Entropy is a critical, but often overlooked, factor in determining the relative stabilities of crystal phases.
The importance of entropy is most pronounced in softer materials, where small changes in free energy can
drive phase transitions, which has recently been demonstrated in the case of organic-inorganic hybrid-formate
perovskites. In this Rapid Communication we demonstrate the interplay between composition and crystal structure
that is responsible for the particularly pronounced role of entropy in determining polymorphism in hybrid
organic-inorganic materials. Using ab initio based lattice dynamics, we probe the origins and effects of vibrational
entropy of four archetype perovskite (ABX3) structures. We consider an inorganic material (SrTiO3), an A-site
hybrid-halide material (CH3NH3)PbI3, a X-site hybrid material KSr(BH4)3, and a mixed A- and X-site hybridformate
material (N2H5)Zn(HCO2)3, comparing the differences in entropy between two common polymorphs.
The results demonstrate the importance of low-frequency intermolecular modes in determining the phase stability
in these materials. The understanding gained allows us to propose a general principle for the relative stability of
different polymorphs of hybrid materials as temperature is increased.