  We systematically investigate, using ab initio density-functional theory
calculations, the properties of interstitial and substitutional Mn in both Si
and Ge, as well as in the Si(1-x)Gex alloy. We show that volume effects are not
the main reason Mn prefers to be a subsitutional impurity in pure Ge, and
chemical effects, therefore, play an important role. Using realistic models of
Si(1-x)Gex, we show that for x approximately greater than 0.16 substitutional
Mn in Ge-rich neighborhoods become more stable than interstitial Mn, which may
allow the growth of Si-based diluted magnetic semiconductors.
