Growth of dendritic and needle tungsten oxide crystals studied by high-resolution electron microscopy

Needle crystals of β-WO 2.90 and λ-WO 2.72 are produced by heating α-WO 3 powder in a vacuum apparatus under various air pressures. Whether β-or λ-tungsten oxide is produced depends on the specimen temperature rather than on the air pressure. The initial stages of the growing process of dendritic or needle crystals have been observed using a high-resolution electron microscope. It is found that bumps which are formed on the surface of a mother needle crystal are necessary for the growth of dendrite crystals. When no bumps are formed on the surface, single needle crystals grow. The needle axis is the b-axis on both β- and ω-tungsten oxides which belong to the same monoclinic system. In this study no amorphous drops have been observed on top of any needle crystal.