High pressure phases of alkali ternary borohydrides

Insitu synchrotron x-ray diffraction experiments were carried out on MBH$_{4}$ (M = K and Rb) borohydrides at high pressures up to 26 GPa using diamond anvil cells. KBH$_{4}$ undergoes a structural transition at 4 GPa to a tetragonal phase from cubic and then to an orthorhombic phase around 7 GPa which are very similar to NaBH$_{4}$ investigated earlier [1]. However, RbBH$_{4}$ shows, a direct transition from the ambient cubic to an orthorhombic phase at 2.9 GPa, followed by a monoclinic phase at 8 GPa. Complementary high pressure Raman experiments, support the transitions observed in the diffraction experiments. The results will be presented in detail. [1]. Ravhi S. Kumar and Andrew L. Cornelius, Appl.Phys.Lett., 87,261916 (2005) This work is supported in part by the U.S. Department of Energy (DOE) under Award Number DE-FG36 05GO85028. HPCAT is supported by DOE-BES, DOE-NNSA,NSF, and the W.M. Keck Foundation.