29 Si NMR study on the stoichiometry of the silicon clathrate Na 8 Si 46

The ${}^{29}\mathrm{Si}$ magic angle spinning NMR spectrum of ${\mathrm{Na}}_{8}{\mathrm{Si}}_{46}$ exhibits three resonances with large positive Knight shifts assigned to the inequivalent $(16i,$ $6c,$ and $24k)$ Si sites in the structure. The integrated intensities are in the ratio $[15.7\ifmmode\pm\else\textpm\fi{}0.3]:[6.5\ifmmode\pm\else\textpm\fi{}0.6]:[23.8\ifmmode\pm\else\textpm\fi{}0.5].$ Along with Rietveld refinement of site occupancies, this result demonstrates that the compound is the stoichiometric clathrate ${\mathrm{Na}}_{8}{\mathrm{Si}}_{46},$ rather than an intermetallic Zintl compound containing silicon vacancies. The result is important for interpreting superconducting and thermoelectric characteristics of members of the tetrelide clathrate family.