Synthesis and X-Ray Characterization of Silicon Clathrates

We report on the synthesis and characterization of two silicon clathrates, Na8Si46 and NaxSi136(x=4–23), by powder X-ray diffraction, combined with Rietveld profile analysis. In Na8Si46, no deviation from the ideal stoichiometry is observed. In NaxSi136, systematic changes in X-ray diffraction intensities enable the Na content and site occupancy to be characterized. In the same structure, we observe a ~0.5% increase in the unit cell edge upon progressing from Na4Si136 to Na23Si136. A statistical mechanical model, combined with experimental data for this phase reveals a preference for the removal of sodium from the smaller of the two available cages by 0.190±0.050eV.