Point Defects and Chromium(IV) Formation Mechanism in Gallia‐ and Alumina‐Based Oxide Glasses

Point defects were found in as‐quenched GeO 2 , 65CaO35Al 2 O 3 , and 65SrO35Ga 2 O 3 glasses on the basis of electron paramagnetic resonance (EPR) measurements. These defects were identified as Ge É centers in GeO 2 glass and O ‐ 2 , O ‐ 3 , and M‐OHC (oxygen hole center) (where M = Al, Ga) in 65CaO35Al 2 O 3 and 65SrO35Ga 2 O 3 glasses. The formation of Ge É centers in as‐quenched GeO 2 glass was due to the thermodynamic stability of GeO at the melting temperature. The latter oxygen‐excess defects are supposed to be formed by excess oxygen ions derived from the modifiers in the aluminate and gallate glasses during the formation of these glasses. To investigate some of the properties of the oxygen‐excess defects in the calcium aluminate and strontium gallate glasses, chromium ions were doped in these glasses as a probe and the relationship between the valency state of the chromium ion and the defects was determined. We conclude that the peroxy bonding (‐O‐O‐) oxidizes the Cr 3+ species to Cr 4+ . Similar defects have been identified in host compounds that are used for Cr 4+ tunable lasers. These results reveal that the point defects are necessary to stabilize the Cr 4+ ions in glasses and crystals.