Spectroscopic study of NH₄ZnF₃and NH₄MnF₃perovskites

Abstract

The Raman and IR spectra of NH4ZnF3 and NH4MnF3 have been measured at room temperature in order to assign the lattice modes and the internal vibration modes of the NH4+ in the cubic (Pm3m) phase. The lattice force constants have been obtained in a shell model and compared with those of other perovskites. The dispersion relations in the whole Brillouin zone, obtained with the same model, are also given. Finally, from the analysis of the Raman intensities the strengths of the NH4 force constants have been determined and compared with their values in ammonium halides and in the 'free ion'. The intramolecular polarisability parameters have also been obtained and compared with those of CH4.