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

The effects of electron-beam damage on the Fe³⁺/ΣFe (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 × 10⁴ e/Ų, the Fe ³⁺/ΣFe 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 Fe ³⁺/ΣFe values >0.5. The critical fluence for radiation damage by 100 kV electrons as defined by Fe³⁺ /ΣFe <0.5 for cronstedtite at 300 K, is 1 × 10⁴ e/Ų. The absorbed dose to the speciman during acquisition of a typical EELS spectrum is large, with values around 2.2 × 10¹⁰ Gy (J/kg), equivalent to the deposition of 620 eV/ų. 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⁺/ΣFe >0.7. With increase in sample thickness, the Fe³⁺/ΣFe values decrease to a minimum, consistent with data from the undamaged material. The increase of Fe³⁺/ΣFe 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⁺/ΣFe values to be measured. The cronstedtite study illustrates the care required when using EELS to measure Fe³⁺/ΣFe 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.