New non-linear optical crystals were grown by using the Czochralski method under air in a Pt crucible. The growth mechanism involved typical 2-dimensional nucleation layer grow. Twins and inclusions in the crystals were studied by using optical microscopy. Thus, a <¯201>-oriented crystal exhibited 2 symmetrical growth edges, and on them were hexagonal facets which were determined to be (010) faces by using X-ray techniques. The twins always nucleated at the edge facets which were anchored to the 3-phase boundary at the solid/liquid interface during crystal growth. The formation of twins was deemed to be in good agreement with the Hurle model. When the melt supercooling exceeded a critical value, the twins energetically favoured nucleation at the 3-phase boundary because the nucleation free energy at the 3-phase boundary was lower than that far away from the 3-phase boundary. Moreover, the step absorption energy was large at the 3-phase boundary, and this made it easy to absorb impurities, etc, which were favorable to the incidence of twins. When the steps absorbed impurities, they changed their growth direction and led to twinning under suitable conditions. In addition, the formation of twins was also related to the seed direction. When an <010>-oriented seed was used, no edge facets emerged and there was no twinning throughout the entire growth process. The morphology of the solid/liquid interface could affect the formation of twins. In order that a twinned nucleus be formed at the 3-phase boundary, the meniscus contact angle had to have the specific value for which it was in equilibrium with the edge facet. This critical growth angle depended upon the seed direction. However, it changed greatly if the temperature

fluctuated. The twinned nucleus also formed when the temperature was lowered quickly, or if the pulling-rate was fast during crystal growth.

Studies on the Growth and Defects of GdCa4O(BO3)3 Crystals. S.Zhang, J.Zhang, Z.Cheng, G.Zhou, J.Han, H.Chen: Journal of Crystal Growth, 1999, 203[1-2], 168-72