Dangling bonds definition

In this paper, we presented new information, which should help in optimising disordered carbon materials for anodes of lithium-ion batteries. We clearly proved that the irreversible capacity is essentially due to the presence of active sites at the surface of carbon, which cause the electrolyte decomposition. A perfect linear relationship was shown between the irreversible capacity and the active surface area, i.e. the area corresponding to the sites located at the edge planes. It definitely proves that the BET specific surface area, which represents the surface area of the basal planes, is not a relevant parameter to explain the irreversible capacity, even if some papers showed some correlation with this parameter for rather low BET surface area carbons. The electrolyte may be decomposed by surface functional groups or by dangling bonds. Coating by a thin layer of pyrolytic carbon allows these sites to be efficiently blocked, without reducing the value of reversible capacity. 

Dalton s atomic theory 11 Dangling bond on Si(lOO) 18 Si(lll), on 13 DAS model 12—18 dc dropoff method 282 Decay constant definition 5... 

Taking the different arguments together, it is the author s opinion that the dangling bond model remains the more plausible explanation of the 2.0055 defect. Perhaps within a short time, further studies of the hyperfine interaction or calculations of the defect energy levels, etc. will be able to provide definitive proof one way or the other. In the remainder of this book, for the sake of definiteness, we refer to the 2.0055 ESR spin and the associated deep trap as the dangling bond, recognizing that the interpretation of electrical data involves only the gap state levels and the electron occupancy, not the atomic structure. 

Cluster model scheme and periodical method were used in the molecular model calculations of active sites of zeolite catalysts results of both approaches are presented and discussed in this review. In cluster models of zeolite structures hydrogen boundary atoms (H ) were used to saturate dangling bonds of the Si and Al atoms. Definite restrictions were imposed on the optimization of positions of these boundary H atoms. In the optimization, the geometry of an appropriate fragment of zeolite lattice was taken from the experimental X-ray diffraction data [7]. Only Si-H and Al-H bond distances were optimized, while the positions of other atoms (except M), as well as directions of 0-H bonds, were kept frozen. The M ion was allowed to move freely in the structure. 

As concerns the definition of the prototype molecule, at least two main cases can be distinguished. If the extended system consists of subsystems which are held together by nonbonded interactions (multipolar electrostatic forces, induction and dispersion interactions), it is possible to designate closed shell chemical subsystems without breaking covalent chemical bonds, and the prototype molecules can be the constituent molecules themselves. The situation becomes much more complicated, when the subunits are held together by covalent interactions, chemical bonds. In this case there are no natural frontiers for the representative subsystems, and it is unavoidable that a certain number of chemical bonds are broken. The prototype molecules are formed by saturating the broken (dangling) bonds by some appropriately selected atoms. 

The cage system is treated quantum mechanically. In the original version of the model all valence electrons were included and to allow a natural definition of the cage, orthogonalized atomic hybrid orbitals were used as a basis set [215]. This allows to avoid problems with the saturation of dangling bonds since all hybrids on the same atom may belong to the cage with a wave function obtained by solution of a closed-shell secular equation. 

With respect to standard molecular-cluster techniques, this approach has some attractive features explicit reference is made to the HF LCAO periodic solution for the unperturbed (or perfect) host crystal. In particular, the self-embedding-consistent condition is satisfied, that is, in the absence of defects, the electronic structure in the cluster region coincides with that of the perfect host crystal there is no need to saturate dangling bonds the geometric constraints and the Madelung field of the environment are automatically included. With respect to the supercell technique, this approach does not present the problem of interaction between defects in different supercells, allows a more flexible definition of the cluster subspace, and permits the study of charged defects. The perturbed-cluster approach is implemented in the computer code EMBEDOl [703] and applied in the calculations of the point defects both in the bulk crystal, [704] and on the surface [705]. The difficulties of this approach are connected with the lattice-relaxation calculations. 

We have noted in the case of elemental semiconductors that the saturation of the dangling bonds at the surface within the framework of a (1x1) unit ceU is to a large extent facilitated by the existence of a (nontrivial) atomic basis in this unit ceU (Si bilayers, basis of the Te crystal structure). In the case of a compound semiconductor, such a basis exists by definition, so that one may expect surface relaxations in a (1x1) periodicity. Nevertheless, since the preservation of bond lengths is an equally important factor in the energy minimization, in most cases reconstructed surfaces with superstmctures are observed, as we see in Section 4.4. 

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