Interplay of chemical bonding and magnetism in Fe4N, Fe3N and zeta-Fe2N

Using spin density functional theory we have carried out a comparative stud y of chemical bonding and magnetism in Fe4N, Fe3N and zeta-Fe2N. All of the se compounds form close-packed Fe lattices, while N occupies octahedral int erstitial positions. High-spin FCC Fe and hypothetical FeN with rock salt s tructure have been included in our study as reference systems. We find stro ng, covalent Fe-N bonds as a result of a substantial sigma-type p-d hybridi sation, with some charge transfer to N. Those Fe d orbitals which contribut e to the p-d bonds, do no longer participate in the exchange splitting of t he Fe d bands. Because of the large exchange fields, the majority spin d ba nds are always fully occupied, while the minority spin d bands are close to half-filling, thus optimising the Fe d-d covalent bonding. As a consequenc e, in good approximation the individual Fe moments decrease in steps of 1/2 mu(B) from FCC iron (2.7 mu(B)) via Fe4N (2.7 and 1.97 mu(B)), Fe3N (1.99 mu(B)) to zeta-Fe2N (1.43 mu(B)). (C) 1999 Elsevier Science B.V. All rights reserved.