Density functional study of the polymorphism of Cs2C2 and Rb2C2

By density functional theory-based calculations it is shown that in the athermal limit the orthorhombic polymorph of Cs2C2 is more stable by ≈7 kJ/mol with respect to the hexagonal modification, while the energy difference between the corresponding two Rb2C2-polymorphs is about 4 kJ/mol. The calculations do not corroborate the experimental finding of unusually long and short C-C bond lengths in Cs2C2 at low temperatures. This theoretical result is supported by calculations on monomeric LiCCH, where DFT calculations give all bond lengths within 1%.