Twin defects in thick stoichiometric lithium tantalate crystals prepared by a vapor transport equilibration method

•The twin defects were observed in thick SLT crystals by a VTE method.•The geometry of twins was discussed. Twin composition planes were 011¯2 planes.•The cause of twinning was analyzed and verified by experiment.•The cracks observed at the intersection of twin bands were discussed.

The twins were observed and investigated in vapor transport equilibration (VTE) treated lithium tantalate crystals by burying congruent lithium tantalate crystals (CLT) in a Li-rich polycrystalline powder. Twins and their etched patterns were observed under an optical polarizing microscope, and the geometry of the twins was discussed. Twin composition planes were the 011¯2 planes. The cause of twinning was analyzed and verified by experiment. The results indicate that the emergence of twins is due to sintering stress, which arises from sintered Li-rich polycrystalline powders at high temperature. 3.2 mm thick stoichiometric lithium tantalate (SLT) crystals without twins were obtained by setting corundum crucibles over the top of the crystals to make crystals free from the sintering stress. In addition, cracks were observed at the intersection of twin bands, and the stress caused by the dislocation pile-up was considered to be the reason for the formation of cracks.