COVALENCY IN FEF2 FROM MAGNETIZATION DENSITY DATA AND FROM ABINITIO THEORY

A further analysis of the polarised neutron diffraction data on FeF 2 has been carried out using a valence orbital population model of the spin density. The fit to the data, chi 2=1.8 for all reflections, is considerably better than for ligand field models used previously. The improvement is traced to the fact that the new model emphasises covalent interaction between iron and fluorine, whereas the earlier treatments introduced it only after spin-orbit interactions within the free Fe2+ ion had been taken into account. The amount of covalence required to explain the results is considerable, at least 10% of the spin of the iron atom being delocalised. However, the amount of spin seen on each fluorine atom is not correspondingly large, because cancellation takes place on account of the antiferromagnetic nature of the compound. An ab initio approximate calculation carried out on FeF64-, units using the DV-Xalpha method lends strong support to the picture introduced by the valence orbital population model, and confirms that there are strong elements of covalence in the Fe-F interaction.