The role of chemical disorder and structural freedom in radiation-induced amorphization of silicon carbide deduced from electron spectroscopy and ab initio simulations

Abstract

Chemical disorder has previously been proposed as an explanation for the anomalously facile amorphization of silicon carbide (SiC), on the basis of topological connectivity arguments alone. In this exploratory study, "amorphous" (formally, aperiodic) SiC structures produced in ab initio molecular dynamics simulations were assessed for their connectivity topology and used to compute synthetic electron energyloss spectra (EELS) using the ab initio real-space multiple scattering code FEFF. The synthesized spectra were compared to experimental EELS spectra collected from an ionamorphized SiC specimen. A threshold level of chemical disorder χ (expressed as the ratio of the number of carbon-carbon bonds to the number of carbon-silicon bonds) was found to be χ ≈ 0.38, above which structural relaxation resulted in formally aperiodic structures. Different disordering methodologies resulted in identifiably different aperiodic structures, as assessed by local-cluster analysis and confirmed by collecting near-edge electron energy-loss spectra (ELNES). Such structural differences are predicted to arise for SiC crystals amorphized by irradiations involving different damage mechanismsand therefore differing disordering mechanisms-for example, when contrasting the respective amorphized products of ion irradiation, neutron irradiation, and high-energy electron irradiation. Evidence for sp 2-hybridized carbon bonding is observed, both experimentally in the irradiated sample and in simulations, and related to connectivity topology-based models for the amorphization of silicon carbide. New information about the probable intermediate-range structures present in amorphized silicon carbide is deduced from enumeration of primitive rings and evolution of local cluster configurations during the ab initio-modelled amorphization sequences.

Radiation Effects in the Crystalline–Amorphous SiOC Polymer-Derived Ceramics: Insights from Experiments and Molecular Dynamics Simulation

“…Based on the formalism raised by Gupta and Cooper, 47 the connectivity in crystalline SiC yields a structure freedom parameter value of f = −3, where f is the difference between the degrees of freedom available to a given polytope (in this case, a SiC 4 tetrahedron) and the constraints on that unit by its connections to neighboring units. 48 The f is slightly <0 in graphite. 49 In comparison, the β-SiC structure is far more constrained.…”

“…In graphite, C atoms in the graphene layer are bonded covalently and arranged in a hexagonal structure, with three of the four potential bonding sites satisfied, and the bonding between graphene layers is via weak van der Waals bonds. Based on the formalism raised by Gupta and Cooper, the connectivity in crystalline SiC yields a structure freedom parameter value of f = −3, where f is the difference between the degrees of freedom available to a given polytope (in this case, a SiC 4 tetrahedron) and the constraints on that unit by its connections to neighboring units . The f is slightly <0 in graphite .…”

The effect of irradiation with swift heavy ions on the structural and morphological properties of beryllium oxide ceramics

“…The main requirement for structural materials is to ensure stability of working characteristics, such as thermal conductivity, electrical resistance, high radiation and corrosion resistance to external influences. When ceramics are irradiated with ionizing radiation, in particular by heavy ions, an enormous number of defects appear in their structure, most of which annihilate in a very short time (10 −14 -10 −12 s) as a result of defects radiation annealing [6,7]. However, the remaining part of defects can lead to significant change in physicochemical, structural, and optical properties of the ceramics by creating cascades of secondary defects and displaced atoms.…”

EELS Evidence for Nascent Polymerization of Carbon and Silicon in Amorphization of SiC

“…1). The feature of interest was the peak at ~284 eV loss, attributable to the π* transition expected from sp 2 bond hybridization found in 3-coordinated C atoms with co-planar bond angles of ~120˚, as found for example in graphite [8]. This spectral feature was found in the ion-amorphized regions of the experimental sample, in the melt-quenched and relaxed ab initio modeled assembly, and in the atom-swap ab initio modeled and relaxed assemblies with χ > 0.30 (the χ = 0.54 atom-swap assembly spectrum is illustrated in Fig.…”

“…The present contribution explores the use of ab initio molecular dynamics (MD) computational modeling (VASP, 216 atoms with periodic boundary conditions, thermostatted in the canonical assembly, with a 1 fs timestep [8]. Random atom swaps were made in an initial 3C β-SiC crystalline assembly to induce initial disorder parameters from χ o = 0.09 to χ o = 0.58, which after relaxation resulted in disorder parameters from χ= 0.09 to χ = 0.43.…”