A NEW MODEL FOR TETRAHEDRAL AMORPHOUS-CARBON (TA-C)

We have developed the reverse Monte Carlo (RMC) modeling method to gen erate two physically acceptable models for tetrahedral amorphous carbo n (ta-C). The models have been produced by fitting to experimental neu tron diffraction data and by applying various constraints consistent w ith chemical and physical knowledge of the material, In particular, th ree different carbon atom types (tetrahedral, planar and linear) have been defined and a new approach to applying coordination number constr aints has been developed, The models were generated from an initial ra ndom distribution of 3000 atoms and from these we have determined ring number and coordination number distributions that are statistically a veraged over the whole structure, We are able to look in detail at the distribution of different types of carbon bonding sites within the bo x and at the effects of incorporating 5 at.% H into the structure. The new models show that atoms with sp2 bonds tend to cluster and form po lymer-like chains, which interlink the regions of sp3 or diamond-like bonding. Also, the relative flexibility of these polymeric chains resu lts in more porous, less dense regions of the network. A comparison wi th other models for ta-C is also made. (C) 1998 Elsevier Science B.V. All rights reserved.