High-pressure routes to high oxidation states in fluorides and oxide fluorides

• Applicant: Professor Dr. Holger Kohlmann
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 545146977

Oxidation numbers are amongst the most basic and useful concepts of general chemistry. The quest for the highest possible oxidation states of transition metals has a long tradition in inorganic chemistry. It is is challenging existing boundaries of chemical composition space and driven by the idea of achieving new functionalities in terms of electrical or magnetic properties through new electronic ground states. The preparation of metal fluorides with high oxidation numbers of the metals often requires the use of elemental fluorine, which is extremely reactive and toxic. For technical and for safety reasons the attainable pressure and thus the maximum achievable chemical potential of fluorine µ(F) in a fluorination reaction is limited. We propose to vastly extend the limits of the chemical potential µ(F) in the synthesis of metal fluorides by using in situ fluorination with xenon difluoride under high pressure – high temperature conditions. We aim at transferring a technique developed for hydrogenation reactions in large-volume presses for the use in fluorination reaction. The sample will be sandwiched between or mixed with XeF2 and contained in a fluoride salt capsule (NaF, CaF2) and squeezed in a Kawai-type assembly on a large volume press up to 25 GPa and heated by a graphite resistance heater up to 1800 K. During the high-pressure experiment synchrotron powder diffraction data are collected in real time in order to follow the evolution of phases and develop strategies for their recovery to ambient conditions. One objective (A) is to establish high pressure fluorination reactions with XeF2 as new synthesis method for fluorides with metals in high oxidation numbers. The phase diagram of XeF2 under high pressure and high temperature will be mapped to establish the melting and decomposition temperatures under GPa pressures. Establishing reaction pathways to transition metal fluorides by in situ synchrotron experiments (objective B) will be tested on known fluorides like FeF4 or K2NiF4 as benchmark systems. Higher fluorides and oxide fluorides of Cr, Mn, Fe, Co, Ni, Cu, Pd are target compounds to push the current limits for oxidation states, since they show a large discrepancy between maximum possible and achieved oxidation number in fluorides. Once recovery strategies are known, larger quantities of all new phases will be synthesized for thorough characterization at ambient condition. Most experiments will be performed in large-volume presses at the synchrotron beamlines ID06 and P61B at the ESRF (Grenoble, France) and PETRA III (DESY, Hamburg, Germany), respectively. In situ fluorination using XeF2 will not only establish an alternative synthesis route to difficult to prepare or elusive compounds like FeF4, CoF4 and NiF4, but also pave the way for new fluorides with higher oxidation numbers, possibly hexafluorides of Cr, Mn, Fe, Pd, tetrafluorides of Co, Ni, Cu, or even higher fluorination degrees.

• DFG Programme: Research Grants
• International Connection: Sweden
• Cooperation Partner: Professor Dr. Ulrich Häussermann