The first direct measurements are reported for the structure of the hydrated La3+ ion in an aqueous
solution (containing 0.007 m La) over a range of temperatures from 25 to 300 8C and pressures up to
1600 bars. The radial distribution of atoms around the La3+ ion was measured using the X-ray absorption
fine structure (XAFS) technique. La L3-edge spectra were collected in the fluorescence
mode from nitrate solutions in a modified hydrothermal diamond anvil cell using the PNC-CAT Xray
microprobe at the Advanced Photon Source, Argonne National Laboratory. Analysis of the XAFS
spectra collected at all temperatures indicates that each La3+ ion has a hydration number of nine and
that the solvating waters surround the ion in a tricapped trigonal prismatic arrangement. As temperature
is increased from 25 to 300 8C, the bond distance between the equatorial-plane O atoms and the
La3+ ion increases from 2.59 ± 0.02 to 2.79 ± 0.04 Å, whereas the bond distance between La3+ and the
O atoms at the ends of the prism decrease to 2.48 ± 0.03 Å. This study also demonstrates the unique
capability of the modified hydrothermal diamond anvil cell for in situ low energy X-ray spectroscopic
analysis of elements in dilute aqueous solutions at elevated temperatures and pressures.