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Fe by electron energy-loss spectroscopy: A cautionary note
1 Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287-1404, U.S.A.
2 Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, U.S.A.
3 Department of Physics and Astronomy, Arizona State University, Arizona 85287-1504, U.S.A.
Correspondence: * E-mail: lgarvie{at}asu.edu
The effects of electron-beam damage on the Fe3+/
Fe (total iron) ratio were measured by electron energy-loss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra were acquired from crushed and ion-beam-thinned cronstedtite. For fluences below 1 x 104 e/Å2, the Fe3+/Fe values from crushed grains range between 0.43 and 0.49, consistent with undamaged material. These measurements were acquired from flakes 180 to 1000 Å thick. With increase in fluence, samples <400 Å thick become damaged and exhibit Fe3+/Fe values >0.5. The critical fluence for radiation damage by 100 kV electrons as defined by Fe3+/Fe <0.5 for cronstedtite at 300 K, is 1 x 104 e/Å2. The absorbed dose to the speciman during acquisition of a typical EELS spectrum is large, with values around 2.2 x 1010 Gy (J/kg), equivalent to the deposition of 620 eV/Å3. Cooling to liquid N2 temperature did not significantly slow the damage process. Ion-beam thinning produces an amorphous layer on crystal surfaces. Spectra from the thinnest regions, which are amorphous, exhibit Fe3+/Fe >0.7. With increase in sample thickness, the Fe3+/Fe values decrease to a minimum, consistent with data from the undamaged material. The increase of Fe3+/Fe with respect to electron-beam irradiation is likely caused by loss of H. At low fluences, the loss of H is negligible, thus allowing consistent Fe3+/Fe values to be measured. The cronstedtite study illustrates the care required when using EELS to measure Fe3+/Fe values. Similar damage effects occur for a range of high-valence and mixed-oxidation state metals in minerals. EELS is the only spectroscopic method that can be used routinely to determine mixed-valence ratios at the nanometer scale, but care is required when measuring these data. Consideration needs to be given to the incident beam current, fluence, fluence rate, and sample thickness.
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