186. Deformation of graphite lattices by interstitial carbon atoms

Coulson, C. A.; Senent, S.; Herráez, Marta; Leal, Mrs.M; Santos, Emilio

doi:10.1016/0008-6223(65)90252-6

Cited by 3 publications

(4 citation statements)

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“…Details of the results are given in table 3, where it can be seen that the structures appear to be a close match as well, including the Jahn-Teller distortion that lowers the symmetry of the defect from D 3h to C 2v on both the α and β sites. This is noteworthy owing to the fact that the effect has a quantum-mechanical origin [36], which is not included in the potentials. However, there is a serious problem with the AIREBO potential, which predicts that the splitvacancy structure (where an atom is located midway between two unoccupied lattice sites) has an energy that is 2.32 eV lower than the normal C 2v structure.…”

Section: Vacanciesmentioning

confidence: 99%

On the validity of empirical potentials for simulating radiation damage in graphite: a benchmark

Latham

McKenna

Trevethan

et al. 2015

J. Phys.: Condens. Matter

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In this work, the ability of methods based on empirical potentials to simulate the effects of radiation damage in graphite is examined by comparing results for point defects, found using ab initio calculations based on density functional theory (DFT), with those given by two state of the art potentials: the Environment-Dependent Interatomic Potential (EDIP) and the Adaptive Intermolecular Reactive Empirical Bond Order potential (AIREBO). Formation energies for the interstitial, the vacancy and the Stone-Wales (5775) defect are all reasonably close to DFT values. Both EDIP and AIREBO can thus be suitable for the prompt defects in a cascade, for example. Both potentials suffer from arefacts. One is the pinch defect, where two α-atoms adopt a fourfold-coordinated sp(3) configuration, that forms a cross-link between neighbouring graphene sheets. Another, for AIREBO only, is that its ground state vacancy structure is close to the transition state found by DFT for migration. The EDIP fails to reproduce the ground state self-interstitial structure given by DFT, but has nearly the same formation energy. Also, for both potentials, the energy barriers that control diffusion and the evolution of a damage cascade, are not well reproduced. In particular the EDIP gives a barrier to removal of the Stone-Wales defect as 0.9 eV against DFT's 4.5 eV. The suite of defect structures used is provided as supplementary information as a benchmark set for future potentials.

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Section: Vacanciesmentioning

confidence: 99%

On the validity of empirical potentials for simulating radiation damage in graphite: a benchmark

Latham

McKenna

Trevethan

et al. 2015

J. Phys.: Condens. Matter

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“…An 'ideal' lattice vacancy in graphite on either an α or β site possesses D 3h symmetry, with a threefold axis through its centre. This leaves it with a doubly degenerate 2 E σ state occupied by one electron, making the defect unstable against a Jahn-Teller distortion [101]. The predicted ground state of the defect adopts a structure with C 2v symmetry, where two of the three atoms neighbouring the vacancy form a bond between them, with the remaining atom on the mirror plane of the reconstructed defect (figure 3(a)).…”

Section: Lattice Vacanciesmentioning

confidence: 99%

“…If monovacancies in graphite are mobile at temperatures above about 200 • C, then it is anticipated that they will coalesce as pairs and higher complexes when they encounter one another, because this reduces the number of broken bonds [101]. A simple calculation shows that the energy gained by forming a pair must be of similar magnitude the cohesive energy of graphite [120].…”

Section: Lattice Vacancy Coalescencementioning

confidence: 99%

The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite

Latham

Heggie

Alatalo

et al. 2013

J. Phys.: Condens. Matter

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Models for radiation damage in graphite are reviewed and compared, leading to a re-examination of the contribution made by vacancies to annealing processes. A method based on density functional theory, using large supercells with orthorhombic and hexagonal symmetry, is employed to calculate properties and behaviour of lattice vacancies and displacement defects. It is concluded that annihilation of intimate Frenkel defects marks the onset of recovery in electrical resistivity, which occurs when the temperature exceeds about 160 K. Migration of isolated monovacancies is estimated to have an activation energy of E a ≈ 1.1 eV. Coalescence into divacancy defects occurs in several stages, with different barriers at each stage, depending on the path. The formation of pairs ultimately yields up to 8.9 eV energy, which is nearly 1.0 eV more than the formation energy for an isolated monovacancy. Processes resulting in vacancy coalescence and annihilation appear to be responsible for the main Wigner energy release peak in radiation-damaged graphite, occurring at about 475 K.

