POINT-DEFECTS AND CHROMIUM(IV) FORMATION MECHANISM IN GALLIA-BASED AND ALUMINA-BASED OXIDE GLASSES

Point defects were found in as-quenched GeO2, 65CaO . 35Al(2)O(3), and 65SrO . 35Ga(2)O(3) glasses on the basis of electron paramagnetic res onance (EPR) measurements. These defects were identified as Ge E' cent ers in GeO2 glass and O-2(-), O-3(-) and M-OHC (oxygen hole center) (w here M = Al, Ga) in 65CaO . 35Al(2)O(3) and 65SrO.35Ga(2)O(3) glasses. The formation of Ge E' centers in as-quenched GeO2 glass was due to t he thermodynamic stability of GeO at the melting temperature. The latt er oxygen-excess defects are supposed to be formed by excess oxygen io ns derived from the modifiers in the aluminate and gallate glasses dur ing the formation of these glasses. To investigate some of the propert ies of the oxygen-excess defects in the calcium aluminate and strontiu m gallate glasses, chromium ions were doped in these glasses as a prob e and the relationship between the valency state of the chromium ion a nd the defects was determined. We conclude that the peroxy bonding (-O -O-) oxidizes the Cr3+ species to Cr4+. Similar defects have been iden tified in host compounds that are used for Cr4+ tunable lasers, These results reveal that the point defects are necessary to stabilize the C r4+ ions in glasses and crystals.