NMR studies of Na atoms in silicon clathrate compounds

We observed the NMR spectrum of ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ in the semimetallic clathrate (Na,Ba${)}_{\mathit{x}}$ ${\mathrm{Si}}_{46}$, which becomes superconducting at low temperatures (4 K). We also obtained spectra for isolated sodium atoms encapsulated in cages in the insulating clathrate ${\mathrm{Na}}_{\mathit{x}}$@${\mathrm{Si}}_{136}$. The observed NMR shifts in this phase are almost two times larger than the ordinary Knight shift in metallic sodium. We attribute this large paramagnetic shift to the fact that sodium atoms inside silicon clathrate cages are not ionized. Due to the interaction of unpaired sodium 3s electrons with silicon nuclei, we also observed very large shifts in the NMR spectrum of ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$. © 1996 The American Physical Society.