Vibrational Spectra of Ammonium and Other Scheelite-Type Perrhenates

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Abstract
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.