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“…The model also assumes that the interstitial atom is not bonded to the host crystal and generates an elastic deformation in the surrounding layers. 3,4 Thus, the model is constructed to yield the desired result, which Iwata et al take to be evidence for its correctness.…”

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confidence: 99%

Comment on “Increase in specific heat and possible hindered rotation of interstitialC2molecules in neutron-irradiated graphite”

et al. 2010

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Iwata and Watanabe's model for the observed low-temperature specific heat of neutron-irradiated graphite ͓T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 ͑2010͔͒ assumes that self-interstitial atoms exist as clusters of nearly free C 2 molecules. We suggest that their hypothesis is not supported by other experiments and theory, including our own calculations. Not only is it inconsistent with the long-known kinetics of interstitial prismatic dislocation loop formation, density-functional theory shows that the di-interstitial is covalently bonded to the host crystal. In such calculations no prior assumptions are made about the nature of the bonding, covalent or otherwise. DOI: 10.1103/PhysRevB.82.056101 PACS number͑s͒: 65.40.Ba, 61.72.JϪ, 61.80.Hg, 71.20.Ϫb A recent study by Iwata and Watanabe 1 showed that neutron irradiation of graphite produces an enhancement of the material's low-temperature specific heat. They conclude from these measurements that the hindered rotation of interstitial C 2 molecules is responsible for this effect. These are elegant, precise experiments which provide important evidence about the nature of such materials. Nevertheless, we profoundly disagree with the analysis of results that they put forward because it contains unjustified assumptions which can be refuted in a number of different ways, particularly with regard to the C 2 entities invoked.First, recent calculations reported by us, based on densityfunctional theory ͑DFT͒, using large supercells ͑with four graphite layers and up to 290 atoms͒ conclusively demonstrate that the binding energy between pairs of interstitial atoms to yield C 2 is large ͑3 eV͒, and hence their formation must be irreversible at the temperatures of irradiation cited by Iwata and Watanabe ͑333 K͒.2 It is clear from the covalently bonded structures illustrated in Ref. 2 that any migration path for C 2 is unlikely to exist with as low an energy as suggested by Iwata and Watanabe or that any free rotation could occur. Isolated self-interstitial atoms also form covalent bonds with the host crystal, according to our calculations, and in agreement with others. Certainly, none of the structures obtained give rise to c : a aspect ratios or formation volumes comparable with those inferred in their paper.Our calculations were not based on the assumption of a model, either covalent or noncovalent. They are only conjugate-gradient geometry optimizations starting from various initial positions for two C atoms placed between perfect graphitic planes, as in Iwata and Watanabe's C 2 diagram in their Fig. 5. No special distortions or atomic displacements were applied before optimization. There is nothing to prevent the formation of freely floating C 2 units, if this is a valid outcome. The optimization algorithm cannot traverse any energy barrier it experiences; it only proceeds downhill.However, in every case the system spontaneously reorganized into fully covalently bonded structures, integrated with the host lattice, either in the same layer or between layers. Inde...

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186. Deformation of graphite lattices by interstitial carbon atoms

Cited by 3 publications

References 0 publications

On the validity of empirical potentials for simulating radiation damage in graphite: a benchmark

On the validity of empirical potentials for simulating radiation damage in graphite: a benchmark

The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite

Comment on “Increase in specific heat and possible hindered rotation of interstitialC2molecules in neutron-irradiated graphite”

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