Water hydration at high pressure in Fe3+, Ni2+, and Cu2+ solutions probed by EXAFS

We report the results of an EXAFS (extended x-ray absorption fine structure) study of Fe3+, Ni2+, and Cu2+ aqueous solutions under high pressures. EXAFS experiments were performed using synchrotron radiation at room temperature and up to pressures of about 1.2 GPa using a diamond anvil cell. Data analysis has been performed using advanced multiple-scattering simulations, and information about the evolution of the first hydration shell around the metal ions has been obtained. It is shown that Fe3+ and Ni2+ solutions retain a local octahedral structure up to the highest pressure, while Cu2+ solutions show a predominant distorted pyramidal fivefold structure with two oxygen distances. The first-neighbor metal-oxygen distances show a different behavior with pressure in the three solutions, being gradually shortened for Ni2+ solutions or elongated in Fe3+ solutions (by ∼-0.01 and ∼0.02 Å respectively), while in Cu2+ solutions, the difference between average equatorial and axial Cu-O distances is gradually reduced. The present results show that pressure does not act as a simple isotropic perturbation on ionic hydration, which is found to be dependent on the bonding mechanisms and ligand-field anisotropy of transition-metal ions.