We studied five alkaline earth dicarbide systems $M_{AE}{\text{C}}_2$ (where $M_{AE}=\text{Be-Ba}$) by using ab initio random structure search. For ${\text{BeC}}_2$ and ${\text{MgC}}_2$, the lowest energy and dynamically stable configuration consists of five-membered carbon rings connected to each other via an individual carbon atom, stabilized through the donation of electrons from the surrounding alkaline earth ions. For ${\text{CaC}}_2$, ${\text{SrC}}_2$, and ${\text{BaC}}_2$, our study shows that the chain crystal structure is more stable than the predicted structure due to strains induced by the increasing size of alkaline earth metal ions. The reaction energies of the typical synthesis pathway are comparable to those calculated for the experimental reaction of the known chain-type structure. Finally, the proposed structure should be optically distinguishable due to a significantly narrower band gap.

• Received 5 July 2010

DOI:https://doi.org/10.1103/PhysRevB.82.125439