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Abstract We report on the electronic structure of vanadium in synthetic V-oxides and in natural roscoelite (V-rich phyllosilicate). This study applied electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM), combined with first-principle calculations, to (1) establish relationships between the V oxidation state and EELS L2,3 features and (2) better constrain the oxidation state and crystallographic siting of V in roscoelite, with implications for other V-bearing phyllosilicates. Both EELS measurements and band structure calculations show that the EELS L2/L3 ratio increases as the oxidation state of V increases. We establish a quantitative relationship between the V L2,3 near-edge structure and the V oxidation state by normalizing the L2 maximum peak intensity to the L3 peak intensity. By applying this method to roscoelite, we find that it hosts a mix of trivalent and tetravalent V distributed between the octahedral and tetrahedral sites with a V4+/ΣV = 0.6 ± 0.1. This relationship is applicable to measurements of V oxidation states in oxide and phyllosilicate minerals, which is useful for constraining the conditions of rock and mineral formation and has potential implications for metal extraction from phyllosilicate ores. 

The detection of the fullerenes C60 and C70 in the interstellar medium (ISM) has transformed our understanding of chemical complexity in space. These discoveries also raise the possibility for the presence of even larger molecules in astrophysical environments. Here we report in situ heating of analog silicon carbide (SiC) presolar grains using transmission electron microscopy (TEM). These heating experiments are designed to simulate the temperature conditions occurring in post-AGB stellar envelopes. Our experimental findings reveal that heating the analog SiC grains to the point of decomposition initially yields hemispherical C60-sized nanostructures, with five- and six-membered rings, which transform into multiwalled carbon nanotubes (MWCNTs) if held isothermally >2 min. These MWCNTs are certainly larger than any of the currently observed interstellar fullerene species, both in overall size and number of C atoms. These experimental simulations suggest that such MWCNTs are likely to form in post-AGB circumstellar material, where the structures, along with the smaller fullerenes, are subsequently injected into the ISM. 

Understanding the fundamental mechanisms that underlie the synthesis of fullerene molecules in the interstellar medium (ISM) and in the environments of astrophysical objects is an open question. In this regard, using classical molecular dynamics, we demonstrate the possibility of in situ formation of fullerene molecules, such as C 60 from graphite, which is known to occur in the ISM, in particular, circumstellar environments. Specifically, when graphite is subjected to thermal and mechanical stimuli that are typical of circumstellar shells, we find that the graphite sheet edges undergo significant restructuring and curling, leading to edge-induced interlayer-interactions and formation of mechanically strained five-membered-ring structural units. These units serve as precursors for the formation of fullerene structures, such as pristine and metastable C 60 molecules. The pathways leading to molecular C 60 formation consist of a series of steps that involve bond-breakage and subsequent local rearrangement of atoms, with the activation energy barriers of the rate-limiting step(s) being comparable to the energetics of Stone–Wales rearrangement reactions. The identified chemical pathways provide fundamental insights into the mechanisms that underlie C 60 formation. Moreover, they clearly demonstrate that top-down synthesis of C 60 from graphitic sources is a viable synthesis route at conditions pertaining to circumstellar matter.