Optical absorption of nickel (II) in Na2O-NaCl-B2O3 and Na2O-NaBr-B2O3 glasses

The optical absorption spectra of nickel(II) have been studied in Na₂O-NaX-B₂O₃ glasses X = Cl or Br); the absorption bands have been interpreted in terms of Ligand Field Theory. In halide-free B₂O₃-rich glasses nickel(II) is octahedral, and halide substitution is accompanied by the formation of a tetrahedral nickel(II)-oxide-halide complex. In alkali-rich glasses nickel(II) is square planar and/or tetrahedral and no halide substitution can be observed. It is suggested that the halide ions are unable to substitute for oxygen in the predominantly covalent nickel(II)-oxygen bonds of the complex formed in basic glasses.