Comparison of chain versus sheet crystal structures for the cyanides MCN (M=Cu-Au) and dicarbides MC2 (M=Be-Ba,Zn-Hg)

The cyanides MCN, 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 MCN could have an alternative M3C3N3 sheet structure of comparable energy with the known one. The valence isoelectronic systems MC2 versus M3C6, M=Be - Ba,Zn - Hg, are also studied. Now, the known dicarbides have the CaC2 or MgC2 chain structures, which are well reproduced. The predicted sheets lie energetically below the chains for M=Zn, Cd, and Hg. All these group-12 systems are experimentally unknown. Indeed, they are clearly endothermic, compared to the elements. For some sheet structures, the densities of states suggest rather small band gaps and even metallic character. When available, the experimental geometries agree well with the calculated ones.