Functional hybrid organic-inorganic formate-based frameworks

Abstract

This thesis is focused on the study of formate hybrid organic-inorganic materials, in particular on the ammonium and hydrazine formate compounds. During our studies we investigated the behaviour of these materials, both from the chemical and physical point of view. We developed a new method for the synthesis of the [NH4][M(HCOO)3] family with M = Co2+, Mn2+, Zn2+, Ni2+, Cu2+ based on the mechanochemical approach. We found a correlation between the milling time and the thermal stability of the di-hydrated metal formates used as regents, indicating that the mechanochemical process is triggered by the dehydration of the metal formate. Moreover, PXRD measurements performed on the milled powders demonstrated the presence of the desired phase with hexagonal symmetry and the absence of secondary products. The mechano-synthetized [NH4][M(HCOO)3] compounds show an excellent crystallinity as also revealed by SEM analysis. Concerning the [NH2NH3][M(HCOO)] formate family with M = Mn2+, Fe2+, Co2+ , we discovered and studied the transition between the chiral (crystal structure featured by hexagonal open channels ) and perovskite polymorphs, which is mediated by the absorption of humidity. In this thesis, we also studied for the first time the ionic conduction behaviour of these two classes of materials. The results show that the process is triggered by the absorption of humidity by the polycrystalline samples and involves the acid organic cation present in the structure. Thus, the synthesis of CsMn(HCOO)3 has been crucial to confirm the ionic conduction behaviour of the ammonium and hydrazine formate materials. Indeed, this compound is isostructural with the hexagonal porous phases with NH4+ and NH2NH3+ but it is not characterized by ionic conduction confirming the role played by organic cations in defining such complex conduction mechanism. Moreover, for CsMn(HCOO)3 we detected a low temperature transition not already known in literature. The low temperature PXRD and ED data show weak extra peaks, probably related to a structural modulation. The structural determination of this low temperature distorted structure for CsMn(HCOO)3 requires further structural investigations.