Comparison of chain versus sheet crystal structures for cyanides $M$CN ($M$ = Cu-Au) and dicarbides $M$C$_2$ ($M$ = Be-Ba; Zn-Hg). Alternatives to graphene ?

TL;DR

This study predicts alternative sheet structures for cyanides and dicarbides, which are energetically competitive with known chain structures, suggesting potential new materials with metallic properties, though they are currently experimentally unobserved.

Contribution

It introduces novel sheet structures for cyanides and dicarbides, expanding the understanding of their possible crystal configurations beyond traditional chain forms.

Findings

Predicted sheet structures are energetically comparable or more stable than known chain structures.

Some predicted sheets may exhibit metallic behavior based on density of states.

Experimental geometries align well with theoretical predictions where data is available.

Abstract

The cyanides $M$CN, $M$=Cu, Ag, Au, have experimentally a structure with hexagonally packed, infinite -$M$-CN-$M$-CN- chains. Following our earlier study for AuCN, we now predict that all three $M$CN could have an alternative $M_3$C$_3$N$_3$ sheet structure of comparable energy with the known one. The valence isoelectronic systems $M$C$_2$ versus $M_3$C$_6$, $M$=Be-Ba; Zn-Hg are also studied. Now, the known dicarbides have the CaC$_2$ or MgC$_2$ chain structures. The predicted sheets lie energetically below the chains for $M$ = Zn, Cd, and Hg. All these systems are experimentally unknown. Indeed, they are clearly endothermic, compared to the elements. For some sheet structures the densities of states suggests rather small band gaps and even metallic character. When available, the experimental geometries agree well with the calculated ones for both cyanides and dicarbides.