The effect of pressure on the Raman spectrum of NH4I

The Raman spectra of NH4I and ND4I are reported at pressures up to 130 kbar and the II–IV transition in this compound is observed to occur at 27(5) kbar at room temperature. The Raman spectral changes which characterize this transition are very subtle and it is concluded that a considerable amount of disorder can still be found immediately above the II–IV transition in the presumably fully ordered phase IV. The behavior of both the N–H stretching modes as well as the librational mode of the NH+4 ions show that at both 298 and 100 K the strength of hydrogen bonding increases when the pressure is raised in phase IV. Two different potentials are used to calculate the barrier heights for the librational motions in NH4I and it is found that anharmonicity is of much less significance in this compound than in NH4Cl and NH4Br. At higher pressures in phase IV the behavior of ν6, however, shows that a volume effect becomes important and this most probably gives rise to the IV–V transition which occurs at 54(4) kbar in NH4I at 298 K and 40(1) kbar at 100 K. The Raman spectra of NH4I (V) are characterized by the appearance of a large number of lattice modes as well as by the disappearance of the LO–TO splitting of ν3 and changes in the relative intensities of the N–H stretching modes. The librational mode ν6 shifts upwards in frequency rather rapidly in NH4I (V) but the N–H stretching modes show only a very slight downward frequency shift. It is not possible to elucidate the structure of NH4I (V), from a consideration of the Raman spectra of powdered samples only, however, similarities and differences between phase V spectra and those of the fictitious phase ’’V’’ of NH4Br, as well as those of other well-known structures of NH+4 compounds are pointed out.