Preparation, structure determination and physical properties of formate-based molecular compounds

Orateur : Soutenance de Thèse de Lidia MAZZUCA

The synthesis and the characterization of new functional materials are key challenges in chemistry and physics. In particul ar, metal–organic frameworks (MOFs) with two or more coupled functionalities are still rare and very attractive candidates because of their wide variety of properties and promising applications interesting in many disciplines. The impact of the development of new fascinating materials on our day life might be considerable. This is also one of the reason explaining the intense increase of research in material science and condensed matter physics and chemistry. This thesis is focused on the synthesis and the physical characterization of magnetic metal formate frameworks using the combination of neutron and synchrotron X-Ray diffraction as well as other techniques. Metal-formate compounds belong to a specific subgroup of metal-organic frameworks where the metal centres are linked by the formate molecules to form an anionic framework. The negative charge of the framework is balanced by a counter-cation which is located in the cavities of the framework. In this thesis, the counter-cation molecules are constituted by protonated amines. Typically, these compounds are synthesized by reacting formate or formic acid with a metal salt under solvo-thermal conditions or by slow evaporation or diffusion techniques. In this work, I have investigated the crystal structure, phase transitions and magnetic properties of two families of metal formate frameworks. The first family is represented by hetero-metallic or mixed-valence compounds adopting a niccolit e-like structure, and the second family is constituted by homo-metallic compounds adopting a perovskite-like (ABX3) structure.The compounds synthesized and characterized in this thesis are : [(CH3)2NH2][FeIIIMII(HCOO)6] (MII = MgII, MnII, FeII, CoII and NiII), [(CH3CH2NH3][FeIIIFeII(HCOO)6], [(CH3NH3)[M(HCOO)3] (MII = CoII, MnII, FeII, NiII and CuII), and [(NH4)[Mn(HCOO)3]. The choice of using specific metal ions has been motivated by their different electronic configuration and therefore different physical behaviours, i.e. a large difference in the magnetic anisotropy is well known among the different divalent ions used in this study. Beside the effects on the properties when different divalent metal ions are introduced within the framework, the effects of the nature of the counterions has also been investigated. Even though there is not a clear correlation between the selected counterion and the change of the magnetic behaviour, neutron diffraction allows elucidating the differences in the crystal and in the magnetic structure when different counterions are used. Moreover, the work done in the thesis has helped us to interpret the bulk magnetometry measurements in the light of our results obtained with neutron diffraction, which is a complementary and more precise technique. A variety of magnetic phenomena, such as ferromagnetic behaviour, antiferromagnetic ordering and spin canting, have been observed in the studied compounds. Furthermore, from the crystallographic point of view, different kind of phase transitions were detected involving, for instance, the order-disorder of the counter-ion as in [(CH3)2NH3][FeIIIFeII(HCOO)6] or [(NH4)[Mn(HCOO)3], or the occurrence of modulated crystal structures, as for example in the [(CH3NH3)[M(HCOO)3] compounds. Keywords : Neutron diffraction, X-ray diffraction, Metal‑organic frameworks, Magneto‑electric compounds, Antiferromagnetism.

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