Diborane release and structure distortion in borohydrides

Abstract

Hydrogen desorption from borohydrides is often accompanied by the release of diborane. The amount of diborane released as a byproduct during the decomposition of borohydrides scales inversely with the borohydride stability, which in turn depends on the electronegativity of the corresponding cation. We present a model based on the difference between the symmetric and asymmetric assembly of B₂H₆ units at the surface. The origin of this reaction is the degree of distortion of the BH₄⁻ anions in the bulk, hitherto depending on the degree of ionization of the cation. A practical measure of the distortion is the range in which the stretching vibration modes appear, which is the difference in the energy of the stretching vibrations of hydrogen atoms with maximum different bonding lengths (Badger's rule). We propose from this relation that the diborane released from the surface of the relatively unstable LiZn₂(BH₄)₅ is formed from a recombination of BH₂^δ+ and BH₄^δ− units. Ultra high vacuum mass spectroscopy measurements support the presented model and clarify the decomposition of stable borohydrides, such as LiBH₄. The sublimation of borohydrides in UHV competes with their decomposition.