Infrared spectra of the isotopically dilute HDO and NHsD+ species in (NH~)~CUC~J-~H~O are interpreted as revealing the presence of three phases which exist between room and liquid-helium temperatures. The transiticn between phases II and III appears to be of the order-disorder type. Recently [l] we described the infrared spectrum of the isotopically isolated NH3Df ion in polycrystalline diammonium tetrachiorocuprate(Ii) dihydrate (DTD) at liquid-nitrogen temperature and concluded that, at this temperature, the ammonium ions adopt hue distant orientations in the tetragonal structure. The numbers of ions possessing each orientation were estimated from band intensities to be approximately equal. At the time we were not aware of the existence of a crystal-structure determination of the compound, and so we derived the salient features of the structure of DTD by combining the lattice parameters of the ammonium compound with the positional parameters of the chlorine and oxygen atoms in the isostructural potassium compound at room temperature as determined by neutron diffraction [2]. It has since come to our attention that Matkovid et al. [3] had in fact reported a determination of the structure of DTD at room temperature from two-dimensional neutron diffraction intensities as early as 1969. This determination has furnished a reasonably accurate set of values of the positional parameters of all the atoms, but more importantly, it was found that the ammonium ions adopt a single orienta-