Stretch and relax! – Losing one electron switches magnetism on in dichromium

Dichromium: both chromium atoms "share" the  12 valence electrons which leads to a multible bond.

Dichromium: both chromium atoms "share" the 12 valence electrons which leads to a multible bond. © HZB

Dichromium-Cation: ten out of the eleven remaining valence electrons are localised around an atom. Their spins are aligned thus leading to ferromagnetism. Only one electron is taking care of the molecular bonding.

Dichromium-Cation: ten out of the eleven remaining valence electrons are localised around an atom. Their spins are aligned thus leading to ferromagnetism. Only one electron is taking care of the molecular bonding. © HZB

An international team of scientists from Berlin, Freiburg and Fukuoka has provided the first direct experimental insight into the secret quantum life of dichromium. Whereas in its normal state the 12 valence electrons form a strong multiple bond between the two chromium atoms, removing only one electron changes the situation dramatically: 10 electrons localize and align their spins, thus resulting in ferromagnetic behavior of the dichromium-kation. The bonding is done by one electron only, resulting in a much weaker bond. The scientists used the unique Nanocluster Trap experimental station at the BESSY II synchrotron radiation source at Helmholtz-Zentrum Berlin and published their results in the Journal Angewandte Chemie.

The electronic structure and bonding of seemingly simple diatomic molecules like dichromium has puzzled scientist for decades. In surprisingly many cases, the ground state of these smallest molecules is still unknown even after a century of quantum mechanics. Because of the enormous computational challenge associated with the correct description of low-lying excited states and multiple bonds, the sextuple bond in the low-spin ground state of neutral Cr2 molecules has become a benchmark criterion in electronic structure calculations. In a joint effort, an international team of scientists from Berlin, Freiburg and Fukuoka has now provided the first direct experimental proof of an unexpected high spin ground state of Cr2+, the cationic cousin of Cr2.

Dramatic effect on magnetism

The team studied the effect of x-ray magnetic circular dichroism on free Cr2+ ions that were stored at 18 K in a dedicated cryogenic ion trap. This effect gives direct experimental insight into spin coupling and localization of the relevant valence electrons. For their experiments, the scientists used the unique Nanocluster Trap experimental station that is available at beamline UE52-PGM of the BESSY II synchrotron radiation source at Helmholtz-Zentrum Berlin.

Localisation of ten valence electrons

Even though only one out of twelve electrons is removed when ionizing Cr2 the molecule reacts dramatically, with complete localization of all ten 3d electrons and with maximum spin coupling. This turns an archetypal antiferromagnet ferromagnetic. “It’s a dramatic effect we see,” says team leader Tobias Lau. “Its particular spin configuration can be interpreted as a result of indirect exchange coupling, where the two groups of localized electrons “talk” to each other via a single bonding electron as a messenger that controls the parallel alignment of all their spins,” says Vicente Zamudio-Bayer who conducted this work as part of his PhD thesis at HZB and TU Berlin and who now continues his research as a postdoc in the Freiburg group.

Almost the same bonding energy

While in the neutral molecule all twelve valence electrons participate in bonding and create a short, unusual sextuple bond, the cation is only bound by one single electron with an almost doubled bond distance but almost the same bond energy. These significantly different bonding situations illustrate the fragile and untypically weak multiple bond in dichromium. They can be visualized as a change from a short and tight multiple bond to which all valence electrons contribute, to a long and loose single bond with all electrons except one localized at both ends. Combining their new results with earlier findings, the scientists can now even give relative energies of the excited states that have caused much confusion in the correct description of this molecular ion, a fact that will facilitate future theoretical approaches.

Cooperation and experimental set up

The experimental setup that was used for this research is operated jointly by Helmholtz-Zentrum Berlin, Universität Freiburg, and Kyushu University in Fukuoka, Japan. It is currently the only setup worldwide that provides the opportunity to investigate with x-ray spectroscopy ultralow density samples of a broad range of gaseous and size-selected molecular ions, clusters and complexes trapped at cryogenic temperatures in a strong magnetic field. This unique setup at BESSY II is currently upgraded in a BMBF-funded project of Universität Freiburg for even lower temperature and increased sensitivity, with the promise for more of these exciting results to come.

This important experimental result is published as an  Zeitschrift Angewandte Chemie International Edition. DOI: 10.1002/anie.201411018

TL/arö

You might also be interested in

  • Humboldt Fellow Alexander Gray comes to HZB
    News
    12.08.2022
    Humboldt Fellow Alexander Gray comes to HZB
    Alexander Gray from Temple University in Philadelphia, USA, is working with HZB physicist Florian Kronast to investigate novel 2D quantum materials at BESSY II. With the fellowship from the Alexander von Humboldt Foundation, he can now deepen this cooperation. At BESSY II, he wants to further develop depth-resolved X-ray microscopic and spectroscopic methods in order to investigate 2D quantum materials and devices for new information technologies even more thoroughly.
  • Green hydrogen: Nanostructured nickel silicide shines as a catalyst
    Science Highlight
    11.08.2022
    Green hydrogen: Nanostructured nickel silicide shines as a catalyst
    Electrical energy from wind or sun can be stored as chemical energy in hydrogen, an excellent fuel and energy carrier. The prerequisite for this, however, is efficient electrolysis of water with inexpensive catalysts. For the oxygen evolution reaction at the anode, nanostructured nickel silicide now promises a significant increase in efficiency. This was demonstrated by a group from the HZB, Technical University of Berlin and the Freie Universität Berlin as part of the CatLab research platform with measurements among others at BESSY II.
  • RBB Abendschau on visit at CatLab
    News
    01.08.2022
    RBB Abendschau on visit at CatLab
    CatLab got a visit from the rbb Abendschau.
    Under the title "Der Weg weg vom Erdgas" (The way away from natural gas), the programme was broadcast on Sunday, 31st July in the rbb Abendschau and will be available in the rbb media library for 7 days.