  The chemical selectivity and great sensitivity of the Extended X-ray
Absorption Spectroscopy technique allowed the determination, in the
paramagnetic phase, of the structural distortions induced by doping in the
spin-Peierls CuGeO$_3$ compound. The distorted environments were analyzed as a
function of concentration, magnetic nature of impurity and the substitution
site (Ni, Mn and Zn impurities on the Cu site, Si impurity on the Ge site).
This has led to estimate the variation of the angles and pair distances, and
hence to evaluate the magnetic coupling along the Cu chains in the vicinity of
the impurities. The antiferromagnetic interaction between Cu first neighbors in
the pure sample is found to be weakened around Ni, almost cancelled in the case
of Mn doping, and even to change sign, producing a ferromagnetic coupling for
Si doping. More generally, the structural distortions on a local scale are
shown to be key parameters for the understanding of the magnetic properties of
doped spin-Peierls compounds.
