X-ray absorption spectroscopy and imaging of heterogeneous hydrothermal mixtures using a diamond microreactor cell

Hydrothermal synthesis is an important route to novel materials. Hydrotherm al chemistry is also an important aspect of geochemistry and a variety of w aste remediation technologies. There is a significant lack of information a bout the speciation of inorganic compounds under hydrothermal conditions. F or these reasons we describe a high-temperature, high-pressure cell that al lows one to acquire both x-ray absorption fine structure (XAFS) spectra and x-ray transmission and absorption images of heterogeneous hydrothermal mix tures. We demonstrate the utility of the method by measuring the Cu(I) spec iation in a solution containing both solid and dissolved Cu phases at tempe ratures up to 325 degreesC. X-ray imaging of the various hydrothermal phase s allows micro-XAFS to be collected from different phases within the hetero geneous mixture. The complete structural characterization of a soluble bich loro-cuprous species was determined. In situ XAFS measurements were used to define the oxidation state and the first-shell coordination structure. The Cu-Cl distance was determined to be 2.12 Angstrom for the CuCl2- species a nd the complete loss of tightly bound waters of hydration in the first shel l was observed. The microreactor cell described here can be used to test th ermodynamic models of solubility and redox chemistry of a variety of differ ent hydrothermal mixtures. (C) 2001 American Institute of Physics.