  We propose a mechanism to describe spin relaxation in n-doped III-V
semiconductors close to the Mott metal-insulator transition. Taking into
account the spin-orbit interaction induced spin admixture in the hydrogenic
donor states, we build a tight-binding model for the spin-dependent impurity
band. Since the hopping amplitudes with spin flip are considerably smaller than
the spin conserving counterparts, the resulting spin lifetime is very large. We
estimate the spin lifetime from the diffusive accumulation of spin rotations
associated with the electron hopping. Our result is larger but of the same
order of magnitude than the experimental value. Therefore the proposed
mechanism has to be included when describing spin relaxation in the impurity
band.
