Vibrational spectra of crystalline ammonium nitrate

The low-temperature Raman spectra of NH4NO3 and ND4NO3 give evidence of one phase (phase V) with the phase transition from phase IV being sluggish and occuring over a temperature range of some 100 K with pronounced hysteresis. The spectra were consistent with the reported powder structure of P42. The internal vibrations of the nitrate ion showed both site and factor group components while the internal motions of the ammonium ion showed evidence of two different coulombic sites with a small site splitting evident for both sites. In the higher temperature phase some evidence relating to heteroionic coupling of Ag site components in phase IV is presented and the site splitting of internal nitrate bands in CsCl type structures of phases V, IV, II and I is discussed.