  The thermal conductivity of polycrystalline semiconductors with type-I
clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr
and/or Eu) exhibit lattice thermal conductivities typical of amorphous
materials. Remarkably, this behavior occurs in spite of the well-defined
crystalline structure and relatively high electron mobility ($\sim 100
cm^2/Vs$). The dynamics of dopant ions and their interaction with the
polyhedral cages of the structure are a likely source of the strong phonon
scattering.
