Photochromism and Its Origin of Heat-Treated Na₂O-BaO-Al₂O₃-TiO₂ Glasses

Abstract

Glasses with various compositions in the systems (nNaO_0.5+mBaO)-AlO_1.5-TiO₂, where n/m is 1/0, 2/1, 1/1, 1/2 or 0/1 (Fig. 1), were heated to various temperatures from 650°C to 950°C at a rate of 5°C/min in a SiC furnace, and then cooled outside the furnace. All the glasses examined darkened on 60 min-irradiation of 365nm light of 6 W ultraviolet lamp, after heated to certain temperatures depending on their compositons (Table 1). The darkening of one of the heat-treated specimens, A 3 in Table 1, was enhanced by the addition of a small amount of FeO_1.5 or ZnO, whereas it was suppressed by the addition of CeO₂ (Table 2, Figs. 2 and 3). On the surfaces of all the heat-treated specimens showing photochromism, an unknown crystalline phase named X in Table 3 was detected. All the specimens containing the X phase, however, did not always show the photochromism (Table 3). Consequently, the cause of the photochromism of the heat-treated glasses described above was attributed to defects, probably induced by impurities, in the X phase precipitated on the surfaces of the glasses. The X phase was observed on the surfaces of both the BaO-free glasses and Na₂O-free glasses (Table 3). The fluorescence X-ray analysis of powders of a NaO_0.5-BaO-AlO_1.5-TiO₂ glass (A 9 in Table 1), which had been heated to 700°C (Fig. 5 (A)) and immersed in a 2 N NaOH solution at 95°C for 100h (Fig. 5 (B)) showed that the X phase is deficient in Al and rich in Ti (Table 4). The X phase was, therefore, considered to be mostly composed of TiO₂. The structure of the X phase was identified as one of the cubic structures in Pa 3, P 4₁32, P 4₂32, Pn 3m and P_*3_* diffraction groups with a lattice constant of a=11.54Å by indexing the powder diffraction peaks of this phase (Fig. 4).