  Recent ab-initio calculations showed that oversized solute atoms (OSA) in BCC
and FCC iron exhibit a very strong attraction with a nearby vacancy (V) at
first neighbour distance (1NN). The attraction it is so large that the 1NN pair
OSA+V is no longer stable and relaxes spontaneously towards a new configuration
where the OSA sits in the middle of the bond, the two ends of which are
decorated with two half-vacancies (V/2). Taking into account the formation of
this complex V/2+OSA+V/2 in the migration process has never been done before:
it requires a new formulation of correlation effects. This is the aim of the
present contribution which establishes the expression of the diffusion
coefficient of the OSA and evaluates exactly the correlation factor in BCC and
FCC lattices, thanks to the introduction of macrojumps. The formalism is
applied to the case of Y in BCC iron and its results are compared to those
obtained by previous authors. It is shown that the smallness of the correlation
factor is not due to the transport mechanism of the OSA but only to the jump
frequencies of the vacancy in the vicinity of the OSA. This smallness of the
correlation factor is however overcompensated by a large macrojump frequency,
and, as a result, the Y atom diffuses more rapidly than iron at all
temperatures by orders of magnitude in the two structures.
