Behavior of Cerium Ions in Glasses Exposed to X-rays

Abstract

Silicate glasses exposed to high-energy radiation develop three visible absorption bands similar to the F- and V-band in alkali halide crystals (R. Yokota, Phys. Rev., 91, 1013 (1953), 95, 1145 (1954), 101, 523 (1956); A. Kats and J. M. Stevels, Philip Res. Report, 11, 103, 115 (1956)). The function of cerium in suppressing formation of these bands were studied spectrophotometrically in glasses of composition 20 R₂O, 10 BaO, 70 SiO₂, and 0.03 to 0.3mol% CeO₂ (R₂O: Li₂O, Na₂O, K₂O).
Glass specimens of about 0.1mm in thickness were irradiated with X-rays of 42kV, 10mA, and changes of their absorption spectra in the range 230 to 800mμ were measured with the Beckman photoelectric spectrophotometer. As the absorption changes were found to be caused not only by color centers but also by other sources such as the valency change of cerium ion, the change due to color centers were separated from the others by heating the irradiated specimen at 150°C for 30 min.; the decrease in absorption intensity by the heating was taken as the absolute change of the absorption due to formation of color centers.
The experimental results showed that trivalent cerium ions suppress development of the three absorption bands equally; i.e., trivalent cerium ions are effective in preventing formation of either of those structural imperfections associated with trapped electrons or positive holes. As to the valency changes of cerium ion itself, a small part of trivalent cerium ions was found to loose their electron upon irradiation by the reaction
Ce³⁺+hν→[Ce³⁺ with positive hole]+e^-.