In last decades, lithium niobate (LiNbO₃, LN) and lithium tantalate (LiTaO₃, LT) have attracted scientific and industrial interest due to their outstanding electro-optical, piezoelectric, and acoustic properties. These materials share the same crystal structure (space group R3c) with only minor differences in lattice and atomic position parameters [1]. Using electric field polarization techniques, domain inversion in lithium niobate and lithium tantalate is well known and has become a well-established method for producing high-quality periodically poled structures in LiNbO₃ and LiTaO₃ [2,3]. But studies on basic material properties of LiNb_xTa_1-xO₃ in the domain inversion process are lacking. Since the samples in polydomain state show no defined overall piezoelectric properties, in particular no useful piezoelectrically excited resonance spectra, only single domain samples have to be used for sensing purposes. This ongoing work focuses on electric field poling of LiNb_xTa_1-xO₃ solid solutions at elevated temperatures and to investigate the resulting piezoelectric and acoustic properties.

To determine the optimal parameters and experimental conditions of the polarization process, the first experiments were performed on LT samples, since the Curie temperature is much lower (about 630 °C), comparing to LN (1200 °C). Samples with dimensions of 10x10x0.5 mm³ were coated with screen-printed platinum electrodes (thickness ~ 4 µm). Subsequently, the samples were placed in an aluminum oxide sample holder and heated to the temperatures in the range of 500°C - 650°C. A Keithley electrometer was used to apply voltage to the sample and for electric current measurements. For resonance measurements, a high-speed network analyzer was used.

Figures 1 and 2 show exemplarily the current of a LiTaO₃ sample as a function of time. The measurements were performed at 600 °C. As seen from the figure 1, at 10 V no significant change of current is visible, after applying 15 V strong increase in current is observed (Fig. 2). Subsequently, the current drops with different slopes. The initial peak is assigned to the poling process. The integration of the curve after subtraction of the background current in this region allows to determine the electrical charge, which is equal to Q = 1.01x10^-4 C. According to Q = 2xPsxA with the spontaneous polarization PS(LT) = 0.50 C/m² [4] and A = 1 cm², the calculated value Q = 10^-4 C is consistent with the measured charge during the polarization process. Further experiments on LiNbO₃ and LiNb_xTa_1-xO₃ solid solutions as well as the determination of optimal parameters for polarization of LiTaO₃ will be performed.

References
[1] Weis, R. S., & Gaylord, T. K. (1985). Applied Physics A, 37(4), 191-203.
[2] Missey, M. J., Russell, S., Dominic, V., Batchko, R. G., & Schepler, K. L. (2000), Optics Express, 6(10), 186-195.
[3] Paul T. Brown, Graeme W. Ross, Robert W. Eason, Armen R. Pogosyan, Optics Communications 163 (1999) 310–316.
[4] S. H. Wemple, M. DiDomenico, Jr., and l. Camlibel, Applied Physics Letters, Volume 12, Issue 6, p.209-211