A Transition in the Ni2+Complex Structure from Six- to Four-Coordinate upon Formation of Ion Pair Species in Supercritical Water: An X-ray Absorption Fine Structure, Near-Infrared, and Molecular Dynamics Study

Abstract

The coordination structure about Ni2+ in water at temperatures up to 525 °C was measured by the X-ray absorption fine structure (XAFS) technique. 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 data 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 the 4-coordinate struct...