The main aim of the proposed experiment is to probe the cation distributions in a series of half-Heusler A(MZ) intermetallic phases that are considered for use in thermoelectric power generation devices. The properties of these materials are sensitively dependent on A/M cation inversion but few systematic studies exist. We have prepared three half-Heusler materials: Ti(NiSn), Hf0.5Ti0.5(NiSn) and V(FeSb) and propose to use Polaris to study the A/M cation distribution between room temperature and 1000 degrees Celsius. Neutrons are needed due to the poor X-ray scattering contrast between the first row transition metals. The experiment will also provide insight into the effect of mixed Ti/Hf A-site occupancy on the lattice thermal conductivity through refinement of the thermal parameters.