Comparison of neutron-scattering data for tetrahedral amorphous carbon with structural models

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Abstract
Several physical models are compared with the experimental reduced radial distribution function G(r) derived from high-resolution pulsed-neutron-scattering data for tetrahedral amorphous carbon. These models include tetrahedral models for amorphous silicon in addition to models constructed specifically to model high-density amorphous carbon. While the tetrahedral models provide better overall agreement than some amorphous carbon models containing threefold sites, no model provides an adequate level of agreement, particularly in the breadth of the first-neighbor distribution. The second-neighbor distribution of some of the models is found to be asymmetric, and this removes the inconsistency found between the measured first and second coordination numbers. Refinement of several models is attempted using a modified form of the reverse Monte Carlo algorithm, and some are successfully refined without topological alterations. For the tetrahedral models, introduction of pairs of threefold sites is found to produce structures in very good agreement with the experimental G(r). The high levels of elastic strain in the best models suggests that a high strain energy is an essential property of tetrahedral amorphous carbon.

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