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

Abstract

The local structure and electronic properties were probed of Na-contg. 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 exptl. studies of 8 samples NaxSi136, with x ranging from ∼0 to 21.5, and simulation of extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure spectra (XANES), Na is primarily in the larger Si28 cages at low loadings, and loss of the Na relative to full cage occupancy occurs preferentially from the smaller Si20 cages. The EXAFS results also show that Na is dynamically disordered in the Si28 cages but less disordered at higher Na loadings. Local-orbital d. of states calcns. indicate that the Na in the Si20 cage has a charge of 0.7e- while Na in the Si28 cages has a slightly higher (loading-dependent) charge (0.72e- to 0.8e-).