Ordered deuterium arrangements and order-disorder transformations of the tantalum deuterides in the range TaD_-TaD_ have been studied by neutron diffraction and calorimetry at temperatures between -170 and 120℃. In addition to the disordered phase (α), three ordered phases based on the superstructures Ta_2D_(β_1), Ta_4D_3(γ), and TaD(δ) are clarified. The Ta_2D_ structure is a nonstoichiometric form of the Ta_2D superstructure over the range x changes to the disordered α-phase around 100℃
