Characterization of the Local Titanium Environment in Doped Sodium Aluminum Hydride using X-ray Absorption Spectroscopy

摘要

Ti K-edge x-ray absorption spectroscopy was used to explore the local titanium environment and valence in 2-4 mol% Ti-doped sodium alanate. An estimate of the oxidation state of the dopant, based upon known standards, revealed a zero-valent titanium atom. An analysis of the near-edge and extended fine structures indicates that the Ti does not enter substitutional or interstitial sites in the NaAlH_4 lattice. Rather, the Ti is located onear the surface and is coordinated by 10.2±1 aluminum atoms with an interatomic distance of 2.82±0.01 A, similar to that of TiAl_3. The Fourier transformed EXAFS spectra reveal a lack of long-range order around the Ti dopant indicating that the Ti forms nano-clusters of TiAl_3. The similarity of the spectra in the hydrided and dehydrided samples suggests that the local Ti environment is nearly invariant during hydrogen cycling.