**7. Summary and conclusions**

The coupling of differential interference microscope and the Schlieren technique is an effective method to visualize the mass transport simultaneously with the growing interface for oxide crystal growth. Besides the buoyancy and Marangoni convections induced by temperature gradient, diffusion-induced micro-convection in the boundary layer also exists. The style of mass transfer near the growing interface has great effect on the interfacial kinetics and the interfacial instability. The mass transport governed by pure diffusion can induce inhomogeneous concentration distribution along the growing interface of the single crystal and thus leading to morphological instability, where dendritic or skeletal shape occurs easily. Nevertheless, if the mass transport is governed by both convection and diffusion, the faceted interfacial shape is easy to maintain. The mass transport can be effectively improved by external forces such as magnetic field and vibration, and thus single crystal with high crystallographic perfection can be obtained.
