Guest displacement in silicon clathrates

We study both theoretically and experimentally the structure of the doped silicon clathrate II ${\mathrm{Na}}_x{\mathrm{Si}}_{34}.$ We find that contrary to published works, the sodium atoms do not retain the $T_d$ symmetry inside the ${\mathrm{Si}}_{28}$ cages and move about 1 Å away from the center of the cage. This displacement, in conjunction with that of a sodium atom in an adjacent ${\mathrm{Si}}_{28}$ 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.