Journal of Alloys and Compounds 363 (2004) 126–129
Raman studies of reorientation motions of [BH4 ]− anionsin alkali borohydrides H. Hagemann a,∗ , S. Gomes b , G. Renaudin b , K. Yvon b a b
Département de Chimie Physique, Université de Genève, 30, quai E. Ansermet, CH 1211 Geneva 4, Switzerland Laboratoire de Cristallographie, Université de Genève, 24, quai E. Ansermet, CH 1211 Geneva 4, Switzerland Received 22 April 2003; accepted 29 April 2003
Abstract 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 ∼5 and ∼60 kJ/mol, respectively. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Metal hydrides; Optical spectroscopy
1. Introduction Interest in alkali borohydrides has recently increased due to their potential as hydrogen storage materials [1] and energy carriers for fuel cells [2]. In a previous study, we reported structure data and Raman spectra for LiBH4 [3,4]. Its orthorhombic room-temperature structure was found to contain tetrahedral [BH4 ]− ions that reorient themselves as the compound transforms to a hexagonal high-temperature modification, while the Raman line width increases sharply at the phase transition (376–384 K, see Fig. 3 in Ref. [4]). In order to relate more quantitatively the high-temperature Raman bandwidth to the energy barrier of reorientation (or libration) motion of the [BH4 ]− ion, we extended our previous measurements to temperatures up to 540 K and carried out new measurements on other alkali borohydrides.
2. Experimental The Raman set-up is essentially the same as that described previously [4]. It consists of an argon ion laser (488 ∗ Corresponding author. Tel.: +41-22-702-6539; fax: +41-22-702-6103. E-mail address:
[email protected] (H. Hagemann).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00468-7
nm excitation wavelength) and a Kaiser Optical Holospec monochromator equipped with a liquid nitrogen cooled CCD camera. The spectral resolution is ca. 3–4 cm−1 . The high temperature cell was redesigned and consisted of a cylindrical brass body in which a small sample container was introduced together with a thermocouple for temperature measurements. Heating was achieved by a copper tube that was connected to a thermostatic bath. Some low temperature measurements were carried out using a liquid nitrogen cold finger Dewar. The samples were enclosed in sealed glass capillaries with a diameter of ∅ = 1.0 mm. Polycrystalline samples of LiBH4 , NaBH4 and KBH4 were obtained from Alfa Aesar (purity 95 wt.%, 98 wt.% and 98 wt.%, respectively). RbBH4 and CsBH4 were prepared in the following way. RbOH.xH2 O, respectively CsOH.H2 O, were dissolved in a minimum amount of methanol at room temperature. The commercial hydrated alkaline hydroxides contain a small amount of carbonate impurities (
