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Review date: July 24, 2003
PNNL-SA-27883

A Transition in the Ni²⁺ Complex Structure from Six- to Four-Coordinate upon Formation of Ion Pair Species in Supercritical Water: An XAFS, NIR and MD Study

M. M. Hoffmann, J. G. Darab, B. J. Palmer, and J. L. Fulton, J. Phys. Chem. A, 103, 8471-8482, (1999)

Abstract: The coordination structure about Ni²⁺ in water at temperatures up to 525°C was measured using x-ray absorption fine structure (XAFS). The solutions containing 0.2 m NiBr2 and 0.2 m NiBr2/0.8 m NaBr were explored at pressures up to 720 bar. For certain systems, both Ni and Br XAFS were acquired and a global model was used to fit the two independent sets of XAFS data. These two independent measurements gave excellent agreement on the coordination structure of the Ni²⁺/Br^- contact ion pairing. The result is a complete picture of the coordination structure of the contact ion pairing including the coordination numbers, distances for the water-ion and ion-ion associations and also a high-quality measurement of the binding strength and amount of disorder (Debye-Waller factor and the anharmonicity) of the Ni²⁺/Br^- association.

The XAFS measurements strongly indicate a transitional change in the coordination of Ni2+ from the octahedral Ni²⁺(H2O)6 species at room temperature to four-coordinate structures at supercritical conditions (e.g. T > 375°C). Specifically the equilibrium structure at 425°C is found to be Ni2+(Br-)3.3(H2O)1.0 for the aqueous solution of 0.2 m NiBr2 with 0.8 m NaBr. At 325°C, the octahedral species still exists but it is in equilibrium with new species of lower-coordination. Above 425°C, at moderate pressures up to 700 bar, the stable structures are a family of four-coordinated species (NiBr(H2O)3ïBr, NiBr2(H2O)2, NiBr3(H2O)ïNa) where the degree of Br^- adduction and replacement of H2O in the inner shell depends upon the overall Br^- concentration. The most likely symmetry of these species is a distorted tetrahedron. Measurements of the Ni pre-edge 1s-to-3d and to 1s-to-4p transitions using X-ray absorption confirm that a symmetry change occurs at high temperature and the results are consistent with the XAFS and MD conclusion of a distorted tetrahedral structure. This observation is further confirmed by near-IR spectra of the same system.

Molecular dynamics simulations under identical conditions were used to verify the experimental findings. Although we found qualitative agreement between the experimental and simulated first-shell coordination structure, it is clear that refinements of the intermolecular potentials are required to quantitatively capture the true high-temperature structure.