Design of Geometric Degrees of Freedom in ReO3-type Coordination Polymers

• Applicant: Dr. Gregor Kieslich
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 493871295

Material design is the challenge to rationally synthesise compounds with desired physicochemical properties. For controlling stimuli responsive properties in crystalline materials, a successful strategy is to understand and chemically engineer energies of structural distortions (structural degrees of freedom) so that they are activated by external stimuli. Canonical examples where such distortions play an important role are collective movements of rigid polyhedrons in ZrW2O8 that cause negative thermal expansion and the coupled, temperature activated displacement of cations in the ferroelectric PbTiO3. While some structural degrees of freedom are exciting on their own, it is the design of their interplay which exhibits most promising opportunities for materials with improved, new, and unexpected properties. Therefore, the identification of new structural degrees of freedom and the study of their interplay is in the centre of active research in material development.In this pursuit crystalline coordination polymers are an important material platform. The use of molecules to assemble frameworks introduces new structural degrees of freedom of rotational, conformational, and translational nature. ABX3 molecular perovskites harbour many of new distortion types in one material class, presenting an intriguing starting point for the development of new material design principles. Looking for opportunities to introduce new structural degrees of freedom in molecular perovskites, we can note that all current distortion modes emerge due to the use of molecular building units per se rather from their targeted chemical incorporation.The goal of this research proposal is to integrate a new kind of structural degree of freedom – geometric in nature – in ReO3-type coordination networks. I propose to harness the chemical diversity of coordination polymers to incorporate multivalent A-site cations with spatially separate charge centres into a 3D [BX3]- network. The spatial order of multivalent A-site cations that bridge several pseudocubic ReO3-type cages is introduced as a new geometric degree of freedom while keeping all existing distortion types known from molecular perovskites. The proposal aims to chemically control A2+ and A3+ order pattern, explores their interplay with other distortion types such as octahedral tilt pattern and Jahn-Teller distortions and investigates their impact on the crystal chemistry as a function of temperature and pressure changes. The proposed work will deliver a new material class with the general formula An+(BX3)n and advances the concept of using structural degrees of freedom for rational materials design – in ReO3-type coordination networks and beyond.

• DFG Programme: Research Grants