Temperature behaviour of structural, dielectric and piezoelectric properties of sol-gel processed ceramics of the system LiNbO3-NaNbO3

Ceramics of the system LiNbO3-NaNbO3 were prepared from sol-gel derived powders in the whole range of compositions and structurally characterized by X-ray diffraction. The impedance of thin ceramic disks was measured as a function of the temperature from 1 kHz to 15 MHz up to 800 degrees C. An automatic iterative procedure was used to calculate the electromechanical coupling factor of the thickness mode resonance from impedance data. The solid solution limits of Na in LiNbO3 and Li in NaNbO3 were found to be 7mole% and 15mole%. As a consequence the materials whose nominal composition is Li1-yNaxNbO3 with 0.07 less than or equal to x less than or equal to 0.85 contain mixtures of the two limit solid solutions. For ceramics with Na content exceeding 10mole%, two dielectric anomalies at 300 and 400-450 degrees C are observed as local maximums in the real part of the dielectric permittivity when measurements are made while heating the sample. These are more enhanced as the Na content increases. There is a change of sign in the temperature coefficient of the conductivity of the ceramics giving place to a minimum at low temperature, 100-150 degrees C, which is not present at high frequency (> 5 MHz). There is a shift in the frequency number of the thickness mode of resonance towards higher values together with a minimum in the electromechanical coupling coefficient at about 350 degrees C. For high Na content ceramics (> 90mole%) an irreversible decrease to zero of the coupling coefficient is observed instead. The second anomaly in the real part of the dielectric permittivity and the anomalies in the electromechanical behaviour are related with the antiferroelectric-orthorhombic to paraelectric-orthorhombic transition reported for NaNbO3 at 370 degrees C. The first anomaly in the real part of the dielectric permittivity and the change in the temperature coefficient of the conductivity are linked with a change in the conductivity mechanisms of localized charge carriers, allowed by a certain degree of structural disorder caused by the Na+ substitution of the smaller Li+ ion, or viceversa, as reported for similar systems. (C) 1997 Elsevier Science Ltd. All rights reserved.