Nanometer-scale measurements of Fe3+/ΣFe by electron energy-loss spectroscopy:: A cautionary note

The effects of electron-beam damage on the Fe3+/SigmaFe (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 cronstedfite. For fluences below 1 x 10(4) e/Angstrom(2), the Fe3+/SigmaFe values from crushed grains range between 0.43 and 0.49, consistent with undamaged material. These measurements were acquired from flakes 180 to 1000 A thick. With increase in fluence, samples <400 A thick become damaged and exhibit Fe3+/SigmaFe values >0.5. The critical fluence for radiation damage by 100 kV electrons as defined by Fe3+/SigmaFe <0.5 for cronstedfite at 300 K, is I X 101 e/Angstrom(2). The absorbed dose to the speciman during acquisition of a typical EELS spectrum is large, with values around 2.2 x 10(10) Gy (J/kg), equivalent to the deposition of 620 eV/Angstrom(3). Cooling to liquid N, 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+/SigmaFe >0.7. With increase in sample thickness, the Fe3+/SigmaFe values decrease to a minimum, consistent with data from the undamaged material. The increase of Fe3+/SigmaFe 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+/SigmaFe values to be measured. The cronstedfite study illustrates the care required when using EELS to measure Fe3+/SigmaFe 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.