Mg K-XANES studies in magnesium halides

Abstract

A new experimental set-up to measure in-situ X-ray absorption spectra under ambient conditions in the 400–2000-eV photon-energy region using both transmission and total-fluorescence-yield detection techniques has been developed, and its utility has been demonstrated by studying optically switchable and amorphous metal thin films (Mg–TM, where TM=Ti, Mn, Co and Ni) in the solid state, as well as Al3+ and Br− ions in aqueous solution. The structural and electronic properties of the freshly co-sputtered Mg–TM thin films in metallic and hydride states were investigated by X-ray diffraction and Mg K- and TM L-edge X-ray absorption spectra. The optical switching of Mg–TM thin films from a mirror-like metallic state to an optically transparent state by exposure to 4% H2 in He is demonstrated by optical transmission and reflectivity in the 350–2500-nm region. The corresponding edge shifts in the Mg K- and TM L-edge X-ray absorption spectra indicate the presence of hydrogen as near neighbors, and its influence on the chemical states of metal thin films is demonstrated using Mg–Ni thin films as a representative example. Aluminum K-edge X-ray absorption spectra in the near-edge region were investigated to study the coordination of Al in crystalline and 0.1 M aqueous solutions of NaAlO2, Al(NO3)3 and Al2(SO4)3. Aluminum in NaAlO2 occupies both tetrahedral and octahedral sites unlike Al in Al(NO3)3 and Al2(SO4)3, where it retains its six-fold coordination irrespective of hydration. Bromine L-edge X-ray absorption spectra for crystalline and 0.1 M aqueous solutions of NaBr, KBr and KBrO3 were carried out to study the ligand-field splittings of core excitations which depend on inter-atomic distances, dissociation in aqueous solution, and the resulting hydration.