Role of entropic effects in controlling the polymorphism in formate ABX 3 metal–organic frameworks

Polymorphism in formate-based dense metal–organic frameworks with the general formula ABX 3 is predicted by quantum chemical calculations and confirmed experimentally. In particular [NH 3 NH 2 ]Zn(HCOO) 3 crystallizes in two different polymorphs, a perovskite-like framework and a chiral structure with hexagonal channels. A detailed thermodynamic analysis reveals that both structures are very close in free energy and that entropy driven effects are responsible for stabilizing the channel structure.