THE DYNAMICS OF NH4+ IN THE NH(4)MF(3) PEROVSKITES .2. INELASTIC NEUTRON-SCATTERING STUDY

Inelastic neutron scattering experiments have been performed to study the librational motion of the NH4+ ion in the perovskite family NH(4)MF(3) (M = Zn, Mn and Cd). The first excited librational band appears as a broad peak in the high-temperature cubic phase. Below the structural phase transition, to a lower symmetry, the band splits into three narrow features. The overtone bands are also detected in the Cd compound. The frequencies of the fundamentals increase for increasing cell size (Zn > Mn > Cd), the same trend as the activation energies. The INS spectra are analysed quantitatively by expanding the potential in symmetry adapted rotational wavefunctions.