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Thermischer Transport in inorganisch-organischen Gerüstverbindungen

Antragsteller Dr. Gregor Kieslich
Fachliche Zuordnung Physikalische Chemie von Festkörpern und Oberflächen, Materialcharakterisierung
Förderung Förderung von 2014 bis 2016
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 253095348
 
Erstellungsjahr 2016

Zusammenfassung der Projektergebnisse

The ultimate goal of materials science is to design of materials with distinct physical properties. For purely inorganic materials, there are guidelines to explain the stability and formation of certain structural features. In organic-inorganic hybrids, however, the range of different chemical interactions and bonds makes crystal engineering a more complex challenge. This was therefore the focus of the conducted research project. Emphasis was laid in particular on the large family of hybrid materials with a perovskite architecture, which exhibit a wide range of fascinating properties such as ferroelectricity, ferroelasticitiy, antiferromagnetic coupling, semiconductivity and glassy phase transitions. In such ABX3 materials, the (transition) metal B and the linker X form a ReO3-like framework. The cation A, often a protonated amine, is then located within the open void of the cavity to form the perovskite-structure motif. The large variation, due to A, B or X site substitution, permits the manipulation of many properties in such materials and opens a wide range of opportunities for various research areas. In order to deepen our understanding of the crystal chemistry of ABX3 hybrid perovskites, the building principles of these compounds were investigated from the outset. By the application of well-established solid-state principles to hybrid perovskites, strong relationships to their inorganic counterparts were identified. Speficially, we estimated ionic radii for organic cations and molecular anions. These were then used to modify Goldschmidt’s concept of ionic Tolerance Factors (TFs), which has been a central mantra in the search for new perovskite oxides. The sound agreement between the TFs and experimental observations suggests that, similarly to solely inorganic ions, density plays an important role in the formation of hybrid perovskites. Based on these findings, a TF screening study of potential amine-metal-anion combinations was performed, pointing at the possible existence of over 600 undiscovered organic-inorganic perovskites (Kieslich et al. Chem. Sci. 2015). The limitations of the TF concept were then studied using lattice dynamic calculations. In hybrid materials, the variety of different bond strengths intensifies the impact of entropy and leads to a relatively shallow energy surface. For instance, vibrational entropy related to hydrogen bonds leads to the occurrence of different structure types in the series [NH3NH2]M(HCOO)3 with M = Zn2+, Mg2+, Mn2+ and Co2+. By using state-of-the-art lattice dynamic calculations, vibrational entropy was identified as a driving factor for polymorphism in [NH3NH2]Zn(HCOO)3. The role of hydrogen bonds between the protonated amine [NH3NH2]+ and the metal-formate cavity in [Zn(HCOO)3]- was then analysed using 1H magic angle spinning nuclear magnetic resonance spectroscopy. We could show that salt bride-like interactions, similar to helical polymers and proteins, are present in [NH3NH2]Zn(HCOO)3. These interactions significantly strengthen the material and lead to enhanced mechanical properties, which have been probed using nanoindenation and high pressure powder X-ray diffraction. I have established a profound understanding of the crystal chemistry of hybrid perovskites, which include formate-based frameworks and metal-halide hybrids. The research project benefited from the availability of first-class synchrotron facilities and the collaboration with C. P. Grey and A. Walsh. The positive response from the physics and chemical community fortifies our belief, that hybrid frameworks with a perovskite-structure will have a prominent influence on materials science in the future.

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