X-ray absorption spectroscopy studies of local structure and electronic properties of NaxSi136 (0 < x < 24) clathrates

Abstract

We have probed both the local structure and the electronic properties of Na-containing type II Si clathrates by x-ray absorption spectroscopy. The near-edge region of the spectrum is particularly sensitive to the local structure surrounding the Na atom. Through experimental investigations of a series of eight samples with general formula NaxSi136, with x ranging from similar to 0 to 21.5, and simulation of extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure spectra (XANES), we find that Na is primarily in the larger Si-28 cages at low loadings, and loss of the Na relative to full cage occupancy occurs preferentially from the smaller Si-20 cages. Our EXAFS results also show that Na is dynamically disordered in the Si-28 cages but less disordered at higher Na loadings. Local-orbital density of states calculations indicate that the Na in the Si-20 cage has a charge of 0.7e(-) while Na in the Si-28 cages has a slightly higher (loading-dependent) charge (0.72e(-) to 0.8e(-)).