X-RAY SPECTROSCOPIC INVESTIGATIONS OF FLUIDS IN THE HYDROTHERMAL DIAMOND ANVIL CELL: THE HYDRATION STRUCTURE OF AQUEOUS LA3+ UP TO 300 °C AND 1600 BARS

Abstract

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 300degreesC and pressures LIP to 1600 bars. The radial distribution of atoms around the La3+ ion was measured using the X-ray absorption fine structure (XAFS) technique. La L-3-edge spectra were collected in the fluorescence mode from nitrate solutions in a modified hydrothermal diamond anvil cell using the PNC-CAT X-ray 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 300degreesC, the bond distance between the equatorial-plane O atoms and the La3+ ion increases from 2.59 +/- 0.02 to 2.79 +/- 0.04 Angstrom, whereas the bond distance between La3+ and the O atoms at the ends of the prism decrease to 2.48 +/- 0.03 Angstrom. 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.