Optical absorption of cobalt (II) in binary thallium borate glasses

Abstract

The optical absorption spectra of cobalt (II) in Tl 2OB 2O 3 glasses have been studied and compared with those in binary alkali borate glasses. In thallium borate glasses cobalt (II) may be present in octahedral and/or in tetrahedral symmetry depending upon the composition of the glass. In low thallium borate glasses cobalt (II) is octahedral while the concentration of tetrahedral cobalt (II) increases with increasing Tl 2O content of the glass; the formation of tetrahedral cobalt (II) becomes noticeable when the concentration of Tl 2O reaches above the critical concentration of about 19 mol %. The ligand field parameters: 10Dq and B have been calculated from the absorption spectra of cobalt (II) in different glasses and it has been found that the Racah parameter, B, is more in Tl 2OB 2O 3 glasses than those in Na 2OB 2O 3 or K 2OB 2O 3 glasses of corresponding molar composition. This indicates that the donor capacity of the BO 4 group in thallium borate glasses is lower than that in alkali borate glasses; this is consistent with the NMR results in Tl 2OB 2O 3 glasses containing less than 20 mol % Tl 2O where three BO 4 groups have been found to form with each Tl 2O unit added.