Anomalies in the Raman scattering spectra at the phase transitions of BaMnF4 and BaCoF4 crystals

The Raman scattering spectra of BaMnF4 and BaCoF4 crystals are studied in the temperature range 4.2 to 300°K. Additional lines are found at the structural phase transition BaMnF4 crystals (Tc = 255°K), which are due to the expansion of the unit cell and the appearance of additional phonon modes at the center of the Brillouin zone. The changes observed are characteristic of a second-order phase transition. In the spectrum of BaCoF4 crystals a line (331 cm−1 at 4.2°K) is found whose frequency and half-width undergo sharp changes at the antiferromagnetic ordering (TN = 69.6°K). Investigations show that the behavior of this line can be explained by exciton scattering of the light, due to the transition of Co2+ ions to the first excited state.

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