LiNb_1-xTa_xO3 solid solutions (LNT) are expected to possess enhanced temperature operation ranges as compared to the edge compositions of the system (LiNbO₃ (LN) and LiTaO₃ (LT)) by combining the high Curie temperature of LN and the better thermal stability of LT.  Further, both LN and LT are commonly used in the congruent composition with about 48.4 mol% of Li₂O. On the other hand, it is established that the stoichiometric crystals exhibit improved high-temperature stability, with respect to their electromechanical properties [1]. In the current work, the electrical and acoustic properties of nearly stoichiometric LiNb_xTa_1-xO₃ solid solutions are studied as a function of temperature and compared to those having the congruent composition. Further focus of this work is to determine the application limits of LiNb_xTa_1-xO₃ at high temperatures for both, congruent and stoichiometric compositions.
Congruent bulk crystals for the study were grown by the Czochralski technique. Subsequently, samples with stoichiometric composition were prepared by means of the vapor transport equilibration (VTE) method. The electrical and electromechanical properties are studied by impedance spectroscopy and resonant ultrasound spectroscopy, respectively. For this purpose, Pt-electrodes were deposited on sample surfaces by the pulsed laser deposition (PLD) technique. The analysis of the long-term stability was performed by conductivity measurements at high temperatures. These results were supplemented by UV optical spectroscopy to observe changes in the Li content of the samples.
The conductivity of the LT samples at 500 °C remains nearly constant after initial period of about 2 days. In contrast, X-cut and Z-cut LN samples show a decrease in conductivity of 14.5 % and 19.5 %, respectively. After about 2 weeks at 500 °C, the absorption edges were found to shift by 1.2 nm (LN) and 3.2 nm (LT) towards higher wavelengths. This corresponds to Li evaporation of ~0.3 and 0.8 mol %, respectively. The Li-content of the VTE-treated LN sample is estimated to be 49.3 mol.%. It also demonstrates reduced decrease of conductivity as compared to congruent samples: ~5 % after 2 weeks at 525 °C. Finally, the study of electromechanical properties revealed a rapid increase of loss above 500 °C, which is suggested to arise from the piezoelectric/carrier relaxation mechanism.