Phase Behavior in the LiBH₄-LiBr System and Structure of the Anion-Stabilized Fast Ionic, High Temperature Phase

The fast ionic, high temperature (HT) phase of LiBH₄ can be stabilized by Br^- substitution. Lithium borohydride bromide compounds, Li(BH₄)_1-xBr_x, have been synthesized mechanochemically, with and without thermal treatment and the resulting phase behavior determined as a function of composition. Single phase materials exist for 0.29 ≤ x ≤ 0.50 with conductivity 2 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)_0.667Br_0.333 retains the HT structure (hexagonal space group P6₃mc, a ≈ 4.2 Å, c ≈ 6.7 Å) from 293 to 573 K. The borohydride bromide exhibits considerable static and dynamic disorder, the latter invoking complex rotational motion of the (BH₄)^- anions.