A computational study on the effect of fluorine substitution in LiBH ₄

Hydrogen substitution by fluorine in the orthorhombic phase of LiBH ₄ has been investigated with quantum-mechanics calculations aiming at describing thermodynamic properties of LiB(H,F)₄ solid solutions for hydrogen storage applications. Excess enthalpy of the mixed compounds was computed with the periodic ab initio CRYSTAL09 code, within the density functional approach and localised Gaussian basis sets, and used for Calphad thermodynamic modelling. The large number of possible mixed configurations for a given fluorine content were reduced by symmetry equivalence criteria. Deep analysis of the results highlights the relevance of structures in which, for a given H/F ratio, fluorine ions are likely to belong to the same BH₄ tetrahedron, rather than be dispersed over the available tetrahedra. This "locality principle" dramatically reduced the configurational space to be explored by expensive quantum-mechanical calculations. Our data show that, at room temperature, the formation of solid solutions between lithium borohydride and borofluoride is not thermodynamically favoured, so that the fluorine substitution destabilizes the pure hydride.