Progress in modeling the chemical bonding in tetrahedral amorphous carbon

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Walters, J.K. ; Gilkes, K.W.R. ; Wicks, James D ; Newport, Robert J. (1998)

The application of the reverse Monte Carlo method to modeling a covalently bonded amorphous material has been investigated, with the aim of generating a physically acceptable model for tetrahedral amorphous carbon (ra-C). Four different models, each containing approximately 3000 atoms, have been produced by fitting to experimental neutron diffraction data and by applying various constraints consistent with prior chemical and physical knowledge of the material. Particular attention has been paid to the development of coordination constraints that are a realistic representation of the local bonding environments in the material. A sufficiently large model (with realistic chemical bonding) has been produced for reliable comparison with experimental diffraction data and determination of medium-long range structural characteristics, e.g., clustering. The results show that better agreement with the experimental data is achieved if the model is allowed to include three- and four-membered rings, and that atoms with sp(2) bonds tend to form small clusters and polymerlike chains interlinking regions of sp(3) or ''diamondlike'' bonding. The inclusion of 5 at. % hydrogen results in a homogeneous distribution of H atoms throughout the network; no preferential bonding to a particular C atom site is revealed.
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