The optical, magnetic, and electronic properties of functional solids, such as oxides or fluorides, are governed by a delicate interplay between atomic arrangement (crystal structure), chemical bonding, and electronic properties of the involved transition metals. In the FLI-AVAI project, we study a special group of fluorides that are not previously investigated in great detail due to challenges with preparation (synthesis) and handling (air sensitivity). The studied fluorides have as a common denominator that they contain transition metals with the special d4 or d9 electron configurations, which induce a structural Jahn-Teller deformation and thereof interesting physical properties. Similar electron configurations are found in important oxides, based on Mn(III) and Cu(II), which display properties such as magnetoresistance and superconductivity. The FLI-AVAI project is focused on fluoroperovskites of Cr(II) and Cu(II) with Jahn-Teller active electron configurations. We have developed a new synthesis method that allows access to these compounds in large quantities, which therefore enables the project. We will study how chemical modifications of the compounds, and how external stimuli, as temperature, magnetic field, and pressure, influence the crystal structure and properties.
The first period of the project is completed at the Paul Scherrer Institute (PSI) in Switzerland. This has laid the foundation for several neutron scattering experiments at the Swiss neutron source at PSI to understand the ordering of magnetic moments in Cr(II) and Cu(II) fluoroperovskites. The work is also integrated with studies of the atomic arrangements at the European synchrotron radiation facility. These have given insight into the dynamical behavior of ammonium in Cr(II) og Cu(II) fluoroperovskites and structural changes in the atomic arrangement under high pressures. A new collaboration with the University of Bern for the synthesis of single crystals of Cu(II) fluoroperovskites is also established. Neutron spectroscopy experiments, in collaboration with École Polytechnique Fédérale de Lausanne, are planned to further our insight into the dynamical magnetic behavior of the single crystals at low temperatures.
The primary purposes of this project are to develop strong skills in advanced neutron and synchrotron techniques through a mobility stay at the Swiss Spallation Neutron Source (SINQ) at the Paul Scherrer Institut (PSI), and to use a battery of sharp methods to investigate unresolved aspects of Jahn-Teller distorted Cr(II) and Cu(II) fluoroperovskites. The triggering factor that enables this research field to push ahead is the availability of a new synthesis protocol that has opened up for the synthesis of such air-sensitive materials, not earlier explored. The interesting properties of fluoroperovskites emerge from their distorted crystal structure induced by Jahn-Teller active states for Cr(II) and Cu(II), d4 and d9. By now being able to explore a wider series of such compounds, fundamental insight can be gained on effects due to structural distortions, volume, and chemical bonding. Corresponding manganese and copper oxides, which also have a d4/d9 high spin state, show a multitude of exciting physical properties. A key question is how these may also emerge in Cr(II) and Cu(II) fluorides. An important part of this project is competence building at SINQ. Hence, in this project, we will investigate so far scarcely explored compounds as well as novel Cr(II) and Cu(II) fluoroperovskites by advanced neutron techniques, accessed through the host institution, PSI. We will study the response of fluoroperovskites to external stimuli (temperature, pressure, magnetic field) to obtain a fundamental understanding of the compounds. This project combines a two-year stay at PSI with synthesis activities at UiO, and a subsequent one year at IFE with the aim to further develop the research project, strengthen links to PSI staff and NcNeutron, and bring the unique competence learned at PSI to Norway for dissemination and integration into research projects at IFE and UiO.