Estimation of Thermodynamic Properties of Metal Hydroborates

Metal hydroborates from M(BH4)(n) to M-x(B12H12)(y) currently attract a strong interest for potential applications in the field of hydrogen storage and more recently as solid ionic conductors. While thermodynamic data for the alkali borohydrides (MBH4) are well known, experimental data for other compounds are cruelly lacking. Using a combination of theoretical (DFT) and experimental data, we calculate the lattice enthalpy for a series of borohydrides. The comparison with literature data show that these lattice enthalpies are very similar to those of the corresponding metal bromides. This similarity is at the origin of a good correlation between the formation enthalpy of bromides and borohydrides which is then used to estimate the previously unknown formation enthalpy of M-II(BH4)(2) (M-II=divalent cation) and Ln(BH4)(3) (Ln=lanthanide). The Gibbs free energy of formation of M-2(B12H12) (M=alkali metal) is also estimated.