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

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