Guest displacement in silicon clathrates

We study both theoretically and experimentally the structure of the doped silicon clathrate II NaₓSi₃₄. We find that contrary to published works, the sodium atoms do not retain the Td symmetry inside the Si₂₈ cages and move about 1 {} away from the center of the cage. This displacement, in conjunction with that of a sodium atom in an adjacent Si₂₈ cage, leads to a ``dimerization'' of sodium atoms. As a consequence, Rietveld refinements of x-ray diffraction spectra and transport, vibrational, and electronic properties must be revisited.