Raman studies of reorientation motions of [BH4]- anionsin alkali borohydrides

Raman spectra of the alkali borohydride series MBH4 (M = Li, Na, K, Rb, Cs) have been measured as a function of temperature in the range 300-540 K. For the cubic modification of M = Na, K, Rb and Cs, the analysis of the Raman line widths suggests that the energy barrier of reorientation of the [BH4](-) anions decreases as a function of cation size in the sequence Na: 12.1(5), K: 9.2(4), Rb: 8.8(3) and Cs: 8.2(4) kJ/mol. For the hexagonal high temperature modification of LiBH4, the data suggest two energy barriers of reorientation at similar to5 and similar to60 kJ/mol, respectively. (C) 2003 Elsevier B.V. All rights reserved.