Ab initio study of iron and iron hydride: II. Structural and magnetic properties of close-packed Fe and FeH

The energetical ordering and magnetic states of hexagonal and double-hexagonal close-packed (hcp, dhcp) as well as face-centred cubic (fcc) Fe and FeH crystals are studied via spin-polarized ab initio total-energy calculations in the local spin-density approximation and with generalized gradient corrections by means of the mixed-basis pseudopotential and the all-electron LMTO-ASA methods. In all three structures, the magnetic spin moments go to zero under volume compression. For pure Fe in the compressed non-magnetic state, the hcp structure is found to have the lowest energy, fee the highest, and the dhcp structure lies in between. The two hexagonal structures have significantly smaller than ideal c/a ratios. For compressed non-magnetic FeH the energetical ordering of the structures is reversed, compared to that for pure Fe, with fee ground-state structure and almost ideal c/a ratios for both hexagonal structures. In the ferromagnetic states at expanded volumes, the energetical orderings are again opposite to those of the non-magnetic states both for Fe and FeH. In ferromagnetic FeH these energy differences are particularly small, yielding almost an energetical degeneracy of all three close-packed structures.