  The problem of phonon scattering by grain boundaries is studied within the
wedge disclination dipole (WDD) model. It is shown that a specific q-dependence
of the phonon mean free path for biaxial WDD results in a low-temperature
crossover of the thermal conductivity, $\kappa$. The obtained results allow to
explain the experimentally observed deviation of $\kappa$ from a $T^3$
dependence below $0.1K$ in $LiF$ and $NaCl$.
