Si-29 NMR study on the stoichiometry of the silicon clathrate Na8Si46

The Si-29 magic angle spinning NMR spectrum of Na8Si46 exhibits three reson ances with large positive Knight shifts assigned to the inequivalent (16i, 6c, and 24k) Si sites in the structure. The integrated intensities are in t he ratio [15.7+/- 0.3]:[6.5+/-0.6]:[23.8+/-0.5]. Along with Rietveld refine ment of site occupancies,this result demonstrates that the compound is the stoichiometric clathrate Na8Si46, rather than an intermetallic Zintl compou nd containing silicon vacancies. The result is important for interpreting s uperconducting and thermoelectric characteristics of members of the tetreli de clathrate family. [S0163-1829(99)00842-5].