  We consider a superconductor hosting mobile impurities, which locally change
the superconducting transition temperature. The BCS interaction at the impurity
is different both in magnitude and in sign from the BCS interaction in the
bulk. It is shown that due to the attraction between impurities, they tend to
form more condensed state. We also consider the distribution of mobile
impurities with local BCS attraction or repulsion at the vicinity of the
superconductor-normal metal interface.
