Temperature dependence of IR spectra of zinc and magnesium fluoride

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Abstract
The reflective infrared spectra of zinc and magnesium fluoride are investigated from 40 to 5000 cm-1 between 4.2 and 300 K. These spectra are analysed by two methods: the first is a classical analysis of an oscillator response and the second is a Kramers-Kronig analysis. This second method shows in the MgF2 spectrum the presence of a supplementary band near 350 cm-1 which is not explained by the group theory in this case. For ZnF2, the comparison of the two methods shows a three-oscillators simple model gives an imperfect representation of the spectrum of Eu symmetry. In the case of MgF2, the frequency shifts of the transverse modes with temperature confirm the Raman scattering data. The transversal frequencies of ZnF2 for the A2u and E3u modes present a maximum near 80 K. It could be possible that this result was the consequence of the presence of a para-ferroelectric transition situated at lower temperature.