Raman scattering study of the three structural phases of KMnF3

The Raman spectra of the three phases of KMnF 3 have been measured and a detailed group theoretical analysis has been carried out in order to identify the observed peaks. The results for the intermediate temperature phase are consistent with a tetragonal structure of D 4h 18 symmetry but the results for the low-temperature phase do not unambiguously determine the crystal structure. Particular attention was paid to the temperature-dependent modes. The soft modes associated with the upper phase transition were fitted to the response functions for a damped simple harmonic oscillator and for a model including damping by phonon density fluctuations. The results favour the latter model and suggest that non-classical critical fluctuations play an important role near the upper phase transition.