Effect of alkali and alkaline-earth cations on the neodymium environment in a rare-earth rich aluminoborosilicate glass

The local structure around neodymium in an aluminoborosilicate glass bearing 3.6 mol% Nd2O3 is studied by optical absorption spectroscopy and EXAFS at the Nd LIII- and K-edges. The influence of the nature of alkalis (M+ = Li+, Na+, K+, Rb+, Cs+) and alkaline-earths (M2+ = Mg2+, Ca2+, Sr2+, Ba2+) on the coordination sphere of Nd3+ ions in the glass is particularly investigated. The Nd3+ sites are well-defined with NdO mean distances of 2.46 ± 0.03 Å, whatever the alkali and alkaline-earth ion type except Li+ and Mg2+, for which glasses exhibit slightly more disordered Nd sites and longer NdO distances (2.49 ± 0.03 Å). Using bond valence considerations, a model is proposed for the Nd site, and consists in 7–8 non-bridging oxygens (NBO), every NBO being charge compensated by 2–3 alkalis and alkaline-earths. The NdO mean distance is adjusted according to the mean field strength of these cations, to avoid overbonding of the NBO’s. A glass series with varying Ca2+/Na+ concentration ratio shows that Nd3+ cations are able to maintain this average coordination site even at high alkaline-earth content.