INFLUENCE OF ONE-FOLD-COORDINATED ATOMS ON MECHANICAL-PROPERTIES OF COVALENT NETWORKS

One-fold-coordinated atoms in a covalent network mechanically soften a network if it is overcoordinated, and conversely, mechanically stiffe n the network if it is undercoordinated. They play no part in network rigidity if the network is optimally coordinated. These features follo w directly from constraint theory and provide a quantitative basis to understand the observed nanoindentation hardness of diamond and silico n carbide networks upon progressive hydrogenation when the contributio n of one-fold-coordinated atoms is taken into account explicitly.