Abstract
First-principles calculations have been employed to examine the possible use of electron energy loss spectroscopy (EELS) as a tool for determining the presence of OH groups and hence hydrogen content in compounds. Our density functional theory (DFT) based calculations describe accurately the experimental EELS results for forsterite (Mg2SiO4), hambergite (Be2BO3(OH)), brucite (Mg(OH)2) and diaspore (α-AlOOH). DFT calculations were complemented by an experimental time resolved study of the oxygen K-edge in diaspore. The results show unambiguously that there is no connection between a pre-edge feature in the oxygen K-edge spectrum of diaspore and the presence of OH groups in the structure. Instead, the experimental study shows that the pre-edge feature in diaspore is transient. It can be explained by the presence of molecular O2, which is produced as a result of the electron irradiation.
Highlights
(Some figures may appear in colour only in the online journal). It was first claimed by Wirth (1997) that the presence of hydrogen bound to oxygen could be detected in compounds by electron energy loss spectroscopy (EELS) through the observation of a pre-edge feature at energies of ≈528 eV
Time resolved EELS studies seem to indicate that under electron irradiation in the TEM, a transient peak near 530 eV may be observed in H-bearing as well as anhydrous minerals. This peak has been associated with an electron transition in molecular O2, which is thought to be liberated due to damage induced by the incident electron beam (Jiang 2006, Garvie 2010)
The EEL spectra were acquired with a FEI Tecnai F30 transmission electron microscope equipped with a Gatan image filter (GIF)
Summary
It was first claimed by Wirth (1997) that the presence of hydrogen bound to oxygen could be detected in compounds by electron energy loss spectroscopy (EELS) through the observation of a pre-edge feature at energies of ≈528 eV. This interpretation of the electron energy loss (EEL) spectra was immediately challenged (van Aken et al 1998, Wirth 1998). After discussing the reliability of our approach, we compute the oxygen K-edge spectrum of diaspore, α-AlOOH, and complement the modeling study by the experimental determination of time resolved EEL spectra of diaspore. The rest of the settings (exchange–correlation functional, energy cut-off, k-point sampling) were the same as for the solid state calculations; instrumental broadening for the single molecule EELS calculation was set at Gaussian FWHM of 0.1 eV
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