Photoelectron spectroscopy of the iron borides Structure, bonding and magnetic behaviour

Abstract

The structures of the interstitial compounds iron semiboride/Fe₂B and iron mono boride FeB are discussed in relation to the changes which occur in the bonding between the iron and the boron atoms with increasing boron content. Photoelectron spectroscopy provides a powerful method for observing the evolution of the various electronic states. The bonding is found to be strongly boron 2sp² hybridization in the monoboride with accompanying changes in the iron 3d valence states which reflect changes in the density of states at the Fermi level. Much weaker boron 2sp² hybridization occurs in the semiboride, which has a different crystallographic (tetragonal) structure. The stronger boron 2sp² hybridization in FeB accounts for the chain like structure of the B atoms in a highly distorted [orthorhombic) iron lattice. These results are very different from those predicted by previous ‘rigid hand models’ and show that very little charge transfer occurs between the Fe and B atoms. The covalent bonding in these metallic alloys has repercussions on their ferromagnetic properties. Multiplet splitting in the Fe(3s) core level is observed in both the semiboride and monoboride to he of the order of magnitude of that in elemental iron. Strong correlation effects complicate the usual picture of spin exchange which determines the actual magnitudes of Fe(3s) core level multiplet splitting due to the iron atomic, 3d magnetic moments.