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We measured the temperature dependent infrared reflectivity spectra of MnF2between 4 K and room temperature. We show that the phonon spectrum undergoes astrong renormalization at TN. The ab-initio calculation we performed on thiscompound accurately predict the magnitude and the direction of the phononparameters changes across the antiferromagnetic transition, showing that theyare mainly induced by the magnetic order. In this material, we found that thedielectric constant is mostly from phonon origin. The large change in thelattice parameters with temperature seen by X-ray diffraction as well as theA2u phonon softening below TN indicate that magnetic order induced distortionsin MnF2 are compatible with the ferroelectric instabilities observed in TiO2,FeF2 and other rutile-type fluorides. This study also shows the anomaloustemperature evolution of the lower energy Eu mode in the paramagnetic phase,which can be compared to that of the B1g one seen by Raman spectroscopy in manyisostructural materials. This was interpreted as being a precursor of a phasetransition from rutile to CaCl2 structure which was observed under pressure inZnF2.

Elastic and magnetic effects on the infrared phonon spectra of MnF2