PNNL

Abstract

X-ray absorption fine-structure (XAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd3+, Sm3+, Dy3+, Yb3+) coprecipitated with calcite in minor concentrations from room-temperature aqueous solutions. Fitting results confirm substitution in the Ca site, but first-shell Nd-O and Sm-O distances are longer than the Ca-O distance in calcite, and longer than consistent with ionic radii sums for 6-fold coordination in the octahedral Ca site. In contrast, first-shell Dy-O and Yb-O distances are shorter than the Ca-O distance and consistent with ionic radii sums for 6-fold coordination. Comparison of Nd-O and Sm-O bond lengths with those in lanthanide sesquioxides and with ionic radii trends across the lanthanide series suggested that Nd3+ and Sm3+ impurities have 7-fold coordination in a modified Ca site in calcite. This would require some disruption of the local structure, with an expected decrease in stability and possibly a different charge compensation mechanism. A possible explanation for the increased coordination for the larger rare earth elements involves bidenate ligation from a CO3 group. Because trivalent actinides such as Am3+ and Cm3+ have ionic radii similar to Nd3+, their incorporation in calcite may result in similar defect structure.