Soft phonon mode behavior in the KMnF3 crystal

The low-frequency region of the Raman spectrum is studied in KMnF3 over the temperature range 4.2–300°K. At low temperatures four lines with frequencies 28, 39.5, 50, and 66 cm−1 were observed. Changes in the spectrum with temperature allow these lines to be ascribed to components of the soft phonon modes Γ25 and M3, which are responsible for phase transitions at 187.6 and 102 °K, respectively. Moreover, an abrupt change in Raman spectrum line frequencies was observed at the temperature of antiferromagnetic ordering, TN = 88°K. Group-theoretical analysis of the spectrum is performed with consideration of condensation of the oscillatory modes Γ25 and M3. On the basis of theoretical and experimental data conclusions are reached as to the symmetry of the KMnF3 crystal in low temperature phases.

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