State of the chromium ion in soda silicate glasses under various oxygen pressures

Optical absorption and ESR spectra of chromium in soda silicate glasses were measured to characterize the electronic environment of the chromium ion in these glasses. Glasses produced in very strongly reducing conditions showed a broad optical absorption in the wavenumber range of 10000-25000 cm-1 without any ESR absorption, which suggested the formation of Cr2+ ions. Glasses produced in air or moderately reducing conditions showed ESR signals suggesting that there are three different states of Cr3+ ions, the strongly coupled Cr3+ ion pairs, the weakly coupled Cr3+ ion pairs and the isolated Cr3+ ions in elongated tetragonal sites or sites with lower symmetry. The presence of Cr5+ ions in glasses produced in air was also suggested. It was indicated that the critical partial oxygen pressure of the formation of Cr2+ ions is in the vicinity of Psub(O)2 = 10-8 atm and that Cr2+ ions do not coexist with Cr3+ ions homogeneously in soda silicate glasses. (orig.)