Potential behaviour of (Fe, Y) sites due to self-irradiation, as resolved through XAFS of natural metamict gadolinite

Vitrified high level nuclear waste matrices with Fe as network modifier often get associated with lanthanides produced due to fission reactions within irradiated nuclear fuel. During its service period, Fe-lanthanide associations undergo disruptions due to ongoing self-irradiation as well as recovery due to generated heat. Depending on the heat, short-range-order (SRO)/medium-range-order (MRO) of the glass-network, localized crystallization within the amorphous matrix cannot be ruled out. Under this scheme, we investigated atomic-scale structure and chemistry of (Fe, Y) cations (Y as representative of lanthanides) within natural glass, through (Fe, Y) K-edge XAFS measurement of "metamict" gadolinite mineral (FeBe2Y2Si2O10). The glass is observed to display structural and chemical stability under natural environment over millions of years. We have logistically attributed this superior stability to the advantageous co-existence of (Fe, Y) atoms [vis-`a-vis in-dependent Fe and Y atoms]. The importance of (Fe, Y)-site heterogeneity also emerges from our results. Amorphous structure is found to be better stabilized around Fe than Y. Disparate crystallization rates around (Fe, Y) sites can be utilized for separation and recovery of rare earths (e.g. Y) from waste. These conclusions may be generalized to comprehend the control of generic amorphous structure with co-doping of transition metal and rare earth elements.