Structural and Spectroscopic Studies on the Alkali Borohydrides MBH₄ (M: Na, K, Rb, Cs).

Abstract

Alkali borohydrides MBH 4 and their deuterides have been investigated by X-ray and neutron powder diffraction (M = K, Rb, Cs) and by infrared and Raman spectroscopy (M = Na, K, Rb, Cs). At room temperature the compounds crystallize with a cubic high temperature (HT) structure having Fm3m symmetry in which the [BH 4 ] − complexes are disordered. At low temperature (LT) the potassium compound transforms into a tetragonal low temperature structure having P4 2 /n mc symmetry in which the [BH 4 ] − complexes are ordered such as in the isotypic sodium congener. The B-H distances within the complex as measured on the deuteride at 1.5 K are 1.205(3) Å. Indications for a partial ordering in the rubidium and cesium compounds exist but are not sufficient for a full structural characterization. Infrared and Raman spectra at room temperature are fully assigned for both hydrides and deuterides, including the overtones and combination bands, the Fermi resonance type interactions and the 10 B to 11 B splitting due to the presence of natural boron in the samples.

Quantitative Spectra–Structure Relations for Borohydrides

“…Once the bond length dependence of the vibrational frequencies is considered, the F matrix cannot be kept constant anymore and its dependence on the B−H distances should be taken into account. 17,18 It has been shown previously 21,22 that it is possible to apply Badger's rule 17,18 to correlate the B−D stretching frequencies with the experimental B−D bond lengths obtained from neutron diffraction for the cubic alkali borohydrides.…”

“…Using the experimental vibrational frequencies and bond lengths for the cubic alkali borohydrides, 21,22 the elements of the stretch−stretch block of the F matrix were adjusted to reproduce the experimental values of the stretching frequencies.…”

“…The values are expressed in internal units (hartrees−bohrs−radians). The experimental B-D distance (Å)21,22 and the experimental 21−23 and GF calculated frequencies (cm −1 ) are also shown.…”

Dehydrogenation and rehydrogenation of a 0.62LiBH4-0.38NaBH4 mixture with nano-sized Ni

“…The measured wavenumbers of the total symmetric stretching mode (ν1) for NaBH4 in the asmilled LiNa-Ni and LiNa mixtures are found to be 2323 cm -1 and 2324 cm -1 , respectively. These wavenumbers are 7-8 cm -1 lower than the 2331 cm -1 measured for as-milled pure NaBH4, suggesting again the volume expansion via an increase in B-H bond length according to Badger's rule[73,74]. This expansion was proposed due to the substitution of Li + into NaBH4[37], suggesting the formation of a Na(Li)BH4 solid solution in the as-milled LiNa-Ni mixture.…”

Powder diffraction methods for studies of borohydride-based energy storage materials

“…This substance shows as imilar behaviour to NaBH 4 .I ts cubic phase has been studied at room temperature by X-ray diffractiono ns ingle crystals (Luck, Schelter, 1999) and by neutron powder diffraction (Renaudin et al, 2004): in both cases it was described in the space group Fm " 3 3 m .The cubic phase transformstotetragonal at 65-70K with as tructure equivalent to the tetragonal NaBH 4 .T he cation size in the cubic alkali borohydrides, MBH 4 increases as Mg oes from Na to Cs, thus the unit cell expands and the shortest H ...Hd istances between the neighbouring BH 4 anions increase. As ar esult,t he weaker H ...Hr epulsion leads to lower temperatures of the cubicto-tetragonal transition ( Renaudin et al,2 004).…”