Bromide substitution in lithium borohydride, LiBH4–LiBr

By means of in situ synchrotron radiation powder X-ray diffraction, powder neutron diffraction, attenuated total reflectance infrared spectroscopy, differential scanning calorimetry and the Sieverts techniques we have investigated how anion substitution in the LiBH4–LiBr system leads to changes in the structural, physical, chemical and hydrogen storage properties of this material. Mechano-chemical treatment facilitates formation of a hexagonal solid solution h-Li(BH4)1−xBrx whereas heating at elevated temperatures, i.e. T > 112 °C appears to allow full solubility in the system LiBH4−LiBr. The first step in the anion substitution process may be dissolution of small amounts of LiBH4 in α-LiBr deduced from observation of a hexagonal solid solution with unit cell volume similar to β-LiBr for a hand-mixed sample. The solid solution, Li(BH4)1−xBrx, is isostructural to the hexagonal high temperature polymorph of LiBH4. This solid solution melts at a significantly higher temperature depending on the composition as compared to h-LiBH4. Furthermore, a new hexalithium borate tribromide, Li6(BO3)Br3 was discovered and structurally characterized.