Growth of column-shaped and plate-like langasite-type piezoelectric single crystals and their physical properties

Column-shaped Ca3TaGa3Si2O14 (CTGS) and plate-like Ca3NbGa3Si2O14 (CNGS) single crystals were grown by micro-pulling-down method using crucibles of original shape. The column-shaped CTGS crystals oriented along a- and c-axes were produced using the seeds of corresponding orientations. The plate-like CNGS crystal was grown along a-axis from the crucible equipped with a plate-shaped die at the bottom. The X-ray diffraction patterns of the grown CTGS and CNGS crystals corresponded to langasite-type structure and the lattice parameters were consistent with those of the crystals grown by Czochralski (Cz) method. The X-ray rocking curve analysis indicated that structural perfection of the grown CTGS and CNGS shaped crystals was comparable to that of the crystals produced by Cz method. The central part of the CTGS crystal was single phase material and had excess of Ca and Si with corresponding deficit of Ta and Ga. However, peripheral parts of the crystal contained secondary phase of calcium tantalate. In the transmission spectrum, two broad absorption peaks around 360 and 490 nm were observed that was associated with the color centers relevant to the oxygen content in the crystal.