Optical and ESR investigation of borate glasses containing single and mixed transition metal oxides

Abstract

The optical and electron spin resonance (ESR) spectra of barium borate glasses, containing the oxides of V, Fe and Cu separately and in mixed proportions, have been studied. The optical spectra of the single transition metal (TM) oxide glasses showed the usual features, while those for the mixed glasses showed single bands without showing individual features of the single TM oxide glasses. However, the linear plots of optical density against composition revealed the presence of two valence states for each TM element, and this was confirmed by ESR results as well. The ESR spectra of the mixed glasses showed a complicated interaction pattern for two different TM ions, in comparison with those of the glasses containing a single TM ion. For the Fe-V glasses, the progressively vanishing hyperfine structure of the VO²⁺ complex with increasing addition of iron oxide is discussed in terms of nuclear spin relaxation, cross-relaxation between two spin systems and spin diffusion within the vanadium spin system. The covalency of the VO²⁺ complex and the number of distorted Fe³⁺ ions were found to decrease with increasing addition of Fe₂O₃ replacing V₂O₅. Similar features were noted for the Cu-V glasses; the spectra of Cu-Fe glass also showed a strong interaction between two different TM ions. It has been suggested that all the possible four valence states (for a given mixed glass) from two different TM elements are present, and that pairing of two different TM ions from two dissimilar TM elements occurs, facilitating the formation of “associates” (e.g. V⁴⁺-O-Fe³⁺).