Properties and mechanism of photochromism in reduced calcium aluminate glasses

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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.