Polymeric forms of carbon in dense lithium carbide

The immense interest in carbon nanomaterials continues to stimulate intense research activities aimed at realizing carbon nanowires, since linear chains of carbon atoms are expected to display novel and technologically relevant optical, electrical and mechanical properties. Although various allotropes of carbon (e. g., diamond, nanotubes, graphene, etc) are among the best-known materials, it remains challenging to stabilize carbon in the one-dimensional form because of the difficulty of suitably saturating the dangling bonds of carbon. Here, we show through first-principles calculations that ordered polymeric carbon chains can be stabilized in solid Li2C2 under moderate pressure. This pressure-induced phase (above 5 GPa) consists of parallel arrays of twofold zigzag carbon chains embedded in lithium cages, which display a metallic character due to the formation of partially occupied carbon lone-pair states in sp(2)-like hybrids. It is found that this phase remains the most favorable one in a wide range of pressures. At extreme pressure (larger than 215 GPa) a structural and electronic phase transition towards an insulating single-bonded threefold-coordinated carbon network is predicted.