Morphotropic Phase Transitions in Mixed Sodium-Potassium Niobate System

Abstract

Ceramic pellets of different compositions of Na _1−xK_xNbO ₃ system (x = 0 to 0.5) have been prepared using ‘solid-state reaction’ method. Pellets of 6 mm diameter have been prepared under two conditions; first, the calcined mixtures were pressed at 0.2 MPa and sintered in a tube furnace opened at both ends; and second, the calcined mixtures were pressed at 0.02 MPa and sintered in a closed furnace. Prepared samples were characterized by x-ray diffraction (XRD) and energy dispersive x-ray analysis (EDAX) method. In the XRD pattern, a continuous shifting of most intense peak (002) to lower angle was observed with increasing potassium concentration (x), except for x = 0.5, where a break in the peak shifting pattern was observed. Lattice parameters and crystallite size for different compositions have been calculated from the XRD data. EDAX results show marked sodium deficiency in the prepared samples. Frequency and x dependence of dielectric constant, loss tangent and dielectric conductivity have been investigated, at room temperature, in the frequency range from 0.1 KHz to 10 MHz. With increasing frequency dielectric constant and loss tangent were found decreasing, which indicate relaxational behavior of this system. Morphotropic regions have been observed near the compositions with x = 0.1, 0.3 and 0.5. Dielectric constant, loss and conductivity have been observed higher, for the samples pressed at 0.2 MPa and sintered in a tube furnace opened at both ends, than the reported values and that for the samples pressed at 0.02 MPa and sintered in a closed furnace. Higher density and the improved oxygen stoichiometry are held responsible for the observed higher dielectric constant, loss and conductivity, in the former types of samples.

Keywords:

• Ferroelectrics
• antiferroelectrics
• perovskite
• dielectric constant
• loss tangent
• sintering
• morphotropic phase

Acknowledgements

Authors are grateful to the Department of Science & Technology, New Delhi for financial support to carry out these investigations.