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 [NH3NH2]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.