A new class of complex solids: orbitally active transition metal fluorides

The project has focused on structural, electronic, and magnetic phenomena in solids. Such topics are at the forefront of contemporary multidisciplinary condensed matter research, not just because of remarkable physical properties that challenge existing theories but also due to their technological significance. Prominent examples are the transition metal oxides, however, similar behaviour is also displayed by other families of strongly correlated solids such as sulfides, nitrides and fluorides. Transition metal fluorides have up to now been scarcely studied, though some have been explored in the past as model systems in magnetism. We have now undertaken systematic studies of such transition metal fluorides and uncovered novel behaviours by tuning the interplay of the many degrees of freedom and ordering parameters. Pressure induced structural phase transition in NaFeF3 have been studied using synchrotron radiation at the ESRF, Grenoble. Likewise structural rearrangements have been studied as function of temperature for the KMnCrF6 ordered magnetic ferroelectric. The project has successfully developed a new soft synthesis route to fluorides, which has given us access to high quality samples and compositions not earlier investigated. The new route is under evaluation for patenting at the inven2 office at UiO.In this way we have synthesized the entire family of Na-(and partly K-)based ABF3 fluorides with B being a divalent transition metal cation, including related type of compounds. The structural and physical properties of these have been studied experimentally by synchrotron and neutron diffraction, whereas magnetic, electronic and optical properties have been measured in our home labs. The observed properties have been compared with predications and results from extensive DFT modeling. The main focus for a PhD work has been the Na-Cr-F system. The success in this respect opens for promising future research at UiO, probably extending the fundamental studies to applications within photon conversion (up-/down- conversion for photovoltaics) and possible application within medical technology. In addition to these studies on fluorides, the project has delivered new insight in other strongly correlated systems, for instance Fe-Se based high-Tc superconductors. The project has involved SINTEF, IFE and ESRF as partners, with joint publications appearing.

Modern technology (eg. communication, memory recording, computing) strongly rely on the conducting and magnetic properties of solid state materials. Semiconducting silicon chips and magnetic iron oxide memories are prominent examples of inorganic material s which are constantly used in our everyday life. Due to the fast growing need for more efficient technologies and devices, the performance requirements for materials are becoming increasingly tougher. Research in this area is focusing on complex material s -the so called "strongly correlated electron systems"- where the electrons couple with each other affecting the conducting and magnetic properties. This high degree of electronic correlations gives rise to a broad range of novel and technological exploi table phenomena such as colossal magnetoresistance (CMR) where enormous variation in resistance are produced by small magnetic field changes and high-temperature superconductivity (HTSC) where the electrons can travel freely within the material with zero resistance. The observation of these phenomena has dramatically challenged our understanding of solids. It follows that the design and experimental characterization of novel families of strongly electron correlated systems are absolutely essential to guid e theory, to understand the important ingredients for the observation of interesting properties and discover new phenomenology. The aim of the project is to undertake a systematic exploration with the goal of uncovering novel and/or rare behaviours throug h the synthesis and detailed physical characterization of selected families of transition metal fluorides. At the end of this study we would have achieved a systematic understanding of the role played by the strong electronic correlations in a rich family of fluoride systems. The fine tuning of the interplay of the many available degrees of freedom in these systems should hold unforeseen surprises and observation of unprecedented phenomena.