Charge ordering in cationic mixed-valence compounds is of crucial importance for materials science. The prototypic example for a transition from a charge-disordered to a charge-ordered state has been magnetite, Fe3O4, where Evert Verwey observed a sudden jump in resistivity near -150°C. In the journal Science Advances now a research team of scientists from Germany and Slovenia reports a Verwey-type charge-ordering transition in a different class of mixed-valence compounds that is composed of negatively charged dioxygen molecules.
Charge ordering in mixed-valence compounds, which usually contain positively charged cations in more than one formal charge state, is of crucial importance for materials science. Many functional properties of materials like magnetism, magnetoresistance, ionic conductivity and superconductivity are found in mixed valence compounds.
One of the very first attempts to understand the mechanism of a charge ordering transition dates back to 1939 when Evert Verwey, a Dutch chemist, observed a sudden jump in resistivity in the prototypic mixed valence compound magnetite, Fe3O4, near -150°C. He proposed that in the nearly metallic state of Fe3O4 at temperatures higher than -150°C, certain iron atoms are indistinguishable in the crystal lattice due to a full delocalization of charges.
However, below this temperature a semiconducting state with a complex ordering pattern of charges over the available iron sites was established. In general, a transition from a charge-disordered into a charge-ordered state which is accompanied by a jump in electrical resistivity is nowadays called Verwey transition.
In the journal Science Advances now a research team of scientists from Germany and Slovenia reports a Verwey-type transition in a completely different class of mixed-valence compounds which is composed of negatively charged dioxygen molecules.
The cesium compound Cs4O6 undergoes a phase transition from a state with indistinguishable molecular O2x- entities to a state with well- defined singly charged superoxide O2- and doubly charged peroxide O22- anions, which is accompanied by a corresponding change in charge transport dynamics. These results are supposed to shed new light on the mechanism of Verwey-type charge ordering phenomena.
“The Verwey-type transition in the cesium oxide Cs4O6 is conceptually more simple than the original one in Fe3O4” states Peter Adler, the first author of the study. Whereas in Fe3O4 the charge order state is complex and it is difficult to identify individual charge states the situation is more unambiguous in Cs4O6 as the molecular entities in the charge ordered state reveal the typical structural and magnetic features of the constituting paramagnetic superoxide and diamagnetic peroxide units.
The authors have established the properties of Cs4O6 by using several experimental techniques, namely neutron diffraction for the study of the crystal structure as well several spectroscopic techniques for verifying a charge localization transition which is a typical hallmark for Verwey-type charge ordering processes.
By contrast to Fe3O4 the present compound Cs4O6 remains magnetically disordered down to a temperature of -271°C which is only two degrees above the absolute zero point and it is much less conducting than Fe3O4. The molecular nature of the basic building units and the peculiar features of Cs4O6 should be favorable for disentangling the complex interplay of the charge, spin, and lattice contributions to Verwey-type charge ordering phenomena. The intertwining of the different degrees of freedom may in the future even be used to control new materials functionalities through the Verwey transition.
Contemporary examples for functional materials properties related to mixed-valence are high-temperature superconductivity in copper oxide-based superconductors where the superconducting state competes with a magnetic and charge order state or very large (“colossal”) magnetoresistance effects in certain manganese oxides which occur in the vicinity of a transition from a delocalized to a more localized mixed-valence state.
Accordingly, unravelling the mechanism of charge ordering processes in mixed-valence compounds is an important subject in materials science. The breakthrough of the present study is the observation of such charge ordering in a relatively simple crystal structure where novel physical phenomena is expected to emerge from intertwining of degrees of freedom pertinent to the well-defined electronically active oxygen molecular units.
The work was partly performed within the project LEMSUPER (Light Element Molecular Superconductivity) which was supported by the EU Seventh Framework Programme (FP7). Further on, the work was supported by the Slovenian Research Agency. The research involved cooperation of several institutions in Germany and Slovenia, namely the Max Planck institutes for Chemical Physics of Solids in Dresden and for Solid State Research in Stuttgart, the Helmholtz Centre for Materials and Energy in Berlin, the Justus-Liebig University in Giessen, the Jožef Stefan Institute in Ljubljana and the University of Ljubljana.
The research at the Max Planck Institute for Chemical Physics of Solids (MPI CPfS) in Dresden aims to discover and understand new materials with unusual properties.
In close cooperation, chemists and physicists (including chemists working on synthesis, experimentalists and theoreticians) use the most modern tools and methods to examine how the chemical composition and arrangement of atoms, as well as external forces, affect the magnetic, electronic and chemical properties of the compounds.
New quantum materials, physical phenomena and materials for energy conversion are the result of this interdisciplinary collaboration.
The MPI CPfS is part of the Max Planck Society and was founded in 1995 in Dresden. It consists of around 280 employees, of which about 180 are scientists, including 70 doctoral students.
Dipl.-Übers. Ingrid Rothe | Max-Planck-Institut für Chemische Physik fester Stoffe
New yeast species discovered in Braunschweig, Germany
13.12.2019 | Leibniz-Institut DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH
Saliva test shows promise for earlier and easier detection of mouth and throat cancer
13.12.2019 | Elsevier
Vaccinia viruses serve as a vaccine against human smallpox and as the basis of new cancer therapies. Two studies now provide fascinating insights into their unusual propagation strategy at the atomic level.
For viruses to multiply, they usually need the support of the cells they infect. In many cases, only in their host’s nucleus can they find the machines,...
More than one hundred and fifty years have passed since the publication of James Clerk Maxwell's "A Dynamical Theory of the Electromagnetic Field" (1865). What would our lives be without this publication?
It is difficult to imagine, as this treatise revolutionized our fundamental understanding of electric fields, magnetic fields, and light. The twenty original...
In a joint experimental and theoretical work performed at the Heidelberg Max Planck Institute for Nuclear Physics, an international team of physicists detected for the first time an orbital crossing in the highly charged ion Pr⁹⁺. Optical spectra were recorded employing an electron beam ion trap and analysed with the aid of atomic structure calculations. A proposed nHz-wide transition has been identified and its energy was determined with high precision. Theory predicts a very high sensitivity to new physics and extremely low susceptibility to external perturbations for this “clock line” making it a unique candidate for proposed precision studies.
Laser spectroscopy of neutral atoms and singly charged ions has reached astonishing precision by merit of a chain of technological advances during the past...
The ability to investigate the dynamics of single particle at the nano-scale and femtosecond level remained an unfathomed dream for years. It was not until the dawn of the 21st century that nanotechnology and femtoscience gradually merged together and the first ultrafast microscopy of individual quantum dots (QDs) and molecules was accomplished.
Ultrafast microscopy studies entirely rely on detecting nanoparticles or single molecules with luminescence techniques, which require efficient emitters to...
Graphene, a two-dimensional structure made of carbon, is a material with excellent mechanical, electronic and optical properties. However, it did not seem suitable for magnetic applications. Together with international partners, Empa researchers have now succeeded in synthesizing a unique nanographene predicted in the 1970s, which conclusively demonstrates that carbon in very specific forms has magnetic properties that could permit future spintronic applications. The results have just been published in the renowned journal Nature Nanotechnology.
Depending on the shape and orientation of their edges, graphene nanostructures (also known as nanographenes) can have very different properties – for example,...
03.12.2019 | Event News
15.11.2019 | Event News
15.11.2019 | Event News
13.12.2019 | Physics and Astronomy
13.12.2019 | Physics and Astronomy
13.12.2019 | Materials Sciences