First-principles self-consistent field electronic-state calculations were carried out for isolated interstitial and substitutional Er. The results of the calculations showed that the 4f orbitals of Er had to be treated as valence states in order to describe the interactions between impurity and host atoms. The tetrahedral interstitial Er3+ configuration was found to be the most stable one among those studied.

F.Gan, L.V.C.Assali, L.C.Kimerling: Materials Science Forum, 1995, 196-201, 579-84