EXAFS study of rare-earth element coordination in calcite

Details EXAFS study of rare-earth element coordination in calcite EXAFS study of rare-earth element coordination in calcite

Sep 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002gecoa..66.2875e&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 66, Issue 16, pp.2875-2885

Computer Science

4

Scientific paper

Extended X-ray absorption fine-structure (EXAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd 3+ , Sm 3+ , Dy 3+ , Yb 3+ ) 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 what is consistent with ionic radii sums for sixfold coordination in the octahedral Ca site. In contrast, first-shell Dy-O and Yb-O distances are shorter than the Ca-O distance and are consistent with ionic radii sums for sixfold coordination. Comparison of Nd-O and Sm-O bond lengths with those in lanthanide sesquioxides and with ionic radii trends across the lanthanide series suggests that Nd 3+ and Sm 3+ have sevenfold 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 between Nd and Sm vs. Yb and Dy. A possible explanation for the increased coordination for the larger rare-earth elements involves bidentate ligation from a CO 3 group. Because trivalent actinides such as Am 3+ and Cm 3+ have ionic radii similar to Nd 3+ , their incorporation in calcite may result in a similar defect structure.

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