Examples of a dynamic Jahn-Teller system: Raman scattering from electronic excitations and phonons in paramagnetic and antiferromagnetic FeCl2and FeBr2

Raman spectra of FeCl 2 and FeBr 2 have been measured above and below their Neel temperature within an energy range of 0 to 900 cm -1 . Spectral features due to scattering from phonons and electronic excitations are analysed and assigned to symmetry species. A dynamic Jahn-Teller calculation using a full-lattice model has been extended, for the first time, to describe all levels within the 5 T 2g ( 5 D) manifold and gives the best, and an accurate description of the observed electronic transitions. Justification for the use of this model comes from the good agreement between calculated and experimentally determined ground-state g values and its successful extension through a molecular-field calculation to predict the energies of transitions observed in the antiferromagnetically ordered crystal. The results of this work indicate that even though the dynamic Jahn-Teller effect is weak, it nevertheless plays an important role in FeCl 2 and FeBr 2 . A weak temperature-dependent band at 127.7 cm -1 in the spectrum of FeBr 2 is assigned to second-order scattering from a magnon-phonon combination mode.