Study of the iron borides. IV. Relation of bonding to structure and magnetic behavior from photoemission experiments andab initiocalculations

Angle-integrated-photoelectron-spectroscopy measurements with synchrotron radiation ($15<h\nu <160\mathrm{}$ eV) have been performed on the interstitial compounds ${\mathrm{Fe}}_2$B (iron subboride) and FeB (iron monoboride). They show the iron states for FeB within ∼4 eV of $E_F$ and boron states at 6.6 eV (B $2p$), 10.5 eV (B $2sp$ bonding), and 14.0 eV (B $2s$). The boron states for ${\mathrm{Fe}}_2$B are less intense, with B $2s$ nearer $E_F$, and with greater overlap of B $2p$-Fe $3d$ states. When corrections are made for relaxation effects there is a good agreement between the experimental photoelectron-emission data, the boron states observed in soft-x-ray-emission boron-$K$ measurements, and the Fe and B states from an ab initio calculation. Electronic-structure models are proposed for ${\mathrm{Fe}}_2$B and FeB which differ from earlier charge-transfer models. These include a change in the Fe $3d$ configuration (and valence band), a provision for covalent bonding in the boron chain of FeB, greater Fe $3d$-B $2p$ interaction in ${\mathrm{Fe}}_2$B than in FeB, and a predominantly metallic binding in both borides. This model accounts for the metallic nature, the brittleness, the ferromagnetism, and the local moments of the iron borides, and also appears applicable to amorphous Fe-B compounds (metallic glasses).