Exploring Biases in Atom Probe Tomography Compositional Analysis of Minerals

Abstract

Atom probe tomography (APT) is emerging as a key nanogeochemistry technique for diverse geoscience applications. Estimating stoichiometric mineral compositions is particularly challenging for features at nanoscale. APT provides reliable measurements for metallic systems but the reliability for oxides is problematic, notably due to oxygen deficit. Here, we use laser-assisted APT to compare results for spinel and garnet crystals. APT compositional results were compared with those by electron microprobe to determine the possible APT analytical inaccuracy. Extensive data processing was accomplished, including correlation histograms, 2D ion distribution maps and 1D elements concentration profiles, to disclose the possible mechanisms leading to mineral stoichiometry biases. Multiple events and neutral molecules formation are probable the main processes responsible for atom deficit. In particular, the amount of the same isotope-same charge state ion pairs correlated with aluminium and oxygen deficits suggests that the co-evaporation in a dead space-dead time window could lead to a significant decrease of detected ions. Also, molecular species dissociation and direct current evaporation could partially account for further atom loss. Overall, better APT compositional estimation was obtained for spinel, which has lesser variation in lattice sites and greater overall lattice symmetry, higher thermal conductivity and lower band gap compared with garnet.

Atom Probe Tomography Analysis of Mica

“…Mg appears always in excess. This behavior has been already observed during spinel and garnet APT characterization (Cappelli et al, 2021) and for Mg oxide at specific laser pulse energy conditions (Devaraj et al, 2013). This reveals a higher apparent APT efficiency in Mg detection or, conversely, the deficit of other structural elements of the mineral, which bias the estimate of the chemical composition.…”

Quantitative Mapping of Chemical Defects at Charged Grain Boundaries in a Ferroelectric Oxide

“…It is well-known for APT that some elements have a lower detection efficiency due to molecular dissociation, generating neutral species or preferential evaporation. [30] Interestingly, a change in composition is detected in the lower part of the needle, which coincides with the location of the grain boundary as measured by TEM (Figure S5A, Supporting Information). A zoom-in to the region of interest is presented in Figure 3C, where the grain boundary is visualized through an isosurface, highlighting regions with increased Er atomic density.…”

Atomic spectrometry update – a review of advances in environmental analysis

“…The study also demonstrated that APT could be applied more broadly to other hydrous mineralogies. Cappelli et al 289 investigated the problems of atom loss and inaccurate estimates of stoichiometric composition when applying laser-assisted APT to garnet and spinel. By studying oxygen quantification and issues related to uneven ion desorption and variation in charge state ratios, a better understanding was obtained of how measured and true mineral stoichiometries diverged due to the influence of mineral properties and crystal structure on the atom probe field evaporation process.…”