Raman Spectroscopy of the Ammonium Ion in NH$_4$ZnF$_3$ and NH$_4$MnF$_3$ Perovskites: Temperature Dependence

Abstract

The Raman spectra corresponding to the internal vibrations of the ammonium ion in NH₄MnF₃ and NH₄ZnF₃ single crystals have been measured from 10 to 300 K. The spectra clearly show the existence of a structural phase transition from cubic to tetragonal symmetry. The splitting of the internal modes is compatible with effective symmetries of the ammonium ion of $D_{2d}$ and $C_s$ in the cubic and tetragonal phases, respectively. The temperature dependence of the symmetric stretching mode in the cubic phase is linear with a slope of 0.231 cm$^{-1}$ K$^{-1}$ for NH$_4$MnF$_3$. This has the opposite sign to that due only to the crystal expansion as obtained by applying hydrostatic pressure. A sharp band appearing only in the spectrum of NH$_4$MnF$_3$ just below the crystallographic phase transition originates in a strong symmetry-induced $\nu_2 + \nu_4:\nu_1$ Fermi resonance.