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

Using spin density functional theory we have carried out a comparative study of chemical bonding and magnetism in Fe4N, Fe3N and zeta-Fe2N. All of these compounds form close-packed Fe lattices, while N occupies octahedral interstitial positions. High-spin FCC Fe and hypothetical FeN with rock salt structure have been included in our study as reference systems. We find strong, covalent Fe-N bonds as a result of a substantial sigma-type p-d hybridisation, with some charge transfer to N. Those Fe d orbitals which contribute to the p-d bonds, do no longer participate in the exchange splitting of the Fe d bands. Because of the large exchange fields, the majority spin d bands 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 consequence, 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.