Evolution of the local environment of cerium and neodymium during simplified SON68 glass alteration

The evolution of the sites occupied by cerium and neodymium (coordination numbers and Ce, Nd-O distances) during alteration of simplified SON68 glass specimens was determined by LIII-edge XAS. Cerium and neodymium are situated in a silicate environment in the glass, surrounded by eight oxygen atoms at an average distance of 2.44 and 2.48 Å, respectively. These two rare earth elements exhibit different leaching behavior, however. The main environment of cerium becomes a silicate (dCe-O = 2.19 Å) with a second oxide or more probably oxyhydroxide site (dCe-O = 2.32 Å). The cerium coordination number increases by 1 to 3 compared with the glass, depending on the leaching conditions. Neodymium is found mainly in a hydroxycarbonate environment (dNd-O = 2.46 Å); the second site is a silicate (dNd-O = 2.54 Å). The neodymium coordination number increases by 1 compared with the glass. When glass containing neodymium is doped with phosphorus, Nd is situated in a phosphate environment; this change is also reflected in the coordination number and Nd-O distance (seven oxygen atoms at 2.42 Å). During glass leaching, neodymium is present at two different sites, phosphate (dNd-O = 2.52 Å) and hydroxycarbonate (dNd-O = 2.40 Å).