A thermodynamic investigation of the LiBH4-NaBH4 system

The LiBH4–NaBH4 pseudo-binary system has been investigated by X-ray diffraction, temperature-programmed photographic analysis, and differential scanning calorimetry, in order to establish the phase diagram. The polymorphic orthorhombic-to-hexagonal phase transition of LiBH4 was observed at 94 °C in samples containing NaBH4, i.e. 15 °C lower than for pure LiBH4, which indicates the dissolution of sodium into LiBH4. The formation of solid solutions was confirmed by powder X-ray diffraction measurements performed as a function of temperature. A new eutectic composition between Li0.65Na0.35BH4 and Li0.70Na0.30BH4, with a melting temperature of 216 °C, is observed. Ab initio calculations have been performed to establish the relative stabilities of the pure compounds in orthorhombic, hexagonal and cubic structures. The obtained experimental and calculated data were compared with available literature values and they were used for a thermodynamic assessment of the LiBH4–NaBH4 system by the calphad method. The enthalpy of mixing for solid and liquid solutions has been estimated on the basis of experimental data.