A study on the migration of Co(II) and Fe(III) ions in the process of thermal phase separation of an alkali-borosilicate glass containing cobalt and ubiquitous trace iron

Abstract

Absorption spectra of added Co(II) ions and EPR spectra of both Co(II) and ubiquitous Fe(III) ions in an alkali-borosilicate glass of vycor-type composition are studied both before and after thermal phase separation of the glass as well as in a silica glass derived from the phase-separated glass through vycorization. The results show that although almost all Co(II) ions migrate to the separated alkali-borate phase at the time of phase separation, only the interstitial ions in the case of Fe(III) take part in similar migration. The Fe(III) ions in the substitutional silicon sites, however, remain unaffected. It is also revealed that as a consequence of the process of phase separation a small fraction of the interstitial Fe(III) ions gets converted into substitutional ions leading to a net increase in the concentration of substitutional Fe(III) ions in the glass. Analysis of both absorption and EPR spectra of the Co(II) ions suggests that a change of host of the ion due its migration does not bring about any significant change in its coordination structure, which is typically a distorted tetrahedron both in the parent glass as well as in the phase-separated borate glass.