Polymeric forms of carbon in dense lithium carbide

TL;DR

This study uses first-principles calculations to demonstrate that polymeric carbon chains can be stabilized in dense lithium carbide under moderate pressure, revealing a metallic phase with potential for novel properties and a transition to an insulating network at very high pressures.

Contribution

It introduces a new pressure-induced phase of polymeric carbon in Li₂C₂, showing stabilization of one-dimensional carbon chains and their electronic properties through computational methods.

Findings

Polymeric carbon chains are stabilized in Li₂C₂ above 5 GPa.

The phase exhibits metallic character due to partially occupied lone-pair states.

A transition to an insulating carbon network occurs above 215 GPa.

Abstract

The immense interest in carbon nanomaterials continues to stimulate intense research activities aimed to realize 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 to suitably saturate the dangling bonds of carbon. Here, we show through first-principles calculations that ordered polymeric carbon chains can be stabilized in solid Li$_2$C$_2$ 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 \emph{sp}$^2$-like hybrids. It is found that this phase remains the most favorable one in a wide range of pressure. At extreme pressure (larger the 215 GPa) a structural and electronic phase transition towards an insulating single-bonded threefold-coordinated carbon network is predicted.