Al K Edge XANES Measurements in NaAlH₄ Doped with TiCl₃ by Ball Milling

Abstract

Al K edge X-ray absorption spectroscopy has been applied to investigate pure sodium alanate and sodium alanate doped with 5 mol % Ti on the basis of TiCl3 by ball milling. These first experimental data indicate that the chemical state of Al in NaAlH4 does not change upon cycling. It confirms also that the valence state of Al is higher than that of metallic Al in NaAlH4. Moreover, a change in the orbital occupancy of Al atoms or structural disorder in the local Al surroundings is occurring most probably after doping sodium alanate and subsequent cycling under hydrogen. Indeed, an evolution of the X-ray absorption near-edge structure (XANES) features can be observed with increasing number of cycles under hydrogen compared to the pure alanate sample. After several cycles, a new phase containing metallic Al is formed, which corresponds to the mixture of Al and the formation of small Ti−Al clusters already observed in Ti K edge measurements. These preliminary results suggest the presence of molecular-scale inhomogeneities within the average structure of sodium alanate.