Thermoelectric (TE) materials are set to play an increasingly important role in the development of new energy efficient technologies by generating electrical current from waste heat. Control of the thermal conductivity is critical for the development of such materials. We have synthesised a new multiple-anion compound which has an extremely low thermal conductivity. We wish to investigate the physical origin of this property, which may arise from (i) the intrinsic complexity of its layered structure, (ii) highly mobile liquid-like Cu+ ions within the structure, or (iii) rattling Se/Cl ions within the structure. By studying the crystal structure at a range of temperatures from 20 - 663 K, we will identify which of these structural features dominates the thermal conductivity, and use this knowledge to design new materials with thermally insulating structures.