Lithium niobate (LiNbO₃, LN) and lithium tantalate (LiTaO₃, LT) belong to the most widely used ferroelectric materials, with applications that range from novel electronic and micro-mechanical devices to nonlinear optics. Lithium niobate-tantalate (LiTa_xNb_1−xO₃, LNT) solid solution single crystals are expected to allow for implementation of unique material’s properties such as the tailored birefringence combined with large temperature stability. To explore the behaviour of solid solutions, single crystals of high chemical homogeneity are required.
The phase diagram of the pseudo-binary system LN—LT was studied by means of differential thermal analysis using single crystalline specimens of different compositions covering the entire composition range. Here it was assumed that the congruently melting compositions of both, LN and LT form a binary system so that all solid compositions also lay exactly on the straight line connecting LN and LT in the ternary system Li₂O—Nb₂O₅—Ta₂O₅.
We have grown LNT single crystals using the Czochralski technique with induction heating. For LN-rich compositions active heating elements (crucible, afterheater) from platinum where used, whereas owing to significantly higher melting temperatures for LT-rich compositions iridium parts were employed. Distribution of Nb and Ta in the crystals was investigated by X-ray fluorescence analysis with high spatial resolution. It was found that as a result of the small crystallized fraction of the melt, the macro distribution along the pulling direction was fairly homogeneous whereas, in contrast, showed pronounced microscopic inhomogeneities resulting from unsteady and cellular growth.
Transport of heat and mass in melt and crystal was modelled by means of numerical analysis using a finite element method. Focus was put on the thermal gradient near the growth interface which is of greatest importance for interface stability. These experiments were used to was used to evaluate various variations of the thermal setups and one was selected for realization. Crystals grown under improved conditions showed a higher homogeneity and less inclusions compared to those grown in default conditions.
Impact of local inhomogeneities on electromechanical losses and nonlinear optical effects was tested and demonstrated the necessity of homogeneous samples for practical application.