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CALCULATION OF THE RAMAN FREQUENCIES AS A FUNCTION OF TEMPERATURE IN PHASE III OF NH4I

We study here the temperature dependence of the Raman frequencies for the overtone 2 nu(6) of the librational mode nu(6) in phase III of NH4I. By using the experimental infrared frequencies of the 2 nu(6) mode, we predict the Raman frequencies of this overtone 2 nu(6) mode as a function of temperature at atmospheric pressure in phase III of NH4I. Our calculations indicate that the Raman frequencies of 2 nu(6) exhibit anomalous behavior at around 110 K (P = 0) in phase III of NH4I, as observed experimentally.