Thermodynamics of hybrid manganese formate perovskites

Hybrid organic-inorganic materials, both dense and porous, have gained significant attention in recent years due to their extreme tunability in terms of compositions and functional properties. A deep understanding of their intrinsic stability is crucial to accelerate the discovery of new compositions that are not only functional but also thermodynamically stable. Here, we report the first systematic experimental study of the effect of A-site cations on the thermodynamic stability of a series of hybrid manganese formate perovskites [AH]Mn[HCOO]3 with AH+ = CH3NH3+, (CH3)2NH2+, (CH2)3NH2+, CH(NH2)2+, and C(NH2)3+ using acid solution calorimetry. Our studies show that the thermodynamic stability among these does not directly correlate with their tolerance factors, in contrast to trends seen among inorganic perovskites. On the other hand the enthalpy of formation correlates linearly with the enthalpy of dissolution in aqueous hydrochloric acid of the corresponding A-site cation salt, suggesting that the interactions between the A-site cation and the framework, rather than geometric factors, dominate the energetics of these perovskites. Thermodynamic stability of hybrid perovskites was found to be strongly influenced by the interaction between the A-site cation and the framework. Calorimetric measurements were able to estimate the relative strength of this interaction.