AbstractRaman spectra of NH4IO4and ND4IO4have been recorded at several temperatures between 83 K and 371 K. As the temperature is increased, several bands due to both internal and external modes of the periodate ion are shifted to lower wavenumbers. There is a crossing of theAgandEglibrational modes of the IO4−ion near 200 K. The bands due to the ammonium ion vibrations are broadened as the temperature is increased and the librational lattice mode is no longer observable above about 200 K. The present results are correlated with observations from NQR, NMR and X‐ray diffraction stud