PROGRESS IN MODELING THE CHEMICAL BONDING IN TETRAHEDRAL AMORPHOUS-CARBON

The application of the reverse Monte Carlo method to modeling a covale ntly bonded amorphous material has been investigated, with the aim of generating a physically acceptable model for tetrahedral amorphous car bon (ra-C). Four different models, each containing approximately 3000 atoms, have been produced by fitting to experimental neutron diffracti on data and by applying various constraints consistent with prior chem ical and physical knowledge of the material. Particular attention has been paid to the development of coordination constraints that are a re alistic representation of the local bonding environments in the materi al. A sufficiently large model (with realistic chemical bonding) has b een produced for reliable comparison with experimental diffraction dat a and determination of medium-long range structural characteristics, e .g., clustering. The results show that better agreement with the exper imental 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 sma ll 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 prefere ntial bonding to a particular C atom site is revealed.