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-x)Br-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 Li-7((BD4)-B-11)(0.667)Br-0.333 retains the HT structure (hexagonal space group P6(3)mc, a approximate to 4.2 angstrom, c approximate to 6.7 angstrom) from 293 to 573 K. The borohydride bromide exhibits considerable static and dynamic disorder, the latter invoking complex rotational motion of the (BH4)(-) anions.