GROWTH OF DENDRITIC AND NEEDLE TUNGSTEN-OXIDE CRYSTALS STUDIED BY HIGH-RESOLUTION ELECTRON-MICROSCOPY

Needle crystals of beta-WO2.90 and gamma-WO2.72 are produced by heatin g alpha-WO3 powder in a vacuum apparatus under various air pressures. Whether beta- or gamma-tungsten oxide is produced depends on the speci men temperature rather than on the air pressure. The initial stages of the growing process of dendritic or needle crystals have been observe d using a high-resolution electron microscope. It is found that bumps which are formed on the surface of a mother needle crystal are necessa ry far the growth of dendrite crystals. When no bumps are formed on th e surface, single needle crystals grow. The needle axis is the b-axis on both beta- and gamma-tungsten oxides which belong to the same monoc linic system. In this study no amorphous drops have been observed on t op of any needle crystal.