more microwave dielectric ceramics with a high positive tau(f) value.