Estimation of Thermodynamic Properties of Metal Hydroborates

Metal hydroborates from M(BH 4 ) n to M x (B 12 H 12 ) 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 (MBH 4 ) 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 (BH 4 ) 2 (M II =divalent cation) and Ln(BH 4 ) 3 (Ln=lanthanide). The Gibbs free energy of formation of M 2 (B 12 H 12 ) (M=alkali metal) is also estimated.