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 stoichio metry is observed. In NaxSi136, systematic changes in X-ray diffraction int ensities enable the Na content and site occupancy to be characterized. In t he same structure, we observe a similar to 0.5% increase in the unit cell e dge upon progressing from Na4Si136 to Na23Si136. A statistical mechanical m odel, combined with experimental data for this phase reveals a preference f or the removal of sodium from the smaller of the two available cages by 0.1 90 +/- 0.050 eV. (C) 1999 Academic Press.