First-principles study on thermodynamical stability of metal borohydrides: Aluminum borohydride Al(BH4)3

TL;DR

This study uses first-principles calculations to analyze the thermodynamical stability of aluminum borohydride, revealing phase stability, weak intermolecular interactions, and a correlation between heat of formation and cation electronegativity.

Contribution

It provides a detailed first-principles analysis of Al(BH4)3's stability, confirming phase preferences and electronic interactions, and extends previous correlations between heat of formation and electronegativity.

Findings

α phase is more stable than β phase

Weak interactions between molecules in solid phases

Heat of formation correlates with cation electronegativity

Abstract

The thermodynamical stability of ${\rm Al(BH_4)_3}$ has been investigated using first-principles calculations based on density functional theory. The heats of formation are obtained to be -132 and $-131 {\rm kJ/mol}$ without the zero-point energy corrections for $\alpha$- and $\beta$-${\rm Al(BH_4)_3}$, respectively, which are made up of discrete molecular ${\rm Al(BH_4)_3}$ units. It is predicted correctly that the $\alpha$ phase is more stable than the $\beta$ phase. The energy difference between the solid phases and the isolated molecule is only about 10 kJ/mol. An analysis of the electronic structure also suggests the weak interaction between ${\rm Al(BH_4)_3}$ molecules in the solid phases. It is confirmed that ${\rm Al(BH_4)_3}$ obeys the linear relationship between the heat of formation and the Pauling electronegativity of the cation, which has been proposed in our previous study [Nakamori {\it et al.}, Phys. Rev. B {\bf 74}, 045126 (2006)].