Silver deficiency and excess effects on quality, dielectric properties and phase transitions of AgNbO3 ceramics

Silver niobate AgNbO3 is a promising basic compound for high permittivity microwave dielectrics and lead-free piezoelectrics. Stoichiometric and nonstoichiometric AgxNbO2.5+x/2 ceramics were obtained for x equal to 0.95, 0.98, 1.00, 1.02, and 1.05. Scanning electron microscopy and X-ray diffraction studies showed that the homogeneity of the perovskite phase and thus the ceramics quality improved with the increase of x. Dielectric and thermal measurements (DSC) were carried out in wide temperature range. They revealed that the phase transitions become less diffused and shift towards higher temperatures with the increase of silver concentration. These temperature shifts are more significant for the low temperature phase transitions, which are connected with appearance of the polar ferroelectric and antiferroelectric states, than for high temperature transitions related to oxygen octahedra tilts. Influence of silver concentration on interaction between electric dipoles and on stability of polar phases is discussed. A few percent silver excess improve significantly ceramic quality.