DYNAMICS AND TRENDS OF ALKALI METAL DODECAHYDRO-CLOSO-DODECABORATES
Nina Verdal1, Terrence J. Udovic1, Vitalie Stavila2, John J. Rush1,3
2Sandia National Laboratories, Livermore, Livermore, CA 94551-0969
3Department of Materials Science
and Engineering, University of Maryland,
College Park, MD 20742-2115
The icosahedron dodecahydro-closo-dodecaborate anion (B12H122-) is a known intermediate in alkali and alkaline-earth borohydride hydrogen desorption reactions used in hydrogen storage. The chemical stability of this anion inhibits the reversibility of the desorption reaction. Characterization of this material and its intermolecular behavior has been the focus of neutron spectroscopic studies. It has been found that the neutron vibrational spectra of these materials are closely related to the crystallographic space group, which is determined by the alkali or alkaline-earth counter-ion. The size of the counter-ion determines the activation energy for icosahedron reorientation and seemingly determines the presence and temperature of phase transitions. Reorientational dynamics of Cs2B12H12 show that in spite of the near-spherical shape of the icosahedron and the cubic crystal environment, the anion reorients about a single axis at 430 K, and two axes at higher temperatures. Even at temperatures as high as 625 K, the icosahedron does not undergo a multi-axis tumbling motion. Our studies indicate that the anion is sensitive to local defects and probably undergoes cooperative motions with its neighbors.