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 stiffen the network if it is undercoordinated. They play no part in network rigidity if the network is optimally coordinated. These features follow directly from constraint theory and provide a quantitative basis to understand the observed nanoindentation hardness of diamond and silicon carbide networks upon progressive hydrogenation when the contribution of one-fold-coordinated atoms is taken into account explicitly.