Abstract: 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 2mm. The Pt wire (φ～0.2mm) was used to heat and suspend the solution. A mixture of KNbO₃
(20wt%) 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 the 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 looses its polyhedral stability. The thickness of this shape destabilizing solution layer is about 60μm. 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 10μm. 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.

Key words: high-temperature oxide, polyhedral instability, KNbO₃, high-temperature in situ observation device, scanning electron microscope