The Effect of Chemical Composition on the Structure and Dielectric Properties of the Columbites A(2+)Nb(2)O(6)

This paper summarizes the synthesis and properties of the materials based on the columbites A(1+x)(2+)Nb(2)O(6+x) (A(2+) = Mg, Co, Zn) in the vicinity of their stoichiometric composition. The evolution of the phase composition, microstructure, and microwave dielectric properties of the studied materials have been considered with respect to different sintering atmospheres (air and oxygen). The correlation between the data obtained by electron microscopy, positron lifetime spectroscopy, and microwave characterization at the frequencies of 10 and 50-70 GHz has been presented and discussed. The A-site deficiency (x < 0) always leads to the formation of the secondary Nb-rich phase in the columbite matrix, which significantly deteriorates the microwave quality factor (Q) of the sintered materials. In contrast, the Nb deficiency improves the microstructure of the ceramics but initiates the formation of A(2+)-rich secondary phases. The competing effect of these factors results in the nonlinear variation Q(x). The resulting magnitudes (Qxf) attain values of 100,000-120,000 GHz depending on the A(2+) ion and sintering atmosphere. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3236661] All rights reserved.