Polyhedral instability of high-temperature oxide crystal

Model experiments were designed in order to obtain more reliable data on the diversity of some crystal forms and polyhedral instability - skeletal and dendritic growth in high temperature solution growth. These experiments were performed by high temperature in situ observation technique. Most of our investigations on high temperature solution growth were performed in a loop-shaped Pt wire heater, having a diameter about 2 mm. The Pt wire (Φ0.2 mm) was used to heat and suspend the solution. A mixture of KNbO3 (20 wt%) and Li2B4O7 was chosen for growth. Rapid growth, and hence diffusion mechanism limitations can result in various imperfections: such as the formation of cavities in facets, skeletal and dendritic patterns. Quenching experiments were designed to distinguish some KNbO3 crystal forms, and the morphology of KNbO3 crystals grown in Li2B4O7 solution was studied with a scanning electron microscope. In all cases, when the crystal is nucleated near air/solution interface, it loses its polyhedral stability. The thickness of this shape destabilizing solution layer is about 60 μm. The morphologies of crystals observed by a scanning electron microscope vary in the layer from polyhedrons to dendrites. Anisotropic aspects of skeletal and dendritic structures reflect the cubic nature of KNbO3 as well as the fact that instabilities amplify and propagate along the four (110) crystal edges. The side branches in the four (110) directions provide convincing evidence for the growth shape anisotropy. The value of the critical size for loss of polyhedral stability is about 10 μm. In contrast, the stable shape of the faceted crystal is generally retained with adequate reproducibility for all crystals grown in the solution beneath the shape-destabilizing layer. The polyhedral crystal growth processes were in-situ observed and recorded by HITISOT. The thermoconcentrational driven convection (i.e. microconvection) around the solid-liquid interface of the polyhedral crystal was also visualized.