Argyrodites exhibit promising thermoelectric (TE) properties but a structural transition causes cracking of ingots on cooling to ambient temperature, introducing problems for device construction. We have prepared series of mixed-anion derivatives of Cu8GeS6, containing halide ions that show marked reductions in the phase-transition temperature and the stabilization of the high-temperature structure at room temperature, eliminating problems of cracking. Neutron diffraction will be used to investigate the role of the larger halide ions (Br and I) in suppressing the structural transition and to provide the first structural characterization of the low temperature form of mixed-anion phases. The data will also provide information on the mobility of copper ions and shed light on the origin of the low thermal conductivity that results in promising TE performance even in non-optimized materials.