Properties and mechanism of photochromism in reduced calcium aluminate glasses

Abstract

Calcium aluminate glasses melted in graphite crucibles were found to show photochromism. Upon exposure to ultraviolet radiation, broad absorptions with an apparent peak of around 2 eV and a shoulder around 3.5 eV were induced, and after interruption of the light illumination both bands faded at room temperature. A distinct photobleaching effect was observed in the fading process. Two kinds of electron-paramagnetic-resonance signals, one symmetric and the other asymmetric, were induced near g=2 by exposure to radiation. The growing and fading behaviors of the symmetric and asymmetric components were parallel to those of the 2-eV and 3.5-eV band, respectively. It is strongly suggested that the photochromism originates from an electron trapped at the site of oxygen vacancy surrounded by Ca2+ ions produced in graphite crucibles during melting under strongly reducing conditions.