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 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 molecules. The pathways leading to molecular 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 formation. Moreover, they clearly demonstrate that top-down synthesis of from graphitic sources is a viable synthesis route at conditions pertaining to circumstellar matter.
C60-based molecular solids have shown promise as important
constituents in electrode materials as well as for providing interfacial
stability for solid state electrolytes in alkali ion batteries. At
room temperature, the solid state C60 crystal is characterized
by a face-centered cubic (FCC) structure, with large interstitial
voids, which can accommodate and promote ion transport. In this regard,
using density functional theory (DFT), we examined the diffusion of
lithium and sodium ions within the FCC C60 lattice. The
underlying diffusion mechanism for both ions consisted of motion between
interstitial tetrahedral and octahedral voids within the C60 lattice. Multiple energy minimum sites were located within each
interstitial void, and the diffusion of the ions involved jumps within
voids as well as between voids. The rate-limiting step for ion diffusion
corresponded to the motion between the tetrahedral and octahedral
voids, and the respective activation barriers were determined to be
0.34 eV for lithium and 0.28 eV for sodium. Importantly, the evaluated
activation barriers compare favorably with those of currently used
solid state electrolytes and electrode materials in alkali ion batteries.
These calculations provide insights into the diffusion mechanisms
of alkali ions in C60 lattices and should enable utilizing
C60 as important components for alkali-ion batteries.
The cluster variation method (CVM) is one of the thermodynamic models used to calculate phase diagrams considering short range order (SRO). This method predicts the SRO values through internal variables referred to as correlation functions (CFs), accurately up to the cluster chosen in modeling the system. Determination of these CFs at each thermodynamic state of the system requires solving a set of nonlinear equations using numerical methods. In this communication, a neural network model is proposed to predict the values of the CFs. This network is trained for the BCC phase under tetrahedron approximation for both ordering and phase separating systems. The results show that the network can predict the values of the CFs accurately and thereby Helmholtz energy and the phase diagram with significantly less computational burden than that of conventional methods used.
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