A stable and strongly ferromagnetic Fe17O10- cluster with an accordion-like structure

Isolated clusters are ideal systems for tailoring molecule-based magnets and investigating the evolution of magnetic order from microscopic to macroscopic regime. We have prepared pure Fe_n^- (n = 7-31) clusters and observed their gas-collisional reactions with oxygen in a flow tube reactor. Interestingly, only the larger Fe_n^- (n ≥ 15) clusters support the observation of O₂-intake, while the smaller clusters Fe_n^- (n = 7-14) are nearly nonreactive. What is more interesting is that Fe₁₇O₁₀^- shows up with prominent abundance in the mass spectra indicative of its distinct inertness. In combination with DFT calculations, we unveil the stability of Fe₁₇O₁₀^- within an interesting acordion-like structure and elucidate the spin accommodation in such a strongly ferromagnetic iron cluster oxide.

Oxidized micrometeorites suggest either high pCO2 or low pN2 during the Neoarchean

Tomkins et al. [A. G. Tomkins et al., Nature 533, 235-238 (2016)] suggested that iron oxides contained in 2.7-Ga iron micrometeorites can be used to determine the concentration of O₂ in the Archean upper atmosphere. Specifically, they argued that the presence of magnetite in these objects implies that O₂ must have been near present-day levels (∼21%) within the altitude range where the micrometeorites were melted during entry. Here, we reevaluate their data using a 1D photochemical model. We find that atomic oxygen, O, is the most abundant strong oxidant in the upper atmosphere, rather than O₂ But data from shock tube experiments suggest that CO₂ itself may also serve as the oxidant, in which case micrometeorite oxidation really constrains the CO₂/N₂ ratio, not the total oxidant abundance. For an atmosphere containing 0.8 bar of N₂, like today, the lower limit on the CO₂ mixing ratio is ∼0.23. This would produce a mean surface temperature of ∼300 K at 2.7 Ga, which may be too high, given evidence for glaciation at roughly this time. If pN₂ was half the present value, and warming by other greenhouse gases like methane was not a major factor, the mean surface temperature would drop to ∼291 K, consistent with glaciation. This suggests that surface pressure in the Neoarchean may need to have been lower-closer to 0.6 bar-for CO₂ to have oxidized the micrometeorites. Ultimately, iron micrometeorites may be an indicator for ancient atmospheric CO₂ and surface pressure; and could help resolve discrepancies between climate models and existing CO₂ proxies such as paleosols.