Phase behaviour in the LiBH4-LiBr system and structure of the anion-stabilised fast ionic, high temperature phase

The fast ionic, high temperature (HT) phase of LiBH4 can be stabilised by Br¯ substitution. Lithium borohydride bromide compounds, Li(BH4)1-xBrx have been synthesized mechanochemically, with and without thermal treatment and the resulting phase behaviour determined as a function of composition. Single phase materials exist for 0.29 ≤ x ≤ 0.50 with conductivity two orders of magnitude higher than LiBH4 at 313 K. Powder neutron diffraction has been used to resolve the details of the crystal structure of one such compound. These demonstrate that 7Li(11BD4)2/3Br1/3 retains the HT structure (hexagonal space group P63mc, a ≈ 4.2 Å, c ≈ 6.7 Å) from 293-573 K. The borohydride bromide exhibits considerable static and dynamic disorder, the latter invoking complex rotational motion of the (BH4)¯ anions.