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.

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

“…There are site and factor group splittings: the degenerate ν 3 mode of F 2 symmetry for the free ion (point group T d ) is split into B + E contributions (site group S 4 ) which transform as A u + B g + E g + E u in C 4h (factor group of the crystal). In addition, by analogy with observations for the analogous [BH 4 ] − ion [34], one can expect the presence of Fermi resonances involving overtones and combination bands of the lower frequency modes observed Combination bands -1000 (Sh) - between 680 and 900 cm −1 . The assignment of these lower frequency modes is not straightforward: there appear four bands at 690, 735, 800 (shoulder) and 900 cm −1 , while only three bands are expected (A u + E u ) for ν 4 , and A u for ν 2 .…”

Structure and Properties of NaBH4·2H2O and NaBH4

“…[34,35] The positions of the totally symmetrical stretching mode at 2340 cm -1 in NaBH 4 and 2319 cm -1 in NaBH 4 ·2H 2 O are similar, suggesting a similar B-H bond length in the hydrated and the anhydrous form. Under the experimental conditions of the diffraction experiments, the B-H bond lengths in hydrated and anhydrous NaBH 4 are similar: the average length of the B-H bonds in NaBH 4 ·2H 2 O at 100 K is 1.12(1) Å, whereas in anhydrous NaBH 4 at 200 K the B-H bond length is 1.09(2) Å.…”

Synthesis and Characterization of NaBD3H, A Potential Structural Probe for Hydrogen Storage Materials

“…For each, the character of the fundamental in the overtone is slightly higher than 10%. The two components of 2ν 6 being very close in energy, the theoretical anharmonic IR spectrum shown in Figure 5 exhibits finally two bands corresponding to the signature of strong anharmonic couplings that occur in this system. The 2ν 6 is slightly blue-shifted with respect to the ν 4 , in agreement with the picture provided by the experimental investigation, with a relative intensity of about 1:5.…”

“…This approach was motivated by the study of alkanes with isotopically isolated C-H bonds 5 which revealed both theoretically and experimentally a linear correlation between C-H stretching frequency and bond length. Previously, we have been able to correlate the totally symmetrical B-D stretching mode of the alkali borohydrides with B-D bond length 6 obtained from neutron diffraction data. However, a similar correlation for the B-H stretching mode of the alkali borohydrides fails.…”