  Transition metals (TMs) implanted in oxides with rock-salt crystal structures
(for example MgO and BaO) are assumed to substitute cations (Mg in case of MgO)
from the lattice sites. We show that not all implanted TMs substitute cations
but can be stable in interstitial sites as well. Stability of TM (Sc--Zn)
dopants in various charge states in MgO and BaO has been investigated in the
framework of density functional theory. We propose an effective way to
calculate stability of implanted metals that let us predict site preference
(interstitial or substitution) of the dopant in the host. We find that two
factors govern the preference for an interstitial site: i) relative ionic
radius and ii) relative oxygen affinity of cation and the TM dopants. If the
radius of the cation is much larger than TM dopant, as in BaO, TM atoms always
sit at interstitial sites. On the other hand, if the radius of the cation is
comparable to that of the dopant TM, as in case of MgO, the transition of the
preferred defect site, from substituting lattice Mg atom (Sc to Mn) to
occupying interstitial site (Fe to Zn) is observed. This transition can be
attributed to the change in the oxygen affinity of the TM atoms from Sc to Zn.
Our results also explain experiments on Ni and Fe atoms implanted in MgO. This
is the first-time we have shown that TM dopants can be stable at interstitial
sites in stable compounds, which could potentially give rise to exotic
properties.
