Structure solution of materials with very high ionic mobilities, and/or small populations of moderately mobile ions is hampered because the scattering from these ions is "smeared out" by thermal motion. This makes methods of locating mobile ions difficult, as the scattering by these elements competes less effectively with the background signal. We propose a powder diffraction experiment to solve the structures of two different chemical systems, a new tin/germanium thiophosphate (a potential solid state lithium electrolyte) and a new bismuth-based iodide (a potential lead-free solar absorber), which are united by the problems they pose with mobile scatterers at room temperature (Li and Cu respectively), by using low temperatures (<100K) to quench the thermal motion of these mobile ions. The structures we obtain will be used to understand their exciting physical properties.