Bonding and magnetism in Fe-M(M=B,C,Si,N) alloys

Self-consistent-field, Xα, scattered-wave, molecular-orbital calculations on tetrahedral Fe clusters with (${\mathrm{Fe}}_4$-M) and without (${\mathrm{Fe}}_4$) central M species (M=B,C,Si,N) are described. The results are consistent with similar earlier calculations and go beyond those works in terms of their chemical-physical insights for bonding and magnetism. Two distinct bonding types, polar and covalent, having different consequences are identified. Polar bonding appears to play a greater role in overall stability than does covalent bonding. It mixes s and d character from Fe with s character from the M species forming new s-p-d hybrids localized largely at the M site. Its magnetic effects depend on the amount of d character lost to this bonding. On the other hand, a more covalent bonding strongly mixes Fe d character with M p character. This covalent p-d bond is delocalized relative to the initial states and can dramatically reduce the electronic state density at the Fermi level, thus suppressing moment formation.