Structure of Aqueous Zinc Bromide Solutions by XAFS and Raman Spectroscopy to 500°C and 900 MPa in a Hydrothermal Diamond Anvil Cell

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William Bassett¹, Kenji Mibe^1,2, I-Ming Chou³, Robert A. Mayanovic⁴, and Alan J. Anderson^5
1Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY
²Earthquake Research Institute, University of Tokyo, Japan
³U.S. Geological Survey, Reston, VA
⁴Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO
⁵Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, Canada

Abstract:
We obtained XAFS spectra on a 1m ZnBr₂/6m NaBr aqueous solution up to 500 MPa and 500 ºC using a hydrothermal diamond anvil cell (HDAC) on beamline B-2 at CHESS.  Both Zn and Br K-absorption edges were utilized providing exceptionally good data for determining P-T dependence of the structure of the zinc bromide complex as well as the envelope of solvated water.  We found that the zinc bromide complex remained as a negatively charged (2-) tetrahedral coordination of bromide ions around each zinc ion throughout the range extending from ambient to the highest P-T conditions.  The number of solvated water molecules steadily decreased with increasing temperature, and the zinc-bromide distance steadily decreased as solvated water was lost.  Because an earlier Raman study suggested the existence of complexes with other structures at elevated temperatures and pressures, we decided to make our own Raman study on the same solution as used for the XAFS study.  The results of our Raman study confirmed that the tetrahedral structure of the zinc bromide complex persisted over the same P-T range as in our XAFS study.

abstract (pdf)