Vibrational spectra of the ammonia halides NH4I and NH4F at high pressures

The vibrational spectra of ammonium iodide NH4I at pressures up to 4.1 GPa and ammonium fluoride NH4F at pressures up to 4.7 GPa were investigated by inelastic incoherent neutron scattering. The pressure dependences of the transverse optical translational and librational modes were obtained. The behavior of the rotational potential barrier for the ammonium ion as a function of the lattice parameter for disordered and ordered cubic phases of ammonium halides with CsCl type structure were calculated. The results obtained confirm that the transition from an orientationally disordered cubic phase into an ordered cubic phase in ammonium halides occurs at close critical values of the positional parameter of hydrogen (deuterium). (C) 2000 MAIK "Nauka/Interperiodica".