Vibrational Spectra of Ammonium and Other Scheelite-Type Perrhenates

The far-infrared and laser-excited Raman spectra of several scheelite-type perrhenates have been recorded at 77°K and room temperature in order to obtain frequencies and symmetry assignments for the lattice modes. Both NH4ReO4 and ND4ReO4 were studied to separate ammonium-ion translations and librations. At 77°K the ammonium-ion librational mode is observed at 264 cm−1 in NH4ReO4 and 194 cm−1 in ND4ReO4. Upon warming to 300°K, the ammonium-ion librational mode disappears and is unobservable in both the far-ir and Raman spectra. The Raman-active ammonium-ion fundamental modes have also been observed in both NH4ReO4 and ND4ReO4. The temperature-dependent changes in the intensities and linewidths of the ammonium-ion fundamentals and lattice modes indicate that the ammonium-ion motion undergoes a change between 77 and 300°K.