Spectroscopic study of manganese-containing borate and borosilicate glasses: cluster formation and phase separation

The coordination and bonding of Mn2+ ions in glasses can be probed sensitively and selectively by electron paramagnetic resonance (EPR) and photoluminescence spectroscopy. These methods also give information on Mn-Mn ion interactions and cluster formation. Mn2+ ions were found to be tetrahedrally coordinated in borosilicate glasses of high optical basicity, and octahedrally coordinated in low alkaline borosilicate glasses (Duran-type) as well as in binary borate.glasses. Broad emission bands and multicomponent fluorescence decay curves in Duran glasses indicate very strong Mn-Mn ion interactions and the presence of multiple Mn2+ sites, even at low Mn-levels. The EPR spectra show exchange narrowing with increasing Mn content in the Duran series, which is caused by a decrease in the Mn-Mn distances as edge sharing MnO6 octahedra are formed. The network structure of Mn-containing binary and ternary borate glasses is discussed on the basis of their infrared spectra. Addition of MnO to Duran glasses is found to cause the preferential transformation of [BO3](0) to [BO4](-) groups, and to a lesser extent of silicate Q(4) to Q(3) units. An increase of the relative population of homopolar B-O-B or Si-O-Si bonds, with the simultaneous decrease in the number of mixed B-O-Si bonds, is also observed, and this explains the visible phase separation of Duran glasses when MnO is added in excess of 4 mol%.