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

Abstract

Raman spectra of the alkali borohydride series MBH 4 (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 [BH 4 ] − 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 LiBH 4 , the data suggest two energy barriers of reorientation at ∼5 and ∼60 kJ/mol, respectively.

LiSc(BH4)4: A Novel Salt of Li+ and Discrete Sc(BH4)4− Complex Anions

“…Raman spectra of the alkali borohydrides have shown significant broadening of the B-H bending modes in the disordered phase. 39,40 Finally, we have performed DFT calculations on a model complex comprising two Sc(BH 4 ) 4 units separated by four Li atoms to simulate a truncated tetragonal unit cell. Initially, the Li atoms were placed on the (0 0 0.6) between the two Sc complexes.…”

Structural and spectroscopic studies on the alkali borohydrides MBH4 (M = Na, K, Rb, Cs)

“…Hydride samples MBH 4 (M = K, Rb, Cs) were synthesized as indicated in a previous study [18]. Deuteride samples MBD 4 (M = K, Rb, Cs) were prepared in the same way.…”

“…The Raman set-up is the same as that described previously [3,18]. It consists of an Argon ion laser (488 nm excitation wavelength) and a Kaiser Optical Holospec monochromator equipped with a liquid nitrogen cooled CCD camera.…”

“…revealing internal modes of vibration only, by taking into account the 10 B to 11 B and 1 H to 2 D isotopic dilution and the occurrence of strong Fermi resonance [15][16][17]. Very recently, the reorientation motions of [BH 4 ] − anions in some alkali borohydrides MBH 4 have been studied by Raman spectroscopy [18]. Given the lack of reliable structural data on the HT-structures of M = Rb, Cs and the LT-structures of M = K, Rb, Cs, in particular with respect to hydrogen positions, additional structural and spectroscopic studies were desirable.…”

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

“…The Raman set-up used was the same as that described previously [18,19]. It consisted of an Argon ion laser (488 nm excitation wavelength) and a Kaiser Optical Holospec monochromator equipped with a liquid nitrogen cooled CCD camera.…”