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 Li_0.65Na_0.35BH₄ and Li_0.70Na_0.30BH₄, 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.