Electronic structure of substoichiometric Fe-Al intermetallics

The formation energy, nature of bonding, electron density of states, and magnetic properties of Fe_1-xAlₓ intermetallics have been calculated in the concentration range $0<~x<~0.5$ using the tight-binding linearized muffin-tin orbital method and a super unit cell containing 16 atoms. The various concentration ranges are simulated by successively replacing Fe atoms by Al atoms and studying the electronic structure within the density-functional theory and generalized gradient approximation for exchange and correlation. The stability of the Fe_1-xAlₓ alloys increases monotonically with increasing Al concentration, while the magnetic moment variation reveals exactly the opposite behavior. Stoichiometric FeAl is found to exhibit two nearly degenerate magnetic structures: a nonmagnetic state with zero moment on Fe and a ferromagnetic state with a moment of 0.75μB per Fe atom. Fe₃Al, on the other hand, is ferromagnetic with a calculated moment of 2.45μB at the Fe-I site and 1.95μB at the Fe-II site. The coupling between Fe and Al atoms is antiferromagnetic, although the moment at the Al site is much smaller (-0.17μB) than that at the Fe sites. The bonding between Fe and Al atoms is primarily due to the hybridization between the $3d$ electrons of the former and the sp electrons of the latter. The bonding has a strong local character in that the coupling is between the nearest neighbor atoms. This is further verified by a calculation using small clusters as models of the bulk structure. The density of states at the Fermi energy is dominated by contributions from the Fe $3d$ state although its variation with Al is modulated by subtle interaction with Al $3p$ electrons. The total densities of states at the Fermi energy of Fe_1-xAlₓ alloys show the same variation as the electrical resistivity, suggesting that the increase and then decrease in resistivity with Al concentration with a peak at 33% Al is purely of electronic origin.