Abstract
Directionally solidified ingots of the eutectic composition and its neighboring compositions in the system (Bi2O3)0.85(Nb2O5)0.15-6Bi2O3⋅SiO2 are expected to show chatoyancy like natural cat's eye stone, since they have structures in which fibrous δ-(Bi2O3)0.85(Nb2O5)0.15 crystals of higher refractive index are aligned parallel in a γ-6Bi2O3⋅SiO2 crystal matrix of lower refractive index. The chatoyancy of the ingots was investigated in terms of their microstructure. An eutectic ingot solidified with a single crucible (Ref. 1) showed the chatoyancy only at a low solidification rate, regardless of the composition of the ingot (Figs. 2 and 3). An eutectic ingot solidified with double crucibles (Ref. 2) showed a chatoyancy even at a fairly high solidification rate, and especially sharp and clear one at a low solidification rdte (Fig. 4). Clearness of the chatoyancy of the ingots could be interpreted in terms of average diameter of cells in which the fibrous crystals are aligned almost completely parallel (Fig. 8). The fraction of parallel-aligned fibrous crystal increases with increasing cell diameter, resulting in sharp and clear chatoyancy (Fig. 9).
