Effect of cation field strength on Co2+ speciation in alkali-borate glasses

The speciation of Co2+ in alkali (Li, Na, K) borate glasses is investigated by combining optical and X-ray absorption spectroscopy. The glass color is directly correlated to the Co2+ local structure. It changes from pink for 10 mol% alkali borate glass containing 6-fold coordinated [6]Co2+ clusters to blue for 30 mol% alkali borate glass containing 4-fold coordinated [4]Co2+. The colorimetric parameters L*, a* and b* vary non-linearly as a function of the alkali content, and indicate of the presence of a third Co2+ species. The linear combination analysis of EXAFS and optical absorption data shows the presence of [5]Co2+ in 23 mol% K2O borate glass, as revealed by specific electronic transitions and an intermediate Co-O distance. Alkali field strength governs Co-coordination in high-alkali borate glasses: the 30% K2O-glass shows an almost complete conversion of Co2+ to a tetrahedral environment, while only partial conversion occurs in 30% Li2O-glass. Structural models of the relations between Co2+ sites and the borate glass network are proposed in relation with super-structure units.