We propose 1 day of beam-time on GEM to probe changes in the crystal structure of the new fast Li+ conducting mixed-cation, mixed-anion phase, Li0.5Na0.5(BD4)0.83I0.17 as a function of temperature. Partial substitution of Li by Na and of BH4 by I stabilises the high pressure, rock salt structure of LiBH4 (HP-LiBH4) at room temperature and pressure. The scattering contrast for this composition by powder X-ray diffraction PXD is very low and so the aims of this experiment are to: (a) determine the precise positions and occupancies of the I- and (BD4)- anions and the Li+ cations, (b) establish the dynamic disorder of the (BD4)- anions, (c) resolve the thermal parameters of Li+ and elucidate the ionic conductivity mechanism.